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Quantitative Transmission X-Ray Microanalysis of Ionic Compounds
Abstract
A new absorption correction applicable to EDX spectra of ionic compounds (oxides, ceramics, minerals, …) is presented. The method enables the accurate quantification of oxygen and nitrogen, and does not require parameters such as specimen thickness, X-ray take-off angle and specimen density. Physically this new correction procedure is based on the sole principle of electroneutrality of the specimen: this means that the sum of all anions and cations times their respective valence states must cancel out. An additional benefit of this method is the possibility to calculate the average thickness of the analysed area if, after the quantitative analysis, numbers for the specimen density and X-ray take-off angle are available.
... In this paper, we test methods commonly used in TEM-EDS analysis quantification on several reference samples, namely the standardless method [11], the Cliff-Lorimer approximation [12] and the absorption correction method based on electroneutrality [13]. Moreover, a spreadsheet based on a parameterless absorption correction approach to EDS spectra of ionic compounds is also presented. ...
... To overcome the problem of knowing the thickness and the beam current, an alternative method based on the electron neutrality criterion has been developed by Van Cappellen and Doukhan [13], in which no direct measurement of either thickness or beam current is required. Even the density of the sample and the take-off angle are not required, unless one needs to know the real thickness of the analysed area for some other reasons. ...
... In Table 4 we report TEM-EDS results for these reference samples along with their bulk chemical composition obtained with EMPA (before crushing or ion-milling). For the determination of the thickness with the ACM [13], densities of 3.52, 3.09, 2.60, 2.52, 4.39 and 3.87 g/cm 3 were assumed for johannsenite, biotite, cordierite, antigorite, fayalite and spinel, respectively. ...
Available quantification methods for energy dispersive X-ray microanalysis in transmission electron microscopy, such as the standardless method (SLM), the Cliff-Lorimer approximation (CLA) and the absorption correction method (ACM), are compared. As expected, the CLA and ACM give superior results with respect to the SLM. As far as absorption can be considered negligible, CLA and ACM perform similarly. However, starting from mass-thickness of the order of 22 × 10-6 g/cm2, absorption become significant and the ACM gives better results. More accurate analyses can be obtained with the ACM if distinct kO/Si factors are determined for light and heavy minerals, respectively, placing a divide at 2.90 g/cm3. Caution must be used when k-factors are derived indirectly from minerals with very different structure/chemistry, suggesting that separate k-factors data sets are required for accurate EDS quantification, at least for the major and diverse broad classes of minerals. Element diffusion of monovalent cations and channelling effects may represent a complication, especially in very anisotropic minerals such as phyllosilicates, where these two phenomena may occur together.
... There are two ways to calculate the quantitative chemical composition using the software associated with ATEM: one is to calculate oxygen as an independent element using the X-ray intensity of oxygen in the same way as the other existing elements in Eq. (1) (hereafter referred to as 'Oxygen Independent mode'); and the other is to calculate all the existing elements except oxygen as oxides, without using the X-ray intensity of oxygen (hereafter referred to as 'Oxide mode'). In the former case, the thickness used in the software is often selected so that the calculated atomic percentage of some key element becomes equal to that of the ideal chemical formula (in many cases Si or O is selected for silicate minerals), or the calculated atomic percentage of all the elements satisfies the electrical neutrality (Van Cappellen and Doukhan, 1994). In the latter case, the various thicknesses are inputted into the software until the calculated atomic percentage of some key element becomes equal to that of the ideal chemical formula because the electrical neutrality is already maintained. ...
... In each SSR figure, the data points were approximated by a quadratic least-squares regression, and the minimum value of the SSR(t) was defined as the 'minimum of the SSR(t) function'. For some data points of the SSR(t), the calculated atomic per- Figure 4 show that the minimum values of the curves of the SSR(t) for the Oxide mode are smaller than those of the Oxygen Independent mode, and are equal to or less than 0.05 for systems A and B. This means that the compositions estimated by the method by Van Cappellen, and Doukhan (1994) are not so accurate because the compositions estimated by their method are based on the calculations by the Oxygen Independent mode. The atomic percentage of oxygen at the minimum point of the SSR(t) for the Oxide mode is very close to the ideal value of 60.00 of garnet for systems A and B. ...
... This may reflect the fact that the X-ray intensity of oxygen is not as accurately measured as those of the other elements by the X-ray counter. Therefore, the widely used method by Van Cappellen and Doukhan (1994) is not appropriate to obtain the correct chemical compositions because it uses the absorption correction based on the Oxygen Independent mode. The minimum value of the SSR appropriate for obtaining a reliable chemical composition may depend on the number of ions to determine their compositions and the number of different ionic sites. ...
Analytical transmission electron microscopy (ATEM) is a powerful tool to obtain the chemical compositions of minerals at very small areas of minerals down to the nanometer scale. Most ATEM analytical systems are capable of performing quantitative calculations of chemical compositions by absorption correction. However, an appropriate procedure of obtaining the correct chemical compositions by absorption correction has not yet been established. In this study, we analyzed thin foils of garnet and olivine with known compositions and thicknesses using two different analytical systems to evaluate the certainty of the absorption correction based on the Cliff–Lorimer equation. The results show that the absorption correction using the real sample thicknesses at the analyzed spots did not yield the correct chemical compositions. The correct quantification data were obtained when using the sample thickness t corrected to minimize the value of the function SSR(t) (sum of squared residuals) = Σi [ni⁰－Σj nij (t)]², where ni⁰ is the ideal atomic percentage of the i site, nij(t) is the atomic percentage of the j ion at the i site calculated by the software, and the sum holds all the ionic sites. Also, it was confirmed that the minimum value of the SSR obtained by calculating all existing elements as oxide (oxygen is not quantified) is in most cases smaller than that obtained by quantifying all elements, including oxygen, independently. This method of using the SSR without knowing the real thickness at the analyzed spot can be applied to the analyzed results of absorption correction by any software. Therefore, this SSR has the potential of becoming a universal indicator to assess the results of quantitative chemical analyses by absorption correction in ATEM.
... As mentioned previously, the knowledge of sample density may not be accessible and the measurement of local thickness can be tedious. Van Cappellen et al. [29] proposed a method where the knowledge of thickness is not necessary, and when analyzing stoichiometric samples, Van Cappellen et al. [34] proposed another approach based on electroneutrality of the specimen. No matter which type of acquisition used, ATEM analysis faces a common problem to translating the apparent concentration to the actual concentration at the grain boundary [36]. ...
... The appearing of intergranular fracture for matensite requires only very low phosphorus grain boundary concentration (gray comparison between M and MIA in gure II.5.6). Based on the measured phosphorus concentration, the critical phosphorus grain boundary concentration is between 0.34 and 1.15 atom/nm 2 (0.02 -0.06 monolayer when considering 1 monolayer = 19.41 atom/nm 2 ). ...
Phosphorus intergranular segregation in low alloy steels is known to decrease brittle fracture stress and induce intergranular fracture, which causes reversible temper embrittlement. in the nuclear industry, nominally bainitic steels are subjected to this phenomenon and the question of microstructure susceptibility has been raised as literature shows that tempered martensite is more susceptible than tempered bainite. the objective of this study is to understand if this result is related to a different segregation behavior of the two microstructures. the first part of our work establishes a segregation quantification method using stem-edx, that was cross-compared with four other analytical techniques. then, a comparative study of intergranular segregation in tempered bainitic and martensitic 16mnd5 steel after different aging treatments is conducted based on the developed analytical method. the results show small microstructural influence on segregation. phosphorus, nickel, manganese and molybdenum were observed at all types of grain boundaries in both microstructures. the segregation concentrations are higher in prior austenitic grain boundaries than in other types of boundaries. taking account of the segregation amount in different types of boundaries, the phosphorus bulk depletion is shown negligible. finally, using charpy tests, we confirm the lower susceptibility of tempered bainite to thermal aging by temper embrittlement and conclude that this is not related to a different segregation behavior from tempered martensite.
... Dans le cas d'échantillons minces, tels que ceux utilisés en microscopie à transmission, des méthodes quantitatives ont été développés par Cliff et Lorimer [3], Van Cappellen [4][5], Eibl [6][7], ou Horita [8] 1.1. ...
... Pour réaliser une analyse sur un échantillon de composition inconnue Van Cappellen propose une première méthode de travail [2] comparable à celle utilisée pour l'extraction des kfacteurs. Pour chaque point du profil d'intensité en fonction de l'épaisseur, la concentration massique de chaque élément est calculée grâce à la formule : Dans le cas de composés ioniques, une alternative basée sur l'équation de l'équilibre des charges peut être utilisée [5]. En effectuant pour un même spectre des calculs à différentes épaisseurs (fictives), il est possible de retrouver la seule épaisseur (fictive elle aussi) qui satisfait à l'équilibre des charges positives et négatives. ...
L'utilisation de la microscopie électronique en transmission couplée à un système d'analyse est un outil de choix pour mener à bien une étude quantitative complète (chimie et structure) sur des nano objets ou des nano phases. Dans La première partie de ce travail consistait à doser quantitativement les éléments présents dans un échantillon sous forme de poudre, en s'attachant plus particulièrement au cas de l'oxygène. La méthode d'analyse EDX employée fut celle décrite par Van Cappellen. Puis, une étude a été menée pour déterminer les conditions d'expérience optimales en diffraction électronique en sélection d'aire pour résoudre la structure des matériaux (intégration des intensités). Cette étude a montré les paramètres déterminants pour minimiser les effets dynamiques. La troisième partie a permis de résoudre partiellement la structure du composé YBCO en se basant sur des images haute résolution à partir desquelles les amplitudes et phases des faisceaux diffractés ont été extraites. Les positions des atomes de cuivre, de baryum et d'yttrium dans ce composé ont été " déterminées. Afin d'estimer les épaisseurs des lames analysées, une étude a été faite en faisceau convergent en " condition deux ondes ". Un programme informatique permettant de trouver rapidement les épaisseurs suivant la technique de Kelly et al. et de simuler des clichés de diffraction en conditions " deux ondes " ou de tracer des cartographies d'intensités en fonction de l'épaisseur et de l'écart à l'angle de Bragg a été développé.
... The former elements were deconvoluted and quantified, whereas the latter ones were only deconvoluted as they do not originate from the sample itself. Element concentrations were determined from intensity ratios using the method of Van Cappellen and Doukhan (1994), where absorption and fluorescence were fitted using the thin-foil approximation taking the Al/O ratios determined by EMPA and a sample density of 4.56 g cm −3 into account. Thickness variations of the lamellae, which can potentially cause artifacts, were excluded by thickness mapping using zero-loss-filtered TEM (EFTEM). ...
We studied grain boundary diffusion and segregation of
La, Fe, Mg, and Ti in a crystallographically defined grain boundary in
yttrium aluminum garnet (YAG). Bi-crystals were synthesized by wafer
bonding. Perpendicular to the grain boundary, a thin-film diffusion source
of a La3.60Al4.40O12 was deposited by pulsed laser
deposition. Diffusion anneals were performed at 1000 and 1450 ∘C. Via a gas phase small amounts of elements were added during
the experiment. The element concentration distributions in our bi-crystals
were mapped using analytical transmission electron microscopy (ATEM). Our
results show strong segregation of La and Ti at the grain boundary. However,
in the presence of Ti, the La concentrations dropped below the detection
limit. Quantitative element distribution profiles along and across the grain
boundary were fitted by a numerical diffusion model for our bi-crystal
geometry that considers the segregation of elements into the grain boundary.
The shape of the diffusion profiles of Fe requires the presence of two
diffusion modes, e.g., the co-diffusion of Fe2+ as well as Fe3+.
The absence of a detectable concentration gradient along the grain boundary
in many experiments allows a minimum value to be determined for the product of
sDgb. The resulting sDgb are a minimum of 7 orders of magnitude
larger than their respective volume diffusion coefficient, specifically for La = 10−14 m2 s−1, Fe = 10−11 m2 s−1, Mg = Si = 10−12 m2 s−1, and Ti = 10−14 m2 s−1 at 1450 ∘C. Additionally, we model the effect
of convolution arising from the given spatial resolution of our analysis
with the resolution of our modeled system. Such convolution effects result
in a non-unique solution for the segregation coefficient, e.g., for example
for Mg between 2–3. Based on our data we predict that bulk diffusion of
impurities in a mono-phase polycrystalline aggregate of YAG is effectively
always dominated by grain boundary diffusion.
... Both point analyses and elemental mapping analyses were conducted. EDX analyses were quantified according to a modified Cliff-Lorimer method described by van Cappellen and Doukhan (1994) and Langenhorst et al. (1995). The composition of tissintite was obtained using the Electron Probe Micro-Analyzer (EPMA, JEOL 8100) with wavelength dispersive spectrometers (WDS) at Nanjing University. ...
Shock-induced Ti-rich melt pockets in a basaltic eucrite Northwest Africa (NWA) 8003 were studied using scanning and transmission electron microscopy. Unique mineral assemblages consisting of clinopyroxene, ilmenite, vestaite, corundum, and kyanite are observed. Among them, vestaite and corundum in NWA 8003 are first reported to occur in eucrite meteorites. Petrographic and chemical evidences indicate that the Ti-rich melt pockets have formed by in-situ melting of ilmenite, plagioclase, pyroxene, and possibly minor silica and apatite nearby. The temperature rise and melting were caused by the high shock impedance contrast at interfaces between ilmenite and other phases with a distinctly lower density. Crystallization pressure, temperature and cooling time of the Ti-rich melt pockets in NWA 8003 are constrained to be ˜0.9–˜10 GPa, ˜1300–˜1730 °C, and < 1 ms (5–50 μm in size), respectively.
... The FIB samples were studied using a 200 keV FEI Technai G2 equipped with an Oxford SDD EDS detector and a Fischione HAADF detector for scanning TEM imaging. EDS data of silicates were quantified using the Cliff-Lorimer method with electroneutrality-based thickness constraints for the absorption correction (Van Cappellen and Doukhan 1994). Analyses of metal grains were corrected for absorption using a thickness estimated from SEM images obtained during sample preparation. ...
Based on the high abundance of fine‐grained material and its dark appearance, NWA 11024 was recognized as a CM chondrite, which is also confirmed by oxygen isotope measurements. But contrary to known CM chondrites, the typical phases indicating aqueous alteration (e.g., phyllosilicates, carbonates) are missing. Using multiple analytical techniques, this study reveals the differences and similarities to known CM chondrites and will discuss the possibility that NWA 11024 is the first type 3 CM chondrite. During the investigation, two texturally apparent tochilinite–cronstedtite intergrowths were identified within two thin sections. However, the former phyllosilicates were recrystallized to Fe‐rich olivine during a heating event without changing the textural appearance. A peak temperature of 400–600 °C is estimated, which is not high enough to destroy or recrystallize calcite grains. Thus, calcites were never constituents of the mineral paragenesis. Another remarkable feature of NWA 11024 is the occurrence of unknown clot‐like inclusions (UCLIs) within fine‐grained rims, which are unique in this clarity. Their density and S concentration are significantly higher than of the surrounding fine‐grained rim and UCLIs can be seen as primary objects that were not formed by secondary alteration processes inside the rims. Similarities to chondritic and cometary interplanetary dust particles suggest an ice‐rich first‐generation planetesimal for their origin. In the earliest evolution, NWA 11024 experienced the lowest degree of aqueous alteration of all known CM chondrites and subsequently, a heating event dehydrated the sample. We suggest to classify the meteorite NWA 11024 as the first type 3 CM chondrite similar to the classification of CV3 chondrites (like Allende) that could also have lost their matrix phyllosilicates by thermal dehydration.
... Selected-area electron diffraction (SAED), conventional bright-field (BF) imaging, high-resolution (HR) imaging, and scanning transmission electron microscopy (STEM) mode were used to observe the occurrence of vestaite and associated minerals, and to determine the chemical composition and structural data of vestaite. EDX spectra were recorded using an Oxford X-Max N 80T SDD system and were quantified by taking an X-ray absorption correction into account (Van Cappellen and Doukhan 1994;Langenhorst et al. 1995). Fe and Ti L 3,2 electron energy loss near-edge structure (ELNES) spectra were measured on vestaite and ilmenite using a Gatan GIF Quantum. ...
Our investigations on the shocked eucrite Northwest Africa (NWA) 8003 revealed the occurrence of a new mineral, vestaite [IMA 2017-068; (Ti 4+ Fe 2+)Ti 3 4+ O 9 ]. This mineral coexists with corundum, ilmenite, and Al-Ti-rich pyroxene in shock melt pockets. It has an empirical chemical formula of (Ti 4+ 0.73 Fe 2+ 0.63 Al 0.60 Mn 0.03 Mg 0.02 Cr 0.01)Ti 3 4+ O 9 and the monoclinic C2/c structure of schreyerite. The ideal vestaite structure can be considered as a modular structure with an alternate intergrowth of M 3 O 5-type (M = Ti 4+ ,Fe 2+ ,Al) and Ti 2 O 4-type slabs. Alternatively, it can also be envisaged as a crystallographic shear structure with periodically shearing of rutile or a-PbO 2 units. Streaking and splitting of diffraction spots observed in selected-area electron diffraction patterns indicate planar defects in the modular structure of vestaite. Our observations reveal that vestaite crystallized at high pressure (≤10 GPa) from a melt that represents a mixture of ilmenite and silicate components. A robust constraint on its formation conditions and stability field cannot yet be provided due to the lack of experimental data for these systems. Vestaite is a new, shock-generated mineral first found in a meteorite of the howardite-eucrite-diogenite (HED) clan, the largest achondrite group. Its discovery is not only of significance to the meteoritic mineralogy, but it could also be of interest to materials science.
... Samples were analysed with a TEM-EDX/FIB coupled method and quantitative analyses were deduced and corrected thanks to the t-Oprotocol (Bourdelle et al., 2012;Van Cappellen and Doukhan, 1994). The TEM-EDX used was a JEOL 2100-F at the Physics and Microanalysis Department of IFPEN Lyon-Solaize, using a 200 kV voltage, a counting time of 60 s and a dead time lower than 15%. ...
A combination of adapted chlorite thermometry and high-spatial-resolution analytical techniques (TEM-EDX/FIB) shows that the low-grade metamorphic chlorites of the Glarus Alps, Central Alps (Switzerland), do not record the peak metamorphic conditions as commonly assumed in previous studies of this reference area for low-grade metamorphism. Chlorites have rather recorded several stages of the retrograde path, through an intracrystalline nanometric-scale compositional zoning. The consistency of the nanoscale zoning patterns observed both within sample and along the transect suggests that local equilibrium was achieved at this spatial scale and maintained during growth or re-equilibration. Applying recent thermobarometers, we highlight that chlorites recorded a distinct behaviour between the northern and southern part of the studied transect: the south of the Glarus area displays a regular P-T exhumation path, from 3.0 ± 0.2 kbar and 310 ± 20 °C for maximum P-T (estimated with conventional thermometers), to ~0.8 kbar and ~220 °C according to chlorite crystal-core analysis, and to ~0.3 kbar and ~190 °C according to chlorite crystal-rim analysis. On the contrary, the north of the Glarus area shows an apparent break in the exhumation, with P estimates from chlorite crystal rims (~1.3 kbar) higher than from chlorite crystal cores (~0.8 kbar). Even if the absolute pressure values are fraught with large uncertainty, their contrasting core/rim pattern is not model dependent. We try to correlate these new thermobarometric results with independent data to refine the exhumation scenario of the North-Helvetic flyschs, confirming differential uplift along the transect. This reappraisal of Glarus chlorite thermometry demonstrates that an adapted thermometry/analysis protocol opens new prospects for investigating the evolution of low-grade metamorphic terranes.
... Finally, semi-quantitative EDS analyses were obtained with the standardless method and corrected for absorption following Van Cappellen and Doukhan (1994). ...
The garnierite ore at Campello Monti occurs as dark green colloform concretions covering surfaces, fractures, and filling veins in harzburgite rocks. The representative composition (Ni2.45Mg0.14Cu0.12Co0.05)Σ2.76Si2.10O5(OH)4 is consistent with a 7 Å phase, namely pecoraite or népouite. Relevant chemical features are an exceptionally high Ni/Mg ratio, a significant level of Cu substituting for Ni, and a low content of S, possibly in tetrahedral sites. Olivine and orthopyroxene in the harzburgite host rock are only partially serpentinized, do not contain detectable Ni, and are almost iron free. The green coating probably originated from groundwater solutions that leached nearby weathered peridotites and sulfide ores, and deposited less-mobile elements along fractures and voids of the host peridotite, just outside their provenance area. Bulk techniques such as X-ray powder diffraction and infrared spectroscopy do not confidently distinguish between népouite and pecoraite, although the comparison with synthetic, implicitly pure polymorphs indicates népouite as the best matching phase. On the other hand, HRTEM clearly shows that garnierite is mostly constituted by plumose aggregates made of curved crystals with frayed tips, a few nanometers thick along the stacks and a few tens of nanometers long (nanoflakes). All known lizardite stacking sequences, namely 1T, 2H1, and 2H2, have been locally observed, even though most crystals show stacking disorder. The recorded nanostructure suggests possible explanations for the recurrent anomalies (low oxide totals, high IVT/VIM cation ratios, etc.) found in EMP analyses of garnierites. The small grain size, the high density of defects, and the structural arrangement actually intermediate between lizardite and chrysotile probably explain the ambiguities that occurred during the characterization with bulk techniques. The results obtained in this study may have important implications for ore processing methods.
... All the data are available in the tables and figures in the main text and supporting information. A more detailed discussion of the methodology can be found in the supporting information (Liu et al., 2016;Shcheka et al., 2006;Tange et al., 2009;Van Cappellen & Doukhan, 1994). The Focused Ion Beam instrument (FEI, Scios DualBeam) at Bayerisches Geoinstitut was financed by a DFG grant INST 91/315-1 FUGG. ...
Phase relations in the system MgSiO3-Al2O3 were investigated at pressures of 27-45 GPa and temperatures of 1700, 2000, and 2300 K using sintered diamond and tungsten carbide anvils in a multianvil apparatus. The bulk compositions in the MgSiO3-Al2O3 binary system crystallize a phase assemblage of pyrope and corundum at pressures below 27 GPa and an assemblage of bridgmanite and corundum at pressures above 27 GPa regardless of temperatures. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum increases significantly with increasing temperature. The solubility of Al2O3 in bridgmanite increases from 6.7 mol % at 1700 K to 21.8 mol % at 2500 K under a constant pressure of 27 GPa. Bridgmanite becomes more aluminous with increasing pressure from 27 to 45 GPa at a given temperature. The MgSiO3 content in corundum increases with increasing pressure at pressure lower than 27 GPa, while it decreases at pressure higher than 27 GPa. Our results suggest that bridgmanite can incorporate a considerably higher Al2O3 content than that of the pyrope composition (25 mol % Al2O3). The present study further suggests that the entire Al2O3 component is accommodated into bridgmanite in the pyrolite lower mantle. However, Al2O3 cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs in the shallow part of the lower mantle, and therefore, additional phases such as MgAl2O4 with calcium ferrite-type structure are necessary to host the excess Al2O3.
... TEM-EDX analyses were obtained in STEM mode using beam currents below 0.5 nA and integration times of typically 120 seconds. After background subtraction, the spectra were fitted using Gaussian functions and quantified using the Cliff-Lorimer technique involving an X-ray absorption correction based on charge neutrality ( van Cappellen and Doukhan 1994;Langenhorst et al., 1995 ). Cliff-Lorimer k factors for the elements O, Mg, Si, Mn, Fe, and Ni were determined from a dedicated FIB sample based on the composition obtained by EPMA and estimates of the sample thickness obtained from carbon marks left by the STEM probe. ...
Space weathering is one of the most common surface process occurring on atmosphere-free bodies such as asteroids and the Moon. It is caused mainly by solar wind irradiation and the impact of micrometeoroids. In order to simulate space weathering effects, in particular those produced by hypervelocity impacts, we produced microcraters via ultra-short (∼100 fs) laser irradiation of crystallographically oriented slices of forsterite-rich (Fo94.7) olivine. The main advantages of the application of a femtosecond laser radiation to reproduce the space weathering effects are (1) the high peak irradiance (10¹⁵ W cm⁻²), which generates the propagation of the shock wave at the nanosecond timescale (i.e., timescale of the micrometeoroid impacts); (2) the rapid transfer of energy to the target material, which avoids the interaction of laser light with the developing vapor plume; (3) a small laser beam, which allows the effects of a single impact to be simulated. The results of our spectroscopic and electron microscopic investigation validate this approach: the samples show strong darkening and reddening of the reflectance spectra and structural damages similar to the natural microcraters found on regolith grains of the Moon and asteroid 25143 Itokawa. Detailed investigations of several microcrater cross-sections by transmission electron microscopy allowed the detection of shock-induced defect microstructures. From the top to the bottom of the grain, the shock wave causes evaporation, melting, solid-state recrystallization, misorientation, fracturing, and the propagation of dislocations with Burgers vectors parallel to [001]. The formation of a short-lived vapor plume causes the kinetic fractionation of the gas and the preferential loss of lighter elements, mostly magnesium and oxygen. The high temperatures within the melt layer and the kinetic loss of oxygen promote the thermal reduction of iron and nickel, which leads to the formation of metallic nanoparticles (npFe⁰). The final stage of the microcrater formation is the cooling of the melt layer that results in its partial crystallization and the formation of olivine crystals with a palisade-like texture.
... The beam size was 0.5 nm. The EDS spectra were analyzed by using the method in ref. 43. ...
Significance
This paper reports an unexpected change in the oxidation state of Fe in bridgmanite, the most dominant mineral in the lower mantle. The oxidation state change resolves the discrepancy between laboratory and seismic studies on the chemical composition of the lower mantle, showing that the lower mantle has major element chemistry similar to the upper mantle. The oxidation state change will also lead to a lower Fe content in bridgmanite in the midmantle, whereas the total Fe content remains the same. Such a change can lead to an increase in viscosity at 1,100- to 1,700-km depths, providing a viable mineralogical explanation on possible viscosity elevation suggested by geophysical studies at the same depth range.
... EDX spectra were acquired to obtain compositions of the very small mineral inclusions using a ThermoNoran Ge detector attached to the TEM. These analyses were quantified on the basis of the principle of electroneutrality by taking into account an absorption correction (van Cappellen and Doukhan, 1994;Langenhorst et al., 1995) and the Fe 3+ /RFe ratios measured by electron energy loss spectroscopy (EELS). ...
The redox processes taking place in the portion of the mantle on top of the subducting slab are poorly investigated and the redox potential of crust-derived fluid phases is still poorly constrained. A case study of supra-subduction mantle affected by metasomatism from crust-derived fluid phases is represented by garnet orthopyroxenites from the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatised at ∼4 GPa, 750-800 °C by a silica- and incompatible trace element- rich fluid phase. This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid multiphase primary micro-inclusions in garnet display negative crystal shapes and infilling minerals (spinel, ±orthopyroxene, amphiboles, chlorite, ±talc, ±mica) occur with constant modal proportions, indicating that they derive from trapped solute-rich aqueous fluids. FT-IR hyper spectral imaging analyses and Raman spectroscopy, together with X-Ray microtomography performed on single inclusions indicate that liquid water is still preserved at least in some inclusions (±spinel).
... Local EDXS analyses were performed using STEM mode with a probe size of 5.6 nm. The Doukhan-Van Cappellen method [Van Cappellen and Doukhan, 1994], based on electroneutrality of specimen was applied to have access to the local thin foil thickness and correct the analyses from thickness effects on absorption, After absorption corrections, quantitative microanalysis could be performed with a final accuracy of a few percent. ...
... These chemical signatures are likely the consequence of moderate aqueous alteration on the parent body, oxidation reactions involving metal and sulfides and further redistribution of Table 2 Selected EDS analyses of anhydrous silicates (at.%). Data are normalized to 100% and were quantified using the procedure given by van Cappellen and Doukhan (1994). Uncertainties for the major elements (O, Mg and Si) are typically 3%, and those for the minor elements (<0.5 at.%) are typically 20%. ...
The Paris meteorite is a weakly altered CM chondrite that has been discovered recently (Hewins et al., 2014). Its matrix offers the opportunity to search for well-preserved pristine pre-accretional material, as well as to study the earliest stages of aqueous alteration in the CM parent body. The study of the matrix of Paris has been conducted by analytical transmission electron microscopy on focused ion beam sections extracted from matrix areas showing different degrees of aqueous alteration.The least altered matrix sample consists of amorphous silicate grains, a few hundreds of nm in size, separated from one another by an abundant porosity. The amorphous silicates enclose numerous Fe-sulfide nanograins and their average composition is close to the chondritic composition. They share many similarities with GEMS (glass with embedded metal and sulfides) grains present in chondritic-porous interplanetary dust particles and with primitive type 3.0 carbonaceous chondrites. This first discovery of GEMS-like texture in a CM chondrite suggests that GEMS grains could have been the building blocks of the CM matrices.In more aqueously altered samples, pronounced microstructural heterogeneities were detected at the micrometer scale. The matrix consists mostly of a mixture of amorphous material and Fe-rich, spongy to fine-fibrous, poorly crystalline phyllosilicates. The porosity fraction is significantly reduced and the mixed amorphous-fibrous material frequently forms a continuous groundmass. The close association between these two material types suggests a replacement mechanism due to aqueous alteration. Chemical compositions correlate strongly with the microstructure. The amorphous material has a composition close to the chondritic value while the fine-fibrous phyllosilicate material is Fe-enriched. This Fe enrichment is found to be continuous from weakly to more heavily altered areas, in which the fibrous morphology is coarser and better crystalline. Cronstedtite with intercalated tochilinite is also found, but in pore spaces. This chemical evolution, concomitant with the maturation of the phyllosilicates, demonstrates that the early aqueous fluids that interacted with silicates in the matrix were enriched in Fe. This composition is probably the consequence of the preferential dissolution of metal and iron sulfides during the first stages of alteration. The enrichment of phyllosilicates in Mg seen in more altered CM chondrites is not observed in Paris.
... ‡ Sample was too fine grained (Ͻ1 m) to determine the titanite composition using the electron microprobe; therefore it was determined from TEM using the method of Van Capellen and Doukhan (1994). KSi-factors at zero thickness for Ca and Ti have been determined on the pure titanite, that for O on forsterite. ...
Phase relations on the join CaTiSiO5-CaSi2O5 were determined at 1350 °C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum Si(VI) content of titanite is 3.0 ± 0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO3. The maximum Si(VI) content of the titanite increases with pressure to 21 ± 2 mol% at 7 GPa and 46 ± 2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Å] = 7.040(9) - 0.492(15) X(Si(VI)); b [Å] = 8.713(7) - 0.316(11) X(Si(VI)); c [Å] = 6.564(4) - 0.220(7) X(Si(VI)); β [°] = 113.721(6) - 0.537(12) X2(Si(VI)); V (cell) [Å3] = 367.8(9) - 47.5(1.6) X(Si(VI)). For the CaSi2O5 composition the recovered material has I1̄ symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P21/a CaTiSiO5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350 °C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO5 and CaSi2O5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO3-CaTiO3 join the topology of the phase relations between ~3 and ~13 GPa in the central part of the CaO-TiO2-SiO2 ternary have been deduced.
... It was operated at 200 kV acceleration voltage. EDX analyses were quantified according to the Cliff-Lorimer technique involving an absorption correction based on charge neutrality (van Cappellen and Doukhan 1994). Cliff-Lorimer k factors for the elements O, Mg, Si, Mn, and Fe were determined using an olivine sample previously characterized by WDS-EPMA. ...
We report the results of detailed mineralogical investigations by analytical scanning and transmission electron microscopy of particle RA-QD02-0115 recovered from the surface of asteroid 25143 Itokawa. We divided the 65 mu m 50 mu m small particle into eight individual subsample slices via the focused ion beam method. The particle dominantly consists of olivine and contains inclusions of merrillite, tetrataenite/taenite, troilite, chromite, kamacite, and Cl-bearing apatite (in approx. decreasing order of frequency). The composition of olivine (fayalite 29.8 1.1 mol % and molar Fe/Mn ratio of 57 2) as well as the Ni-rich metal assemblage indicates an LL-type affinity in accord with previous classifications. The particle shows effects of solar wind irradiation on one of its principal faces. Olivine developed an approximately 34 nm wide rim composed of low-angle misoriented, nanometer-sized crystallites accompanied by a small amount of amorphous material. Exposed troilite developed a 4 to 8 nm wide polycrystalline rim with large-angle misorientations of the iron sulfide nanocrystallites. Merrilite shows marginally discernable surface damage but was too unstable under the electron beam for a detailed study. Cl-bearing apatite was found fully crystalline with no discernable rim structure. We discuss the unusual polycrystalline nature of the olivine rim in terms of possible annealing and recrystallization effects, which may have occurred during periods of time when Itokawa's surface temperature may have been warmer due to closer perihelion distances. Model calculations show that the dynamical orbital evolution of near-Earth asteroids could lead to complex space weathering processes, arising from the competing interplay between irradiation-induced damaging and thermally driven annealing. ?
... The K factors (Cliff and Lorimer 1975) were determined experimentally for all elements of interest, following the method of Van Cappellen (1990). For the absorption correction, we used a method developed by Van Cappellen and Doukhan (1994), in which the thickness parameter is adjusted in the quantification program until the resultant composition fits the electroneutrality condition, i.e., the oxygen atomic concentration is equal to the sum of the atomic concentrations of the cations multiplied by their respective valencies. EDX analyses were made in TEM scanning mode with a window of 50 × 50 nm to avoid amorphization of mineral grains and to prevent loss of alkalis. ...
Natural mid-ocean ridge basalt (MORB) samples recovered from diamond-anvil cell (DAC) experiments performed between 33 to 89 GPa and 1700 to 2600 K were studied with a transmission electron microscope (TEM). We used the focused ion beam (FIB) lift-out technique to prepare the recovered high-pressure, laser-heated samples for TEM study. Observations of TEM sections show the presence of five phases for samples transformed at pressures ranging from 33 to 45 GPa: Al-bearing Mg-perovskite, Ca-silicate perovskite, stishovite, and two Al-rich phases. The Al-rich phases were identified by selected area electron diffraction (SAED) patterns and chemical composition analysis, and include the new aluminous (NAL) phase with hexagonal structure and the calcium ferrite (CF) type phase. Chemical analyses obtained by analytical transmission electron microscopy (ATEM) show that Mg-silicate perovskite is the major host for A], with significant amounts also distributed between the CF-type and NAL phases, and less than 1 wt% in stishovite. Beyond pressures of similar to 40 GPa (similar to 1100 km depth), the Al content of Mg-perovskite and CF-type phase increases. Between 45 and 50 GPa, the NAL phase disappears. This mineralogical change may explain reported seismic anomalies in subduction zones at mid-mantle depths.
... The electron dose was maintained constant from place to place by using the same spot size (15 nm), acquisition time (60 s), and the mean counting rate. By using the " electron neutrality criterion " (Van Cappellen and Doukhan 1994), the chemical raw concentrations were corrected for differential absorption effects caused by variations in thickness from point to point in the analyses. Images were recorded with a Gatan MSC794 CCD camera. ...
Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely discussed, have not been explained definitively. Our paper reports the results of comprehensive SEM, EPMA, XRD, TEM, and MAS-NMR studies performed for the first time on a natural pyrochlore, which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A-cation depleted, REE-, U-, and Th-bearing fluornatropyrochlore, locally exceptionally rich in SiO2 (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca2+- and Si4+-rich, and F- moderate fluids, which affected only some domains of the pyrochlore crystals and resulted in filling the A site vacancies mainly by Ca2+, but also Mn2+, Sr2+, and K+. The secondary alteration, induced by the exposure of the host rock to ground water driving fluid-mediated coupled dissolution-reprecipitation process, affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among others, the leaching of some A- and Y-site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases was observed in the TEM images even at very high magnification, it might be concluded that Si4+ can occupy severely alpha-decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any Si-[6](4+) signal around -200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around -84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q((2)) species defined as Si atom with two bridging O atoms, i.e., [Si(OSi)(2)(-)(2)], in the form of finite-length chain-like structures, located in the damaged A and B sites of the primary structure.
... The microscope is equipped with an EDAX energy-dispersive system and a Gatan Erlangshen CCD camera (Gatan, Inc., Pleasanton, CA, USA), and operates with a LaB 6 electron source at 200-kV acceleration voltage. EDX analyses were quantified according to the Cliff-Lorimer technique involving an absorption correction and the use of calibration k x/Si factors for elements X with respect to Si (van Cappellen and Doukhan 1994; Langenhorst et al. 1995). Lattice defects were imaged and identified by combining selected area electron diffraction (SAED) and conventional brightfield and dark-field imaging techniques involving, in particular, the weak-beam imaging technique. ...
We report here detailed analytical scanning and transmission electron microscopic investigations on an olivine-dominated dust particle (RB-QD04-0042) from the surface of asteroid 25143 Itokawa. The dust particle was returned to Earth by the Hayabusa spacecraft and was made available in the context of the first announcement of opportunity for Hayabusa sample investigation. Multiple thin slices were prepared from the precious particle by means of focused ion beam thinning, providing a unique three-dimensional access to its interior. The 40???50 ?m sized olivine particle contains a spherical diopside inclusion and an intimate intergrowth of troilite and tetrataenite. The compositions of olivine (Fo69Fa31) and diopside (En48Wo42Fs10), as well as the high Ni content of the sulfide-metal alloy, indicate a LL ordinary chondrite origin in accord with previous classifications. Although no impact crater exists at the surface of RB-QD04-0042, transmission electron microscopy revealed the presence of various shock defects in constituent minerals. These defects are planar fractures and [001] screw dislocations in olivine, multiple {101} deformation twins in tetrataenite and basal (0001) stacking faults in troilite. These diagnostic shock indicators occur only in a small zone on one concave side of the dust particle characterized by a high fracture density. These observations can be explained by a collisional event that spalled off material from the particle's surface. Alternatively, the dust particle itself could be a spallation fragment of an impact into a larger regolith target. This suggests that Itokawa dust particles lacking visible microcraters on their surfaces might have still experienced shock metamorphism and were involved in collisional fragmentation that resulted in the formation of regolith.
The aluminous calcium-ferrite type phase (CF) and new aluminous phase (NAL) are thought to hold the excess alumina produced by the decomposition of garnet in MORB compositions in the lower mantle. The respective stabilities of CF and NAL in the nepheline-spinel binary (NaAlSiO $$_{4}$$ 4 –MgAl $$_{2}$$ 2 O $$_{4}$$ 4 ) are well established. However with the addition of further components the phase relations at lower mantle conditions remain unclear. Here we investigate a range of compositions around the nepheline apex of the nepheline-kalsilite-spinel compositional join (NaAlSiO $$_{4}$$ 4 –KAlSiO $$_{4}$$ 4 –MgAl $$_{2}$$ 2 O $$_{4}$$ 4 ) at 28–78 GPa and 2000 K. Our experiments indicate that even small amounts of a kalsilite (KAlSiO $$_{4}$$ 4 ) component dramatically impact phase relations. We find NAL to be stable up to at least 71 GPa in potassium-bearing compositions. This demonstrates the stabilizing effect of potassium on NAL, because NAL is not observed at pressures above 48 GPa on the nepheline-spinel binary. We also observe a broadening of the CF stability field to incorporate larger amounts of potassium with increasing pressure. For pressures below 50 GPa only minor amounts ( $$<0.011(1)\frac{K}{K+Na+Mg}$$ < 0.011 ( 1 ) K K + N a + M g ) of potassium are soluble in CF, whereas at 68 GPa, we find a solubility in CF of at least $$0.088(3)\frac{K}{K+Na+Mg}$$ 0.088 ( 3 ) K K + N a + M g . This indicates that CF and NAL are suitable hosts of the alkali content of MORB compositions at lower mantle conditions. For sedimentary compositions at lower mantle pressures, we expect K-Hollandite to be stable in addition to CF and NAL for pressures of 28–48 GPa, based on our simplified compositions.
We present kinetic partitioning data for trace cations measured in zoned clinopyroxene crystals obtained from a variably cooled and decompressed olivine basalt erupted at Mt. Etna volcano in Italy. Supersaturation effects and compositional heterogeneities at the interface melt lead to the development of sector zoning, concentric zoning, and patchy zoning in clinopyroxene crystals. Apparent partition coefficients between compositionally different growth layers and adjacent melts (Di) for isovalent groups of trace elements are tested for internal consistency on the thermodynamic basis of lattice strain (ΔGstrain) and electrostatic (ΔGelec) energies of substitutions. The excess energy of partitioning (ΔGpartitioning) for trace cations in zoned crystals accounts for a kinetic incorporation control leading to large enthalpic effects through distortion of the lattice and changes in the electrostatic forces. ΔGpartitioning depends upon the complementary relationship between ΔGstrain and ΔGelec, which is the most appropriate thermodynamic description for the accommodation of rare earth elements and high field strength elements in the lattice site of zoned crystals. Polyhedral sectors, skeletal forms, and overgrowth zones have Di values settled by the number of charge-balanced and -imbalanced configurations taking place in the lattice site as a function of aluminium in tetrahedral coordination, and crystal structural changes produced by heterovalent cation substitutions. In an energetically unstable macroscopic system ruled by cooling and decompression, thermodynamic requirements for the crystallochemical control of Di encompass the attainment of local equilibrium at the crystal-melt interface via the establishment of small-volume reaction kinetics. The requisite of local interface equilibrium is however susceptible to the anisotropic growth velocity of each specific clinopyroxene surface, thereby giving reason to different energetic properties of the crystallographic site. This axiomatic control requires that transition metal cations partition also in consideration of electronic effects related to the crystal field stabilization energy. The overriding implication is that Di values for trace cations having different size, charge, and electronic configuration serve as sensitive probes of the different crystal growth mechanisms, surface incorporation sites, and arrangements of atoms at the lattice-scale. In this perspective, fractional crystallization modeling of 2011-2013 bulk rock data from lava fountains indicates that the compositional evolution of magmas erupted at Mt. Etna cannot be described by a unique equilibrium value of Di for a given clinopyroxene-melt interface. The leverage of interface kinetics is distinctively dominant along the subvolcanic plumbing system, thereby requiring that values of Di differ for structurally and compositionally distinct zones in clinopyroxene phenocrysts. To successfully interpret the trace element signature of Etnean magmas, the archetypal constancy of partition coefficient at bulk thermodynamic equilibrium must be in some measure reappraised in favor of the establishment of a local interface equilibrium upon highly dynamic crystallization and growth conditions.
In this paper, we evaluated the potentialities of Raman spectroscopy and electron backscattered diffraction (EBSD) in the microscopic characterization of Ca-REE fluorcarbonates (CRFC) belonging to the bastnäsite-synchysite series to provide a “road map” for further investigations with transmission electron microscopy (TEM). EBSD was effective in establishing the sample orientation, setting up the oriented cuts, and ascertaining the effective syntactic relationship among all the detected CRFC phases; however, it failed to distinguish between different polysomes. On samples with different orientations that were preventively ascertained by EBSD and characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), micro-Raman spectroscopy allows for distinguishing between polysomes based on the differences in intensity and position of the symmetric stretching vibration (ν1) of the carbonate group (CO32–) in the region around 1080–1099 cm–1. However, as evidenced by TEM-EDS, what appears as a homogeneous polysome in backscattered electrons (BSE) images may be a disordered intergrowth of compositional faults with a bulk composition being matched with that of a real polysome only by accident. Therefore, we conclude that the Raman signal is sensitive to different Ca/(Ca+REE) ratios but not to any ordered distribution of Ca-poor and Ca-rich lamellae within the analyzed volume, making the unambiguous identification of a polysome tricky. Finally, several ordered polysomes were detected at the TEM scale, including a B2S and a long-range polytype with a 32 nm repeat distance along c. The possible implications of the detected microstructure for ore mineral formation are discussed.
The Earth has been releasing vast amounts of heat from deep Earth's interior to the surface since its formation, which primarily drives mantle convection and a number of tectonic activities. In this heat transport process the core-mantle boundary where hot molten core is in direct contact with solid-state mantle minerals has played an essential role to transfer thermal energies of the core to the overlying mantle. Although the dominant heat transfer mechanisms at the lowermost mantle is believed to be both conduction and radiation of the primary lowermost mantle mineral, bridgmanite, the radiative thermal conductivity of bridgmanite has so far been poorly constrained. Here we revealed the radiative thermal conductivity of bridgmanite at core-mantle boundary is substantially high approaching to ∼5.3±1.2 W/mK based on newly established optical absorption measurement of single-crystal bridgmanite performed in-situ under corresponding deep lower mantle conditions. We found the bulk thermal conductivity at core-mantle boundary becomes ∼1.5 times higher than the conventionally assumed value, which supports higher heat flow from core, hence more vigorous mantle convection than expected. Results suggest the mantle is much more efficiently cooled, which would ultimately weaken many tectonic activities driven by the mantle convection more rapidly than expected from conventionally believed thermal conduction behavior.
The science of x-rays is by now over 125 years old, starting with Wilhelm Röntgen's discovery of x-rays in 1895, for which Röntgen was awarded the first Nobel Prize in Physics. X-rays have fundamentally changed the world in areas, including medical imaging, security scanners, industrial inspection, materials development, and drugs spectroscopy. X-ray science has been so far responsible for over 25 Nobel Prizes in Physics, Chemistry, and Medicine/Physiology. With x-ray generation being a highly commercialized, widely adopted technology, it may appear that there is little left to discover regarding the fundamentals of x-ray science. Contrary to this notion, recent years have shown renewed interest in the research and development of innovative x-ray concepts. We highlight, in this Perspective, promising directions for future research in x-ray science that result from advances in quantum science and in nanomaterials. Specifically, we describe three key opportunities for advancing x-ray science in the near future: (1) emerging material platforms for x-ray generation, especially 2D materials and their heterostructures; (2) free-electron-driven emission of entangled photon–photon and electron–photon pairs for x-ray quantum optics; and (3) shaping free-electron wavepackets for controllable x-ray emission. These research directions could lead to improvements in x-ray resonance fluoroscopy, high-contrast x-ray imaging, stimulated coherent x rays, x-ray superradiance, and other prospects for x-ray quantum optics.
The mechanical properties of olivine-rich rocks are key to determining the mechanical coupling between Earth’s lithosphere and asthenosphere. In crystalline materials, the motion of crystal defects is fundamental to plastic flow1–4. However, because the main constituent of olivine-rich rocks does not have enough slip systems, additional deformation mechanisms are needed to satisfy strain conditions. Experimental studies have suggested a non-Newtonian, grain-size-sensitive mechanism in olivine involving grain-boundary sliding5,6. However, very few microstructural investigations have been conducted on grain-boundary sliding, and there is no consensus on whether a single or multiple physical mechanisms are at play. Most importantly, there are no theoretical frameworks for incorporating the mechanics of grain boundaries in polycrystalline plasticity models. Here we identify a mechanism for deformation at grain boundaries in olivine-rich rocks. We show that, in forsterite, amorphization takes place at grain boundaries under stress and that the onset of ductility of olivine-rich rocks is due to the activation of grain-boundary mobility in these amorphous layers. This mechanism could trigger plastic processes in the deep Earth, where high-stress conditions are encountered (for example, at the brittle–plastic transition). Our proposed mechanism is especially relevant at the lithosphere–asthenosphere boundary, where olivine reaches the glass transition temperature, triggering a decrease in its viscosity and thus promoting grain-boundary sliding.
We report results of a systematic search for fluid/melt microinclusions (<1 μm) in “dusty” garnets from a heavily metasomatized Type-I kyanite-bearing eclogite from the Roberts Victor mine, South Africa. FTIR analyses of “dusty” zones indicate the presence of molecular water, CO2 and carbonate in the inclusions and of hydroxyl groups in the garnet. EPMA analysis of 136 microinclusions reveals the bulk composition of the microinclusions. Relative to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in SiO2, Al2O3 and MgO. Most of the elements form compositional mixing arrays of microinclusion + clear garnet. The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point, with some notable differences, towards the array of silicic to low-Mg carbonatitic high-density fluids trapped in diamonds. The main differences are the presence of molecular CO2 in addition to carbonate, the lower concentration of incompatible major constituents such as water, K2O and TiO2 and higher levels of Al2O3 and FeO. Still, the resemblance is close, indicating the role of diamond forming fluids in mantle metasomatism in general. The trapped melts are secondary and penetrated late in the history of the eclogite. They did not take part in the main metamorphic or metasomatic reactions, however they can account for introduction of Al2O3, FeO and incompatible trace elements into the ecologies.
We have performed an extensive characterization by transmission electron microscopy (including precession electron diffraction tomography and ab initio electron diffraction refinement as well as electron energy loss spectroscopy) of anhydrous phase B (Anh‐B) formed directly from olivine at 14 GPa, 1400 °C. We show that Anh‐B, which can be considered as a superstructure of olivine, exhibits strong topotactic relationships with it. This lowers the interfacial energy between the two phases and the energy barrier for nucleation of Anh‐B, which can form as a metastable phase. We have calculated the elastic and seismic properties of Anh‐B. From the elastic point of view, Anh‐B appears to be more isotropic than olivine. Anh‐B displays only a moderate seismic anisotropy quite similar to the one of wadsleyite.
We demonstrate the feasibility of coincidence measurements on a conventional transmission electron microscope, revealing the temporal correlation between electron energy loss spectroscopy (EELS) and energy dispersive X-ray (EDX) spectroscopy events. We make use of a delay line detector with ps-range time resolution attached to a modified EELS spectrometer. We demonstrate that coincidence between both events, related to the excitation and deexcitation of atoms in a crystal, provides added information not present in the individual EELS or EDX spectra. In particular, the method provides EELS with a significantly suppressed or even removed background, overcoming the many difficulties with conventional parametric background fitting as it uses no assumptions on the shape of the background, requires no user input and does not suffer from counting noise originating from the background signal. This is highly attractive, especially when low concentrations of elements need to be detected in a matrix of other elements.
In this study, we performed leaching experiments for timescales of hours-to-months in deionized water on fresh volcanic ash from Mt. Etna (Italy) and Popocatépetl (Mexico) volcanos to monitor Fe release as a function of ash mineral chemistry and size, with the aim of clarifying Fe release mechanisms and eventually evaluating the impact of volcanic ash on marine and lacustrine environments. To define sample mineralogy and Fe speciation, inclusive characterization was obtained by means of XRF, SEM, XRPD, EELS and Mössbauer spectroscopies. For Etna and Popocatépetl samples, glass proportions were quantified at 73 and 40%, Fe2O3 total contents at 11.6–13.2 and 5.8 wt%, and Fe3+/FeTot ratios at 0.33 and 0.23, respectively. Leaching experiments showed that significant amounts of iron, ~ 30 to 150 and ~ 750 nmol g−1 l−1 for pristine Etna and Popocatépetl samples, respectively, are released within the first 30 min as a function of decreasing particle size (from 1 to 0.125 mm). The Popocatépetl sample showed a very sustained Fe release (up to 10 times Etna samples) all along the first week, with lowest values never below 400 nmol g−1 l−1 and a maximum of 1672 nmol g−1 l−1 recorded after 5 days. This sample, being composed of very small particles (average particle size 0.125 mm) with large surface area, likely accumulated large quantities of Fe-bearing sublimates that quickly dissolved during leaching tests, determining high Fe release and local pH decrease (that contributed to release more Fe from the glass) at short timescale (hours-to-days). The fractional Fe solubility (FeS) was 0.004–0.011 and 0.23% for Etna and Popocatépetl samples, respectively, but no correlation was found between Fe released in solution and either ash Fe content, glass/mineral ratio or mineral assemblage. Results obtained suggest that volcanic ash chemistry, mineralogy and particle size assume a relevant role on Fe release mostly in the medium-to-long timescale, while Fe release in the short timescale is dominated by dissolution of surface sublimates (formed by physicochemical processes occurring within the eruption plume and volcanic cloud) and the effects of such a dissolution on the local pH conditions. For all samples, a moderate to sustained Fe release occurred for leaching times comparable with their residence time within the euphotic zone of marine and lacustrine environments (variable from few minutes to few hours), revealing their possible contribution to increase Fe bioavailability.
We conducted a transmission electron microscope study of the exsolution microstructures of Ca-rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo40, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo40 do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h⁻¹, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h⁻¹. We conclude that the formation of type I and II chondrules in the proto-solar nebula was the result of a common mechanism.
The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels (~320 °C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces.Open image in new windowGraphical abstractSchematic model showing the setting for décollement step-down into the ocean-plate basalts at the subdution plate interface. The diagram also shows textures of intact basalt and its expected frictional properties and behaviors associated with seismogenesis.
Using synchrotron-based X-ray diffraction, we explore characteristic signatures for nonhydrostatic stresses and their effect on the spin state crossover of ferrous iron in (Mg, Fe)O ferropericlase (Fp) upon compression in a two-phase mixture which includes an Al- and Fe-bearing bridgmanite (Bm). We observe an influence of nonhydrostatic stresses on the spin state crossover starting pressure and width. The undesirable stresses discussed here include uniaxial deviatoric stress evolving in the diamond anvil cell and effects of intergrain interaction. While the former leads to a pressure overestimation, the latter one lowers the pressure of the onset for the high-spin to low-spin electronic transition in Fe2+ in ferropericlase (Mg, Fe)O with respect to hydrostatic conditions.
Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near-) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium- and coarse-grained (a few tens to hundreds of micrometers) Ca-rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high-temperature (magmatic) origin as revealed by the well-developed pigeonite-augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2-3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.
Phase relations on the join CaTiSiO5-CaSi2O5 were determined at 1350 degrees C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum Si-VI content of titanite is 3.0 +/- 0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO3. The maximum Si-VI content of the titanite increases with pressure to 21 +/- 2 mol% at 7 GPa and 46 +/- 2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Angstrom] = 7.040(9) - 0.492(15) X(Si-VI); b [Angstrom] = 8.713(7) - 0.316(11) X(Si-VI); c [Angstrom] = 6.564(4) - 0.220(7) X(Si-VI); beta [degrees] = 113.721(6) - 0.537(12) X-2(Si-VI); V (cell) [Angstrom(3)] = 367.8(9) - 47.5(1.6) X(Si-VI). For the CaSi2O5 composition the recovered material has I (1) over bar symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P2(1)/a CaTiSiO5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350 degrees C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO5 and CaSi2O5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO3-CaTiO3 join the topology of the phase relations between similar to 3 and similar to 13 GPa in the central part of the CaO-TiO2-SiO2 ternary have been deduced.
You’ve now got an idea of how to acquire XEDS data from thin foils. You understand what factors may limit the information in them and what false and misleading effects may arise. Also, you know how to be sure that a certain peak is due to the presence of a certain element and the occasions when you may not be so sure. Having obtained a spectrum that is qualitatively interpretable, it turns out to be a remarkably simple procedure to convert that spectrum into quantitative data about the elements in your specimen, and this is what we describe in this chapter.
Breakaway oxidation of alloy 304L at 600 °C was studied in four environments (O2 + H2O + KCl, O2 + H2O + SO2 + KCl, H2 + H2O + Ar, O2 + K2CO3) for up to 168 h. The resulting scales were investigated by FIB/SEM, SEM/EDX, STEM/EELS, STEM/EDS and oxidation was elucidated by thermodynamic calculations (Thermo-Calc). The initial thin protective scale broke down in all cases. After breakaway, the scale consisted of two layers, i.e. an inward growing spinel/reaction zone and an outward growing iron-rich layer. The general features and microstructure of the scales after breakaway were similar in all environments and were explained in terms of: (1) Different diffusivities of Cr3+ and Fe2+ in the spinel oxide. (2) The appearance of a miscibility gap in the FeCr and FeCrNi spinel oxides. (3) The equilibrium composition of the spinel (at low pO2 Ni is not present in the spinel).
Majorite is a high pressure polymorph of enstatite with the garnet structure. The amount of enstatite that can be dissolved in garnet as a majorite component increases significantly with pressure, and therefore, majoritic garnet is thought to be a major constituent of the Earth's transition zone. The transport properties of majoritic garnet are, however, not well constrained at the moment. The magnitude of the diffusivity of the majorite component in garnet influences our understanding of the homogenization time scale of Earth's mantle. This is important in subduction zone settings, where the subducting oceanic crust will form a majorite inhomogeneity in the transition zone because of its higher aluminium content. Reaction kinetics in the dry transition zone are diffusion controlled and therefore an improved dataset on the diffusivity of the majorite component in garnet will enable us to better understand the of role of disequilibrium in subduction zones. This dissertation therefore reports the results of diffusion experiments on garnet. Diffusion experiments have been conducted with diffusion couples of majoritic garnet – Dora Maira pyrope, Dora Maira pyrope and Ötztal almandine and Ötztal almandine and majoritic garnet in a multi-anvil press between 1400 – 1900 °C and 12 – 20 GPa. The diffusion experiments with the majoritic garnet – Dora Maira pyrope garnet couples show that the diffusion of the majorite component in garnet is very slow, comparable to the diffusivity of silicon in wadsleyite and ringwoodite. The activation energy, activation volume and the pre-exponential for diffusion of the majorite component in garnet were determined to be 241 ± 54 kJ mol-1, 3.3 ± 0.1 cm3 mol-1 and 2.3 x 10-7 cm2 s-1, respectively. The diffusivity of the majorite component in garnet was determined to be 2-3 orders of magnitude slower than the self-diffusivity of Mg, Fe and Ca in garnet at the same conditions. Also Fe – Mg interdiffusion appeared to be significantly faster in majoritic garnet than in almandine garnet. Comparison with diffusion data on wadsleyite and ringwoodite shows that the diffusivity of the majorite component in garnet is very similar to that of the silicon self-diffusivity in the high-pressure polymorphs of olivine. The diffusion data obtained in this PhD has been used to determine whether solid state diffusion can homogenize the mantle after subduction. The diffusion distance of majoritic garnet has been calculated assuming grain boundary diffusion is the main mechanism, and it can be concluded that solid state diffusion is not able to homogenize the mantle. Next to this, a numerical model has been developed that determines whether diffusion of the majorite component is fast enough such that enstatite can dissolve into garnet during subduction. The results show that there will be a significant delay in case of the lower lithospheric mantle of the subducted slab. Due to its lower tempeture, the oceanic crust can, however, only dissolve a fraction of its pyroxene content and metastable pyroxene is thus expected to be present during subduction into the transition zone. The metastable presence of pyroxene leads to the question to what will happen to its aluminium contents as it is expected to get exsolved as garnet. Experiments were performed on aluminous enstatite at 1700 °C and 15 GPa. It is shown that majoritic garnet exsolves with the dominant topotactic relation being [001]clinoenstatite parallel to <111>garnet. Also a high density of stacking faults were observed with a displacement vector of R = ½[1 1 1] which can be explained by the transformation of HP high-clinoenstaite to low-clinoenstatite. Using the aluminium concentration profiles in clinoenstatite directly adjacent to the garnet precipitates the aluminium diffusivity in HP high-clinoenstatite was determined to be at least 6 x 10-11 cm2 s-1 at 1700 °C and 15 GPa. Comparison with data in diopside shows there is a discrepancy between diffusion data at high pressure and at low pressure, which might indicate a strong dependence of Al diffusivity in clinopyroxene on Ca contents or a change in diffusion mechanism. The results of the experiments conducted in this PhD study show that the low diffusivity of components in the Earth may severely hamper reaction kinetics in the Earth in the case where mass transport is required.
Johannsenite, along with hedenbergite and ilvaite, occurs in a skarn of the Campiglia Marittima Cu-Pb-Zn metasomatic ore deposit (central Italy). The skarn formed at low pressure, under decreasing temperature and in redox conditions which changed from oxidising to reducing. The clinopyroxenes are arranged in spheroidal bodies consisting of radiating crystals tens of centimetres long which formed under fast crystallisation rates. Optical and scanning electron microscopy (SEM) revealed two types of lamellar features. In one case the lamellae are parallel to [001], the morphological elongation of the crystal, but in other cases they are almost perpendicular. Single-crystal X-ray diffraction patterns of samples with lamellae parallel to [001] contain reflections forbidden for the C2/c space-group and compatible with either (100) or (102̄) twin planes. High Resolution Transmission Electron Microscopy (HRTEM) confirms that (100) is the twin plane. HRTEM also shows the presence of a translational component necessary to achieve the twin structure. At the level of the crystal structure, the twin interface appears as an out of step boundary, with the two opposite portions related by a b-glide. The lamellar features almost perpendicular to [001] also derive from twinning, but in this case (001) is the twin plane. In contrast with the ubiquitous (100) twins, (001) twins are restricted to manganese-rich crystals with more equidimensional habit. The two twin types never occur together in the same crystal. The (001) twins are obtained by reticular pseudo-merohedry (or alternatively, by twin-lattice quasi-symmetry, TLQS). Both (100) and (001) twins are interpreted as primary growth features.
This chapter provides an introductory overview of transmission electron microscope (TEM)-based spectroscopy techniques for use in geochemical sample analysis. The chapter addresses the basic questions of why TEM-based spectroscopy is useful, what the most common types of TEM-based spectroscopy are and what information they provide, critical aspects of TEM and spectrometer instrumentation, and the most common sample preparation techniques. Energy-dispersive x-ray spectroscopy techniques are illustrated with example analyses of the composition of a multiphase, nanoscale cometary dust impact residue. Electron energy-loss spectroscopy examples include elemental analysis with core-loss spectroscopy, distinguishing carbon polytypes with near-edge spectroscopy, and oxidation state analysis.
Osmium-rich Pt group element (PGE) alloys occur worldwide in association with chromite in ultramafic (peridotite) complexes. It has been suggested that these Os-rich alloys formed under extreme P-T conditions in the lowermost mantle, before the metallic core of the Earth formed, or later, in the outer core, and have been transported to the upper mantle as xenoliths in deep-rooted mantle plumes.
The Allende matrix is dominated by micron-sized lath-shaped fayalitic olivine grains with a narrow compositional range (Fa40–50). Fayalitic olivines also occur as rims around forsterite grains in chondrules and isolated forsterite fragments in the matrix or as veins cross-cutting the grains. Allende is a type 3 CV carbonaceous chondrite having experienced a moderate thermal metamorphism. There is therefore a strong chemical disequilibrium between the large forsterite grains and the fayalite-rich fine-grained matrix. Chemical gradients at interfaces are poorly developed and thus not accessible using conventional techniques. Here, we used analytical transmission electron microscopy to study the microstructure of the fayalite-rich matrix grains and interfaces with forsterite fragments. We confirm that fayalitic grains in the matrix and fayalitic rims around forsterite fragments have the same properties, suggesting a common origin after the accretion of the parent body of Allende. Composition profiles at the rim/forsterite interfaces exhibit a plateau in the rim (typically Fa45), a compositional jump of 10 Fa% at the interface, and a concentration gradient in the forsterite grain. Whatever the studied forsterite grain or whatever the nature of the interface, the Fe-Mg profiles in forsterite grains have the same length of about 1.5 μm. This strongly suggests that the composition profiles were formed by solid-state diffusion during the thermal metamorphism episode. Time–temperature couples associated with the diffusion process during thermal metamorphism are deduced from profile modeling. Considering the uncertainties on the diffusion coefficient value, we found that the peak temperature in Allende is ranging from 425 to 505 °C.
Single crystals of San Carlos olivine in contact with graphite were annealed at P = 1 bar, T = 1373 K, for studying the reaction of extraction of (Fe, Ni) metal. Scanning electron microscopy and transmission electron
microscopy were performed on samples recovered after the experiments. Precipitates of (Fe, Ni) and thin amorphous layer of
silica were identified, exclusively on the surface of the single crystals. Mass balance indicates that volatilization of Fe,
Mg, and Si is negligible under these conditions. The reaction can be summarized as:
which occurs at the crystal surface without affecting the interior of the crystal, except for an Fe2+ and Mg2+ compositional profile in the olivine matrix. These chemical profiles are consistent with measured values of Fe2+-Mg2+ interdiffusion coefficients, in agreement with the fact that Si and O are relatively immobile in olivine under such conditions.
This study shows that annealing at relatively moderate temperature under reducing conditions can cause surface modifications
and thus probably can strongly influence the surface evolution of planetary objects exposed directly to space environments
(regoliths, surfaces of asteroids, or interplanetary dust particles).
Energy dispersive X-ray microanalysis of thin specimens in a transmission electron microscope is a valuable method for investigating the chemistry of the nano-scale structure of rocks and minerals. This chapter summarizes first the key elements of this technique, including X-ray generation from the specimen, the detection chain and the quantification of spectra collected. The second part of the chapter is devoted to applications of the technique to extraterrestrial objects (Martian meteorites, cometary dust, primitive chondrites). Through examples, the different EDX working modes are illustrated (spot analysis, line profile, element map). Beam-damage issues are also discussed.
Extensive negative aeromagnetic anomalies in the Modum area, south Norway, derive from rocks containing ilmenite with hematite exsolution, or hematite with ilmenite exsolution, carrying strong/stable reversed remanence. Here we describe a 2.5 cm thick high-temperature metamorphic vein of exsolved titanohematite. Reflected-light and EMP analyses show it contains three types of exsolution: spinel plates on (001); rutile blade satellites on spinel oriented at angles of similar to 60-90 degrees to titanohematite (001); and lamellae 0.1-0.3 mu m thick too fine for EMP analyses, also parallel to (001). Powder XRD gave a = 5.0393 angstrom, c = 13.7687 angstrom, V = 302.81 angstrom(3) for titanohematite (approximate to Ilm9), and unrefined reflections of rutile and geikielite. Overlap EMP analyses showed enrichment in MgO, TiO2, and lack of Al2O3, indicating a mixture of titanohematite and geikielite. Non-overlap analyses showed the titanohematite is 6%Fe2+TiO3, 2%MgTiO3, 92% Fe2O3, generally confirmed by TEM-EDX analyses that also showed the geikielite is 30%Fe2+TiO3, 70%MgTiO3. Orientation and interface relationships between exsolutions and host titanohematite were characterized with TEM, using conventional and high-resolution imaging complemented by selected-area electron diffraction. Spinel shares (111) with (001) of titanohematite and geikielite (001) the same. The epitactic relationship between rutile and titanohematite, previously not well constrained, was estimated from reflected-light and TEM images and lattice-fit studies. The a(1) axis of rutile is parallel to a(1) of hematite and c of rutile is normal to a(2) of hematite, all in the hematite basal plane, which, however is not a phase interface. The rutile appears to occur in blades within prism planes in titanohematite located similar to 69 degrees from a axes of hematite, with long axes of the blades oriented in a minimum strain direction within the planes at similar to 63 degrees from the (001) basal plane. Spinel and rutile, analyzed by EMP, exsolved first. Spinel gave 96%MgAl2O4, 3%FeFe2O4, Mg/total R2+ = 0.98. Magnesian/aluminous spinet lacking Ti exsolved from titanohematite in coupled exsolution with ferrian rutile, where combined components were dissolved as corundum/geikielite components in high-T aluminous magnesian titanohematite. Early exsolution lowered geikielite, and eliminated the corundum component. Later fine exsolution of ferroan geikielite moved the titanohematite closer to Fe2O3. Mg2+ has no magnetic moment, but breaks up linkages between Fe atoms, lowers Neel Ts, and produces unusual low-T properties. This titanohematite has Neel T, 873 K (600 degrees C). Geikielite at 70%MgTiO3, is far beyond its theoretical nearest-neighbor percolation threshold at 30.3%MgTiO3. However, the sample shows a negative magnetic exchange bias below 25 K and low-T remanence lost above similar to 40 K. Such properties are reported in samples containing thin ilmenite lamellae in titanohematite, in theory with odd numbers of Fe layers, where exchange bias is linked to lamellar magnetism at the phase interfaces, when the ilmenite becomes a high-anisotropy magnet in a magnetically softer host. Potential explanations for the behavior of ferroan geikileite are discussed.
Majoritic garnet and silicate perovskite assemblages synthesised from natural pyroxene in the multi-anvil press and diamond anvil cell were studied by energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) to quantify the composition and oxidation state of iron. In these assemblages of the (Mg, Fe)SiO3–Al2O3 system, Fe3+ has a much stronger affinity to silicate perovskite than to majoritic garnet at the same pressure, temperature and oxygen fugacity. To determine the site occupancy of iron in majoritic garnet and silicate perovskite, we have also performed both spectroscopic techniques (EDXS and EELS) under channelling conditions. Electron channelling spectroscopy enables the octahedral sites of both phases to be distinguished from other cation sites by variations in the intensity ratios of the EDXS and EELS spectra acquired systematically at tilting conditions about the channelling-sensitive crystal plane and zone axis. In majorite almost all of ferric iron is partitioned into the octahedral site, whereas in silicate perovskite both ferrous and ferric iron substitute preferentially into the pseudo-dodecahedral site of the perovskite structure. The latter is charge balanced by aluminium in the octahedral site.
The parameterless correction method to perform absorption and fluorescence corrections in X-ray microanalysis of tranparent (S.) TE.M. specimens is presented and supported. This correction procedure requires no external parameters such as foil thickness or density, and no coefficients such as mass absorption coefficients or fluorescence yield coefficents and is therefore suitable for fast routine thin film microanalysis. Furthermore, thickness dependent artefacts such as surface and contamination layers and crystallographic orientation effects (the Borrmann effect) can be detected by the method. Une méthode permettant de corriger les effets d'absorption et de fluorescence en microanalyse X d'échantillons transparents pour la microscopie en transmission (S.) TE.M. est présentée. Cette procédure n'exigeant la connaissance d'aucun paramètre comme l'épaisseur ou la densité de l'échantillon ni d'aucun coefficient comme les coefficients d'absorption de masse ou encore des coefficients de rendement de fluorescence, est extrêmement utile et pratique pour effectuer des analyses de routine d'échantillons minces et a été appelée : "parameterless correction method". De surcroit tout effet dépendant de l'épaisseur de l'échantillon, comme la présence de couches de surface ou l'effet d'orientation crystallographique (l'effet Borrmann), peuvent-être détectés par cette méthode.