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SEM images (30 000×) of common opal-A samples, after HF etching (except e and f). (a) Opaque orange opal-AG from Mexico (Mina Iris, Queretaro, no. 759). The silica spheres vary in diameter from 2 to ~250 nm. Both small and large spheres show concentric structures, and the larger the sphere, the more numerous the layers. (b) Opaque orange opal- AG from France (Saint- Nectaire, no. 950) with spheres ~6.5 to 7.5 µm in diameter. Note the overall botryoidal appearance. (c) Gray opal-AG from Australia (Lightning Ridge mine, New South Wales, no. 235C) showing spheres that are not spherical, but commonly elongated, leading to an imperfect packing, which cannot diffract light. (d) Transparent opal-AG from Honduras (no. 684C) consisting of well-ordered spheres too large (~650 nm) to diffract light. Concentric layering is seen in the center of spheres and a radial structure in the rims. (e) Fire opal-AG from Slovakia (Dubník, no. 637) consisting of spheres that are not ordered and are too small (~80 nm in diameter) to diffract light. (f) White opal-AG from Honduras (no. 671) with spheres ~280 nm in diameter (adequate for a red play-of-color), but do not show a regular arrangement.
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aBstract The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average ~25 n...
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... diameter. This finding is consistent with results from previous studies concluding that polydisperse spheres are the primary cause of lack of play-of- color in opals ( Rau and Amaral 1969;Sanders and Darragh 1971). We observed that a variation of only 5% in sphere diameters is sufficient to preclude diffraction. In one Mexican common opal sample (Fig. 2a), diameters range from ~250 to 2000 nm. Similar, large variations were observed for ~23% of the samples, especially for opals from Honduras, France, and Slovakia. Interestingly, our results revealed that contrary to conclusions from the earlier studies, which were based on a limited number of samples, most Australian common opals have ...
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... to orange opaque opals from Austria (called forcher- ite) and France (Saint-Nectaire) consist of exceptionally large spheres with diameters ranging from ~2000 to 8000 nm, which are the largest we observed. The spheres commonly coalesce to build botryoidal-like structures (Fig. ...
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... shaped spheres. Our study indicates that common opal-AG with imperfectly shaped spheres is more abundant than originally believed (Sanders and Darragh 1971). We observed this type of irregularity in ~25% of our samples, but only in those from Coober Pedy, South Australia and the Lightning Ridge area, New South Wales, Australia (Fig. 2c). Spheres in these opals are commonly elongated, making orderly stacking impossible. The fact that they never appear broken implies that they were soft when they were stretched, or that they originally formed in that ...
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... that are too large or too small. We found only one common opal-AG, from Honduras (sample no. 684), in which spheres are in a well-packed arrangement but have diameters too large (~650 nm) to diffract light (Fig. 2d). In theory (Sanders 1964), spheres that are too small (<140 nm) also would not diffract visible light. We encountered only two opal-AG samples (from Dubník, Slovakia and Andamooka, Australia) with spheres smaller than those required for diffraction (~80 nm in diameter), but these spheres also were not stacked in an ordered array (Fig. ...
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... light (Fig. 2d). In theory (Sanders 1964), spheres that are too small (<140 nm) also would not diffract visible light. We encountered only two opal-AG samples (from Dubník, Slovakia and Andamooka, Australia) with spheres smaller than those required for diffraction (~80 nm in diameter), but these spheres also were not stacked in an ordered array (Fig. ...
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... that are not well ordered. We observed that ~13% of our samples of common opal-AG are made of spheres of ap- propriate diameter for diffraction but are stacked irregularly such that diffraction of light does not occur. These samples include a common white opal-AG from Honduras, in which spheres are ~290 nm in diameter (Fig. 2f), and some opals from Slovakia and ...
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... addition to the more usual concentrically layered spheres, we also observed spheres having radial structures in transparent common opal-AG from Honduras (13 samples). In these opals (Fig. 2d), spheres that are cut through their centers show concen- tric structures in their cores, and radial structures around them. The transition between the concentric and radial structures is abrupt. The radial structures are formed by the alignment of ~25 nm silica grains from the centers to the edges of ...
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... familiar picture of opal-AG with non-cemented, perfectly formed, individual spheres (e.g., Heaney et al. 1994) depicts a situation that is, in fact, not common in nature. We observed spheres without cementation in only a few opals from Slovakia (Fig. 2e) and Honduras (Fig. 2e). These opals are white, chalky, and extremely porous, and would be considered gems only after color treatment. Opal-AG samples from Australia, Brazil, France, Madagascar, Mexico, and the U.S.A. always contain some ...
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... familiar picture of opal-AG with non-cemented, perfectly formed, individual spheres (e.g., Heaney et al. 1994) depicts a situation that is, in fact, not common in nature. We observed spheres without cementation in only a few opals from Slovakia (Fig. 2e) and Honduras (Fig. 2e). These opals are white, chalky, and extremely porous, and would be considered gems only after color treatment. Opal-AG samples from Australia, Brazil, France, Madagascar, Mexico, and the U.S.A. always contain some ...
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... observed in many samples the effects of compaction of the silica spheres, although we did not find mention of it in the literature on natural opals. The structure of opal-AG is generally believed to consist of rela- tively perfect spheres, such as those in Figures 2a, 2e, 2f, or 3c. However, spheres may be distorted through compaction, after they settle, leading to polygonalization (that is, spheres with polygonal, commonly hexagonal cross-sections). ...
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... spheres may be distorted through compaction, after they settle, leading to polygonalization (that is, spheres with polygonal, commonly hexagonal cross-sections). In Figure 2d, the compaction effect is very noticeable, but in Figures 3a and 3b, spheres are not well packed and the compaction is less. There is no one deposit particularly noted for producing opals with compaction features, and within a deposit, compaction varies from sample to sample. ...
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The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average ~25 nm in diam...
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... The samples commonly show well-cemented to uncemented silica spheres, typical of opal-A. This contrasts with opal-CT, which is typically composed of more complex structures such as lepispheres (Lynne et al., 2005(Lynne et al., , 2007Jones and Renaut, 2007;Gaillou et al., 2008). Silica spheres show diameters ranging from less than 100 nm to more than 20 µm. ...
Silica minerals constitute a main target to assess the origin of life or the possibility of its emergence. On Earth, ancient hydrothermal silica deposits have preserved the oldest forms of life. Beyond Earth, such silica-rich hydrothermal systems have been observed on Mars by orbital near-infrared (NIR) remote sensing and in situ rover exploration. This work investigates the variations of texture and NIR properties of opal with temperature, within a single geological context of hot springs. Silica sinters have been sampled in Icelandic hot-spring fields, in the Reykholt region, and at the Hveravellir site, with water temperature ranging from 14 to 101 ∘C. Variations in the NIR spectral features (concavity ratio criteria, CRC) vary with fluid temperature, lithofacies, and microtexture. Only high-temperature samples display high CRC values (CRC5200>0.85), but low CRC values (CRC5200 < 0.75) are measured for any temperature. Hence, temperature is not the only parameter controlling spectral properties of opal. Several other parameters such as the hydrodynamic context, the microbial activity, silica micro-textures, and porosity may also affect silica precipitation, the incorporation and speciation of water in it, and thus its NIR signature. The observations suggest a limitation in the use of NIR spectral features for the interpretation of the geological context of fossil opal on Earth or Mars: only opal with high CRC values can be inferred as being formed by hydrothermal activity. Low CRC values can be attributed to either low-temperature hydrothermal activity (< 50–60 ∘C) or to continental weathering.
... chemical formula An 2-5 Ab 40-58 Or [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] and are sanidine according to the classification diagram (Figures 9(a) and 9(b)). Previous studies have demonstrated that mica compositions are related to the crystallization of magma or hydrothermal processes [39] and that magmatic types have higher TiO 2 and K 2 O and lower Al 2 O 3 contents [40]. ...
... Most previous structural studies of common chalcedony have suggested that they are primary built blocks of platelets without cemented grains, which are not well stacked in order [3]. The microstructure of opal is mainly constructed of spheres or tablets, which are typically cemented by nanograins containing many pores from earlier studies [48]. However, SEM imaging of the Longhua chalcedony indicated that a large number of nanograins irregularly coalesced to build platelet-like structures with many pores. ...
A low-crystallinity index chalcedony was found in the rhyolitic ignimbrite of the Late Jurassic Zhangjiakou Formation, located in Longhua County, Hebei Province, China. This chalcedony occurs as fillings along the fragile fractures of the host rock and is distinct from any other chalcedony deposits, such as the known basalt and carbonate-related types. The host rock is rhyolitic ignimbrite, comprising sanidine (50–70 vol.%), plagioclase (10–15 vol.%), quartz (8–10 vol.%), magnesian biotite (3–5 vol.%), and accessory minerals. The chalcedony appears as long lenticular veins and irregular-shaped bodies, occasionally containing small fragments of the surrounding rock at the boundary. It is colored in yellow, red, and/or white/colorless, with physical properties of specific gravity 2.55–2.56, reflection index of 1.54, Mohs hardness of 6.07–6.34, and weight loss of 1.97%–2.32% by heating. From the boundary to the inner center, its growth structure changes from comb-like macrocrystalline quartz to thin fiber crystallites and then to a relatively uniform cryptocrystalline phase, indicating precipitation from a crystalline to the cryptocrystalline sequence. Electron probe and Raman spectroscopy analyses reveal that the component minerals of the chalcedony are α-quartz and moganite and that the red inclusions are hematite. Quartz in chalcedony exhibits platelet shapes with tiny pores, which are cemented by nanograins, and such a structure is closer to that of opal. It’s crystallinity indexes (CIs) range ~1–3, as indicated by the X-ray diffraction patterns. This low CI and structural features, together with its occurrence, suggest a low temperature of 40°C–80°C during its formation. All these properties show a distinction from those of the most reported chalcedonies. This chalcedony is interpreted as an intermediate transitional type from normal chalcedony to opal, shedding new light on understanding microcrystalline silica mineral aggregate and exploration for a similar gem deposit.
... • A network similar to amorphous silica (opal-AN or "hyalite") with a glass-like structure, formed by the quenching of hot silica-rich solutions on cooler surfaces [13,14]; • Gel-like amorphous silica (opal-AG), exhibiting a structure composed of spheres, precipitated from an aqueous solution, similarly to laboratory-grown silica-gel and well documented in Australian specimens [12,[15][16][17][18][19][20][21]. ...
... A network similar to amorphous silica (opal-A N or "hyalite") with a glass-like structure, formed by the quenching of hot silica-rich solutions on cooler surfaces [13,14]; • Gel-like amorphous silica (opal-A G ), exhibiting a structure composed of spheres, precipitated from an aqueous solution, similarly to laboratory-grown silica-gel and well documented in Australian specimens [12,[15][16][17][18][19][20][21]. ...
... The distinction between opal types, even if firstly defined by X-ray diffraction, can also be achieved by Raman spectroscopy [22][23][24], infrared spectroscopy (either in the midinfrared sensitive to silica framework vibrations [24][25][26] or in the near-infrared sensitive to the hydration state [12,27,28]), nuclear magnetic resonance [29,30], and by observing microstructural features by electron microscopy [9,18,31]. ...
The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.
... Here, we explore the mineralogical properties of biosilicas by combining current and past research on opals and glass in order to compare the silica structure and bonding environments of biosilicas versus X-ray amorphous, non-crystalline, and paracrystalline geological counterparts. Classical sedimentary and volcanic opal-A (especially precious Australian opals) and opal-CT structures have been extensively studied from the perspective of identifying material properties and gemstone provenance via infrared spectroscopy (e.g., Graetsch et al., 1994;Sodo et al., 2016), Raman spectroscopy (e.g., Ostrooumov et al., 1999;Smallwood et al., 1997;Sodo et al., 2016), XRD (e.g., Elzea & Rice, 1996;Graetsch et al., 1994;Jones & Segnit, 1971;Sodo et al., 2016), neutron diffraction (e.g., Graetsch & Ibel, 1997), NMR (e.g., Brown et al., 2003;Graetsch et al., 1994), SEM (e.g., Gaillou et al., 2008), TEM (e.g., Elzea & Rice, 1996;Sanders, 1975), thermal characteristics (e.g., Brown et al., 2002;Smallwood et al., 2008), and nanoindentation analyses (e.g., Thomas et al., 2008). Silica sinter deposits and hydrothermal silica gels have also been investigated in order to understand the first steps of silicification and fossilization (e.g., Handley et al., 2005;Konhauser et al., 2001;Mountain et al., 2003), biosignature preservation (Kaur et al., 2011), and silica diagenesis (Liesegang & Tomaschek, 2020;Rice et al., 1995). ...
... We assume that instead, these silica samples originated via abiotic geological processes. Following the nomenclature used by the X-ray diffraction community, we use descriptors such as X-ray amorphous, noncrystalline, disordered, mineraloid, and paracrystalline to refer to the various hydrated silica phases (reviewed in Smith, 1998 Gaillou et al., 2008) and disordered, paracrystalline low-cristobalite/ tridymite opals (opal-CT; Elzea & Rice, 1996). Both of these opals are sometimes known to have longer-range order in their nanostructures with the size and orderly stacking of silica spheres and lepispheres leading to play-of-color effects in gem-quality stones (e.g., Sanders, 1964). ...
... Instead, these materials produce diffuse, broad X-ray scattering peaks consistent with their structural disorder. Opal-A G (synonymous with opal-A) is composed of stacked hydrated silica spheres (Gaillou et al., 2008), sometimes with long-range order on a nanometer-scale (Sanders, 1964(Sanders, , 1975Smallwood et al., 2008) and ...
Non-crystalline silica mineraloids are essential to life on Earth as they provide architectural structure to dominant primary producers, such as plants and phytoplankton, as well as to protists and sponges. Due to the difficulty in characterizing and quantifying the structure of highly disordered X-ray amorphous silica, relatively little has been done to understand the mineralogy of biogenic silica and how this may impact the material properties of biogenic silica, such as hardness and strength, or how biosilica might be identified and differentiated from its inorganic geological counterparts. Typically, geologically formed opal-A and hyalite opal-AN are regarded as analogs to biogenic silica, however, some spectroscopic and imaging studies suggest that this might not be a reasonable assumption. In this study, we use a variety of techniques (X-ray diffraction, Raman spectroscopy, and scanning electron microscopy) to compare differences in structural disorder and bonding environments of geologically formed hydrous silicas (Opal-A, hyalite, geyserite) and silica glass versus biogenic silicas from an array of organisms. Our results indicate differences in the levels of structural disorder and the Raman-observed bonding environments of the SiO2 network modes (D1 mode) and the Q-species modes (~1015 cm-1 ) between varieties of biogenic silicas and geologically formed silicas, which aligns with previous studies that suggest fundamental differences between biogenic and geologically formed silica. Biosilicas also differ structurally from one another by species of organism. Our mineralogical approach to characterizing biosilicas and differentiating them from other silicas may be expanded to future diagenesis studies, and potentially applied to astrobiology studies of Earth and other planets.
... e) Bright iridescent colors of the opal gemstone [28] with ordered spheres of silica creating the effect. [29] Reproduced with permission. [28] Copyright 2018, Mineralogical Society of America. ...
... Adapted with permission. [29] Copyright 2008, Mineralogical Society of America. f) Colorful hairs of the seamouse [30] arise from structural holes filled with sea water. ...
The ability to selectively redirect specific wavelengths of light has attracted a lot attention for photonic crystal materials. Presently, there is a wealth of research relating to the fabrication and application of photonic crystal materials. There are a number of structures that fall into the category of a photonic crystal; 1D, 2D, and 3D ordered structures can qualify as a photonic crystal, provided there exists ordered repeating lattices of dielectric material with a sufficient refractive index contrast. The optical responses of these structures, namely the associated photonic bandgap or stopband, are of particular interest for any application involving light. The sensitivity of the photonic bandgap to changes in lattice size or refractive index composition creates the possibility for accurate optical sensors. Optical phenomena involving reduced group velocity at wavelengths on the edge of the photonic bandgap are commonly exploited for photocatalytic applications. The inherent reflectivity of the photonic bandgap has created applications in optical waveguides or as solar cell reflector layers. There are countless examples of research attempting to exploit these facets of photonic crystal behavior for improved material design. Here, the role of photonic crystals is reviewed across a wide variety of disciplines; cataloging the ways in which these structures have enhanced specific applications. Particular emphasis is placed on providing an understanding of the specific function of the tunable optical response in photonic crystals in relation to their application.
... In the common C and CT opals, four different types of microstructure are described (Greer, 1969;Sanders and Murray, 1978;Murray and Sanders, 1980;Graetsch, 1994;Rossman, 1994;Fritsch et al., 1999;Gaillou et al., 2008b and references therein; Caucia et al., 2013Caucia et al., , 20152019). 1) massive microtexture composed of siliceous cement and, at times, amorphous silica nano-granules that appear too small to be observed even by SEM; ...
... Similarly, the SEM observations also showed that the microstructures of the samples are quite similar with the presence of lepispheres, and correspond to those observed in magmatic opals. All these evidences suggest that opals formed under similar genetic conditions, with a slow or moderate growth rate (Gaillou et al., 2008b;Caucia et al., 2013b). ...
The recently discovered common green opals from Anosy in Madagascar were studied
for their physical, chemical and gemological properties and gemological relevance.
The color of the opals is yellowish green but not very homogeneous for the presence
of cavities, dark lamellae and spots, the diaphaneity is translucent / opaque with
greasy luster and are inert to long and short wavelength UV radiation (366–254 nm).
Refractive index and specific gravity values range between 1.435–1.460 and 2.03-2.07,
respectively. The opals are CT type with tridymite more abundant than cristobalite and
contain clay minerals (saponite). The main chromophores that determine the color are
Fe and, subordinately, V and Cu. Other detected trace elements are Mg, Al, Ca, K, Na,
Ni and Cr. The high contents of Ba, probably deriving from mica and feldspars, are
noteworthy and can represent a geochemical marker. Three types of microstructures were
observed: homogeneous and very fine microspheres, lepispheric cauliflower structures
and globular aggregates of different sizes formed by laminae. Our analyses suggest that
opals formed under similar genetic conditions, with a slow or moderate growth rate.
Anosy opals would certainly have a good commercial value as semi-precious materials,
although lower than those from other regions of Madagascar (Bemia
... All opals from either Wegel Tena or Mezezo deposits are opal-CT (Gaillou et al., 2008b;Rondeau et al., 2012;Ayalew et al., 2020). ...
... Concentrations of minor and trace elements vary from a sample to another (Table 2). This trend is consistent with the previous studies of Ethiopian opals (Gaillou et al., 2008b;Rondeau et al., 2012;Chauviré et al., 2019). In decreasing order of average concentration, aluminum is the most abundant, between 4275 and 20,160 ppm. ...
... However, we observed significant differences of a few tens of nanometers between different localities. This is consistent with previous studies on orange opal-CT (Fritsch et al., 2001(Fritsch et al., , 2006Gaillou et al., 2008b;Zhao and Bai, 2020). The predominantly random stacking of grains and their sizes imply a rapid growth rate that did not allow the organization or development of larger-scale structures (Gaillou et al., 2008b). ...
Of all the world's gem opal deposits, Ethiopian's ones stand out for their quantitative production of white opals and orange fire opals. They also provide a significantly amount of intriguing specimens displaying a translucent, porous, white periphery covering a transparent, massive, orange core. These “zoned opals” are stable (they do not evolve with time after mining) and are only encountered in Ethiopia thus far. This suggest that the environment of formation and residence of opals, attributed to pedogenetic processes in a strongly weathered environment, play an important role in the formation of those specimens. We hypothesized that “zoned opals” are a transitional state between orange and white opal due to a progressive and centripetal transformation caused by fluids in the soil. First, we document the properties of zoned opals for the first time and compare them to entirely orange opals from Ethiopia (Wegel Tena and Mezezo) and Mexico, by multi-scale techniques: UV–visible absorption spectrometry, Raman spectroscopy, LA-ICP-MS, Atomic Force Microscopy and Scanning Electron Microscopy. White periphery proved to be more porous, depleted in iron, and structurally more polymerized compared to the orange cores. Second, we conduct experimental modeling of the “external whitening” phenomenon as it could occur in a pedogenetic environment rich in both organic and inorganic fluids. For that, we submitted orange opals to various solutions of reactants naturally occurring in soils or acting as good proxies for acidolysis and/or complexolysis: oxalic acid, sulfuric acid, EDTA, pyoverdine and deferoxamine. Some of organic and inorganic solutions at pH = 1, proved to satisfactorily model the natural whitening. As for natural opals, whitened samples were more porous and depleted in iron. However, the polymerization state of the silica network significantly decreased. Moreover, through the short time scale of our experiments, it evolved in a non-linear manner. That is why, depolymerization and increase of porosity may induce repolymerization with longer times after the external whitening destabilization phenomenon, as it is seen for 30–20 million years old naturally whitened samples.
... This phenomenon is called play-of-color, which is caused due to the interaction of light with their pseudo structure composed of regularly spaced layers consisting of submicron sized (average diameter few hundreds of nm) silica spheres. [21][22][23] Since the play-of-color phenomenon does not appear in ordinary fused aluminosilicate glass, it is assumed to be induced by HfO 2 . Figure 3 shows the XRD pattern results of each Sn-doped glass powders. ...
In this study, Sn-doped 10HfO 2 -10Al 2 O 3 -80SiO 2 glasses were prepared using a xenon imaging furnace and their physical, optical, and scintillation properties were investigated. At the composition ratio, the specimens did not completely vitrify, and they were crystallized glasses that contained nanocrystals of c-HfO 2 . Raman spectra show the absorption bands due to Si–O–Hf bonds, and the band clearly indicated an effective molecular mingling of SiO 2 and HfO 2 components in the glass. Moreover, the energy dispersive X-ray spectroscopy maps suggested that the elemental distribution of this glass specimen is heterogeneous. In terms of optical properties of the glass, all of the specimens showed emission due to Sn ²⁺ , and their tendency to increase PL QY with increasing Sn concentration. The estimated luminescence from pulse height spectrum measurements under alpha irradiation was ~2500 ph/MeV, approximately 35% of the GS-20 glass scintillator counterpart.
... Copyright 2009, OPTICA. (e) Bright iridescent colors of the opal gemstone 28 with ordered spheres of silica creating the effect 29 . Reproduced from ref. 28 . ...
... Copyright 2018, Mineralogical Society of America. Adapted from ref. 29 . Copyright 2008, Mineralogical Society of America. ...
The ability to selectively redirect specific wavelengths of light has attracted a lot attention for photonic crystal materials. Presently, there is a wealth of research relating to the fabrication and application of photonic crystal materials. There a number of structures which fallinto the category of a photonic crystal; 1D, 2D and 3D ordered structures can qualify as a photonic crystal, provided there exists ordered repeating lattices of dielectric material with a sufficient refractive index contrast. The optical responses of these structures, namely the associated photonic bandgap or stopband, are of particular interest for any application involving light. The sensitivity of the photonic bandgap to changes in lattice size or refractive index composition creates the possibility foraccurate optical sensors. Optical phenomena involving reduced group velocity at wavelengths on the edge of the photonic bandgap are commonly exploited for photocatalytic applications. The inherent reflectivity of the photonic bandgap has created applications in optical waveguides or as solar cell reflector layers. There are countless examples of research attempting to exploit these facets of photonic crystal behaviorfor improved material design. Here, the role of photonic crystals is reviewed across a wide a variety of disciplines; cataloguing the ways in which these structures have enhanced specific applications. Particular emphasis is placed on providing an understanding of the specific function of the tunable optical response in photonic crystals in relation to their application.
... We suggest that any examination of new sources of material, or a re-interpretation of old sites, should include a 29 Si SP NMR spectrum and a T1 relaxation experiment to build up a corpus of reference material. For instance, other examples of authenticated "white or milky opals" [16] from sites such as Honduras [16,91], Brazil [16], Mexico [28], Madagascar [8] and the USA [33] may prove to have distinct characteristics. ...
Single pulse, solid-state 29 Si nuclear magnetic resonance (NMR) spectroscopy offers an additional method of characterisation of opal-A and opal-CT through spin-lattice (T1) relaxometry. Opal T1 relaxation is characterised by stretched exponential (Weibull) function represented by scale (speed of relaxation) and shape (form of the curve) parameters. Relaxation is at least an order of magnitude faster than for silica glass and quartz, with Q3 (silanol) usually faster than Q4 (fully substituted silicates). 95% relaxation (Q4) is achieved for some Australian seam opals after 50 s though other samples of opal-AG may take 4000 s, while some figures for opal-AN are over 10,000 s. Enhancement is probably mostly due to the presence of water/silanol though the presence of paramag-netic metal ions and molecular motion may also contribute. Shape factors for opal-AG (0.5) and opal-AN (0.7) are significantly different, consistent with varying water and silanol environments, possibly reflecting differences in formation conditions. Opal-CT samples show a trend of shape factors from 0.45 to 0.75 correlated to relaxation rate. Peak position, scale and shape parameter, and Q3 to Q4 ratios offer further differentiating feature to separate opal-AG and opal-AN from other forms of opaline silica. T1 relaxation measurement may have a role for provenance verification. In addition , definitively determined Q3 / Q4 ratios are in the range 0.1 to 0.4 for opal-AG but considerably lower for opal-AN and opal-CT.
