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Irradiation and heating effects in amethyst crystals from Brazil
Abstract
In this work we report optical absorption spectroscopy study of thermal and irradiation effects on samples of amethyst from Minas Gerais and Rio Grande do Sul, Brazil. Three bands were studied: 10500 cm?1 (k), 18300 cm?1 (?) and 28000 cm?1 (?). Thermal and irradiation effects shows that the ? and ? bands belongs to a same center and the k band to another center. The isothermal decay and irradiation growth of these band reveal a complex kinetics. The optical absorption bands of amethyst from Minas Gerais do not recover the prmitive absorbance after being bleached at 470°C and irradiated. This sample heated at 470°C in highly reducing atmosphere gets a yellow-brown color. The amethyst from Rio Grande do Sul treated at 400°C gets, also, a yellow-brown color. We suggest this color is probalbly due to the formation of Fe2O3 submicroscopc segregate crystals due to the diffusion of Fe ions and oxygen vacancies.
... Nassau (1984) propõe um esquema no qual temperaturas em torno de 300 a 400 o C seriam suficientes para extinguir a cor violeta, transformando a ametista em quartzo incolor. Em Isotani et al. (1987) e Dotto & Isotani (1991) são apresentados estudos detalhados de perda/restauração da cor da ametista mediante experimentos térmicos e de irradiação. ...
... No Brasil, devido a abundância de ametistas sejam elas de qualidade gemológica ou não, foram realizados estudos por Baggio et al. (2015), Fisher et al. (2010), Gilg et al. (2002), Hartmann (2015), Hartmann et al. (2010), Juchem (2014), Juchem et al. (2001Juchem et al. ( , 1994 e Proust & Fontaine (2007) envolvendo diversas propriedades, como químicas, petrográficas, acerca dos fluidos em quartzos, das rochas hospedeiras e geodos de ametistas. Estudos mais voltados para a gemologia e mineralogia acerca das ametistas, provenientes do Brasil, podem ser encontrados em trabalhos como Correa (2010Correa ( ,2007, Dotto & Isotani (2006), Guttler & Kohigashi (2006), Tononi et al. (2019). No Ceará, as propriedades gemológicas de ametistas na região de Santa Quitéria foram descritas por Oliveira et al. (2020) Este trabalho trata da caracterização gemológica de cinco cristais de ametistas provenientes do município de Quixeramobim, no estado do Ceará, geologicamente inserido no Distrito Pegmatítico Solonópole-Banabuiú. ...
Since the end of the 20th century, it has become extremely usual for countries with a remarkable exploration of gems to catalog and disseminate the properties of their gems for various purposes, such as market, academic or criminal. In Brazil, there is a gap in this regard; many mineral occurrences do not have their gemological properties tabulated or disclosed. Globally, gemological studies do not cease and gemological data on minerals are updated frequently; as in the case of tourmalines. In addition, the properties of minerals used as gems can help or even reveal new aspects about the geological evolution of the region where a mineral occurs. For the gemological characterization of amethysts from Quixeramobim (CE), the usual gemology equipment was used, such as: refractometer, dicroscope, spectroscope, polariscope, hydrostatic balance, fluorescent lamp and gemological microscope. Quixeramobim amethysts have the standard characteristics of amethysts from different parts of the world, but the abundance of oriented fluid and solid inclusions requires further study. With the exception of this peculiarity, amethysts have no anomalies; unlike the amethysts from Santa Quitéria, also from the State of Ceará, which have a very high birefringence as an anomalous characteristic. This demonstrates the importance of gemological characterization not only at a State / Regional level, but also at the Municipal scale, as in the same State there may be occurrences of the same mineral, but with divergent properties. Therefore, it is important to characterize the occurrences in different geological contexts, as the formation environment can interfere with the characteristics of minerals.
... yellow-brown, although not all samples displayed the entire sequence of changes [33]. Optical absorption spectroscopy of irradiation and thermal effects on samples of amethyst from Brazil has been reported by Dotto and Osotani [13]. The isothermal decay and irradiation growth of the studied three bands, 10,500 cm À1 (j), 18,300 cm À1 (h) and 28,000 cm À1 (n), were considered to reveal a complex kinetics. ...
... Gemological quality crystals of beryl, quartz, spodumene, topaz, and tourmaline among many others, constitute an important economic resource for the Eastern Brazilian Pegmatite Province [4]. The optical absorption spectra of crystals of the Province of Oriental Brazilian Pegmatite were reported by several authors for beryl [5][6][7][8][9][10], amethyst [11], spodumene [12][13][14][15], topaz [16][17][18][19][20][21][22][23][24], and tourmaline [25]. ...
It is well known that crystals of topaz from the Eastern Brazilian Pegmatite Province may turn blue by the irradiation with 60Co gamma rays followed by heat treatment. Also, it is known that the sensation of color changes with the thickness of these crystals. The dependence of the color, given by 1931 CIE chromaticity coordinates, with the thickness of the crystal was analyzed. The absorbance used in the calculation of these coordinates was given by the sum of Gaussian lines. The parameters of these lines were determined through the decomposition of the optical absorption spectra in the ultraviolet and visible regions. The decomposition revealed several lines, whose assignment was made considering studies in spodumene and beryl crystals and highly accurate quantum mechanical calculations. The transmittance becomes very narrow with increasing thickness, and the CIE chromaticity coordinates converge to the borderline of the CIE Chromaticity Diagram at the wavelength of maximum transmittance. Furthermore, the purity of color increases with increasing thickness, and the dominant wavelength reaches the wavelength of maximum transmittance.
The effect of heat treatment on amethyst color was studied from a new perspective of chromaticity of gemstones and the cause of amethyst coloration was discussed based on the results of X-ray diffraction, ultraviolet-visible spectroscopy. The results show that the amethyst color has no significant relationship with cell parameters but the crystallinity index decreases as temperature rises. The absorption band at 545 nm in the UV-visible spectrum can be related to a charge-transfer transition of Fe3+ and O2-, which has a significant relationship with amethyst lightness and chroma. The color at different temperatures can be divided into three stages: The amethyst stage with temperature below 420 °C, the prasiolite stage with temperature between 420 and 440 °C where the color center is the most unstable, the citrine stage with temperature above 440 °C. The color change degree of heated amethyst is related to its initial color. When the initial color is darker, the color difference of heated amethyst is larger, and the easier it is to change the color after heat treatment. A more appropriate heating temperature to obtain citrine by heating amethyst is about 560 °C.
High demand for some rare gems creates pressure on the production line and some gems are consumed quickly in some countries. In order to sustain the gem supply to the most demanding markets, man-made minerals may be considered as an alternative to expensive genuine ones. Non-genuine precious and semiprecious stones can be found as enhanced, reproduced, and counterfeited gems. The former is extremely common among precious gems, while the last one is usually for semiprecious stones. Enhancement methods transform (or recycle) very low-quality (waste) gems into unique jewelry. Fabricated crystals are obtained (or recycled) from different or even irrelevant materials. The identification of such materials is getting harder due to continuous improvement in production technologies. Governing bodies have begun to issue regulations to their members so that misleading information given in retail can be reduced. In addition to limited regulation and enforcement, the buyers will also need some sort of education provided by trustworthy foundations such as reputable labs, mineral museums, and academic institutions. Although the internet provides a vast amount of information about fake minerals, most of which is also fictitious, especially, earth science-related museums should have a special duty in this regard and educate the public through hands-on experiments.
Colorless quartz can develop several colors when submitted to gamma radiation.
One of these colors is grayish green, developed in geode-hosted colorless quartz
crystals in volcanic rocks of the Serra Geral Group. Development of this color is
associated to the water content present in the quartz crystalline structure, being
activated through gamma radiation. Mineralization is related to a hydrothermal
system, which supplies water, and thus generates the crystallochemic conditions for
the development of green color. In this research, colorless quartz crystal samples
hosted in geodes from rhyodacites of the Serra Geral Group were collected,
specifically from the Nova Bréscia and Progresso regions, in Rio Grande do Sul. The
objective is to investigate the color change to green. Samples were analyzed in the
infrared region, followed by applying of the Amethyst Factor (fa) and
thermogravimetric analysis (TGA). All samples presented and absorption peak at
3585 cm-1
and a broad band varying between 3445-3428 cm-1
. Absorption in these
wavelengths indicates the samples will develop green or violet color. After exposition
to the dose of 900 kGy all crystals changed color. Some samples developed zonation
along the crystal: (1) from colorless to slightly greenish and (2) from grayish green to
slightly purplish. Samples from Nova Bréscia (NB1 group) developed color zonation
varying from greenish to colorless, followed by amethyst, which occurs as a ghost
crystal in the apical portion, and again colorless. Samples of the NB3 group
developed a more uniform grayish green color, having just one of the samples
developed pervasive zonation from gray to greenish. The Progresso samples (group
PRa-II) displayed color zonation from colorless/slightly grayish to grayish green; to
other sample groups (PRa-IA; PRb) grayish green color became more uniform,
presenting weak contrast with slight zonation from green to grayish. Therefore,
colorless quartz samples of the Progresso and Nova Bréscia regions present a
potential for development of color under gamma radiation, especially grayish green
and, subordinately, purple. From the irradiation results, it was found that the
composition of the mineralizing fluid varied during quartz crystallization process, as
demonstrated by the color zonation described for most crystals. TGA analyses
indicate high water content in samples which remained colorless/slightly grayish after
irradiation. Thus, it is suggested that the excess of structural water might inhibit the
development of green color through gamma radiation in colorless quartz crystals.
Isothermal decay behaviors, observed at 515, 523, 562, and 693 K, for an optical absorption band at 620 nm in gamma-irradiated Brazilian blue topaz were analyzed using a kinetic model consisting of O− bound small polarons adjacent to recombination centers (electron traps). The kinetic equations obtained on the basis of this model were solved using the method of Runge–Kutta and the fit parameters describing these defects were determined with a grid optimization method. Two activation energies of 0.52±0.08 and 0.88±0.13 eV, corresponding to two different structural configurations of the O− polarons, explained well the isothermal decay curves using first-order kinetics expected from the kinetic model. On the other hand, thermoluminescence (TL) emission spectra measured at various temperatures showed a single band at 400 nm in the temperature range of 373–553 K in which the 620 nm optical absorption band decreased in intensity. Monochromatic TL glow curve data at 400 nm extracted from the TL emission spectra observed were found to be explained reasonably by using the knowledge obtained from the isothermal decay analysis. This suggests that two different structural configurations of O− polarons are responsible for the 620 nm optical absorption band and that the thermal annealing of the polarons causes the 400 nm TL emission band.
The infrared bands of amethyst and prasiolite samples from different origins were correlated to the trace elements contents. Amethysts have an iron content greater than 20 ppm and a low content of sodium and potassium. Prasiolites have an aluminum content greater than 120 ppm and a higher overall trace elements content, which accounts for a strong absorption between 3200 and 3600 cm-1. Colorless samples of quartz that become amethysts and prasiolites after irradiation have infrared spectra at room temperature with a broad band at 3441 cm-1 and a sharp band at 3595 cm-1. The broad band splits in several bands at low temperatures that are related to AlSi and FeSi. The color of amethysts and prasiolites are assigned to [AlSiO4/h+]º and [FeSiO4/h+]º centers formed by the exposure to ionizing irradiation and to the influence of lattice distortions due to the content of iron as a substitute for silicon and a high content of trace elements of large ionic radius like potassium.
The spectral decomposition analysis was applied to the optical absorption spectra of green and colorless beryl crystals from the Brazilian Eastern Pegmatitic province in the natural state, submitted to heat treatment and irradiated with UV light. The attributions of the lines were made taking into account highly accurate quantum mechanical calculations. The deconvolution of the green beryl spectra revealed four lines, two of them around 12,000cm−1 (1.5eV) and two of them around 34,000cm−1 (4.2eV) attributed to Fe2+ and Fe3+, respectively. The deconvolution of the colorless beryl spectra without any treatment, after heating and for the same heat treatment followed by UV light irradiation revealed five lines. The analysis of ratio relations showed that the lines at 36,400cm−1 (4.5eV) and 41,400cm−1 (5.1eV) belongs to a single defect attributed to a silicon dangling bond defect (Si:). Discussions and comparison with reported defects in quartz have supported the allocation of the lines at 61,000cm−1 (7.6eV) and 43,800cm−1 (5.4eV) to diamagnetic oxygen vacancy defect (Si–Si) and unrelaxed (Si⋯Si) defect, respectively. Finally, the line at 39,100cm−1 (4.8eV), quite polarized along the c-axis, was attributed to a (Fe2+OH−) defect in the structural channels.
The development of mineralogy, the evolutionary changes in compre hending the mineral substance of the earth are closely associated with the progress of research methods. Over a space of more than two and half centuries, from the goniometry of the mineral crystals to microscopic petrography and optical mineralogy, to crystal structure determinations, electron micros copy and electron diffraction and finally investigations into their electri cal, magnetic and mechanical properties, all this has led to the formation of the existing system of mineralogy, its notions, theories and to a proper description of minerals. However, no matter how great the variety of methods employed in mineralogy, they all come to a few aspects of substance characteristics. These are methods of determining the composition, structure and proper ties of the minerals. Thus the X-ray micro analyzer, the atom-absorption, neutron-activation, chromatographic and other analyses open up new opportunities for determining nothing else but the elementary com position of minerals.
The biaxial absorption bands in amethyst quartz, with peaks at 2.28 eV and 3.54 eV related to Fe4+ and a peak at 3.02 eV—which is the A3 band related to the [AlO4]° trapped hole center, have orientations of maximum light absorption in the basal plane of Brazil-twinnedr-growth sectors paralleling the planes of Brazil optical twinning. Absorption minima are at 90° to the maxima in all cases. The Brazil twinning planes always parallel thea-axes(1210,etc) of quartz and in many cases also parallel planes perpendicular to ther-faces(1011,etc.). These are directions of channels in the quartz structure. The anisotropy ratio,σmax/σmin, of the Fe4+ band is that of the A2 absorption band in smoky quartz as would be expected if Fe3+ furnishes electrons to quench the trapped holes causing this absorption band. In the absence of the A1 and A2 absorption bands, the A3 absorption band width at half-maximum decreases from 1.43 to 0.36 eV indicating decreased charge-transfer character of the [AlO4]° center in the absence of the other types of Al trapped-hole centers in quartz. The key to the Brazil twinning in α-quartz are the channels which fill with large Fe3+ ions that force twinning to relieve strain in the structure. Amethyst color results only if aluminum is present substitutionally in the quartz as well as the interstitial iron plus ionizing radiation.
The many colors found in quartz crystals are always due to the presence of impurity atoms, whose concentrations rarely exceed 0.1 wt.%. The impurity atoms occupy either silicon sites in the lattice or interstitial sites in the channels parallel to the c axis of the quartz structure; very fine precipitations of foreign phases are sometimes also observed, though this is less common. In addition to intrinsic colors of transition metal ions, one also finds color centers produced by ionizing radiation. Regular distributions of impurity atoms and colors may develop in the crystals as a result of some factor connected with its growth. Most of the natural quartz colors, as well as others that do not occur in nature, have been produced in synthetic quartzes.
Several different paramagnetic centers have been identified and studied in specimens of natural and synthetic α quartz. The dominant feature of the EPR spectrum of natural amethyst and citrine is a center S1, which was previously identified as substitutional Fe3+ with a charge‐compensating alkali‐metal ion on a neighboring interstitial site. Synthetic brown and green quartz also contain a small proportion of S1 centers, but the dominant feature of their EPR spectra is a center I which is identified as an interstitial Fe3+ lying in one of two possible interstitial sites in the structure, with a charge neutralizing ion in a neighboring substitutional site. This spectrum is fitted to a spin Hamiltonian with D=2.333 Gc/sec, E=0.63 Gc/sec, F=0.60 Gc/sec, the z axis of the center coinciding with the c axis of the crystal. Interconversion of S1 and I centers by heat treatment indicates that the latter are more stable. In a crystal of synthetic amethyst, a new EPR center, denoted S2, has been characterized, which is converted to S1 by annealing at 350° and obtained from S1 by irradiation with x rays. Citrine and heat‐treated amethyst display a broad isotropic EPR absorption at g=2, believed to be caused by nuclei of precipitated Fe2O3. The brown color is partly due to these inclusions and partly to a shift of the charge‐transfer band [Fe3++O2—→Fe2++O−] into the visible as S1 centers are converted to less constricted I centers.