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Palke A.C.,Gemological Institute of America | Geiger C.A.,University of Salzburg
European Journal of Mineralogy | Year: 2016

Pyrope-rich garnet is an important "sink" for trivalent transition metals such as Cr3+, V3+, and Fe3+ in Earth's upper mantle. In order to obtain a better crystal-chemical understanding of transition metals in garnet and especially pyrope solid solutions, 27Al and 29Si MAS NMR spectra were collected on synthetic crystals of composition Mg3(Al0.915V0.085)2Si3O12 and Mg3(Al0.98mCr0.02)2Si3O12, and compared with the 29Si MAS NMR spectrum of a Fe3+-bearing grossular garnet [Ca3(Al0.956Fe3+ 0.044)2Si3O12]. The purpose of the study is to explore the nature of paramagnetic resonances, or their lack thereof, arising from interactions of Cr3+, V3+, and Fe3+ with 27Al and 29Si nuclei. The 27Al and 29Si MAS NMR spectra of Cr3+-bearing pyrope show significant resonance broadening, similar to that of Fe3+-bearing grossular, which hinders any crystal-chemical interpretation. The 27Al and 29Si MAS NMR spectra of V3+-bearing pyrope, on the other hand, show several distinct and narrow paramagnetically shifted NMR resonances, even at relatively high concentrations of V3+. The various NMR resonances can be assigned to different local atomic configurations around AlO6 or SiO4 groups having zero, one, two or three next-nearest octahedrally coordinated V3+ neighbors. The intensities of the experimental resonances were measured and compared to those calculated for the hypothetical case of statistically random mixing of V3+ cations to investigate Al-V3+ mixing behavior at the octahedral site. A simple analysis, based on the number of unpaired electrons for a given transition metal (i.e., Fe2+ 3+, Cr3+, and V3+) in pyrope, is made in an attempt to explain the marked differences in their behavior in 27Al and 29Si MAS NMR spectra. © 2015 E. Schweizerbart'sche Verlagsbuchhandlung. Source

Thomas T.,Gemological Institute of America | Rossman G.R.,California Institute of Technology | Sandstrom M.,Deliciously Nerdy Labs
Review of Scientific Instruments | Year: 2014

An optical instrument we refer to as the "biaxial orientation device" has been developed for finding the optical plane, acute bisectrix, and obtuse bisectrix in biaxial crystals by means of optically aligning conoscopically formed melatopes and measuring the angular coordinates of the melatopes, where the angular values allow for determination of the optical plane containing the optical axes using a vector algebra approach. After determination of the optical plane, the instrument allows for the sample to be aligned in the acute bisectrix or obtuse bisectrix orientations and to be transferred to a simple mechanical component for subsequent grinding and polishing, while preserving the orientation of the polished faces relative to the optical plane, acute bisectrix, and obtuse bisectrix during the grinding and polishing process. Biaxial crystalline material samples prepared in the manner are suitable for accurate spectroscopic absorption measurements in the acute bisectrix and obtuse bisectrix directions as well as perpendicular to the optical plane. © 2014 AIP Publishing LLC. Source

Eaton-Magana S.,Gemological Institute of America
Diamond and Related Materials | Year: 2015

Abstract Among gem diamonds, one of the most challenging identifications is distinguishing naturally irradiated diamond from laboratory irradiated diamond. In both cases, the GR1 optical center is created, which in high concentrations imparts a greenish color to the diamond. This research attempted to identify if time-resolved spectroscopy at the nanosecond scale would demonstrate substantive differences between the lifetime behavior of the GR1, in addition to the H3 and NV0 centers due to the type of irradiation. All three centers showed nominally similar behavior and decay times; however, the luminescence decay of treated diamonds showed more complex behavior with additional exponential decay components able to be resolved. To our knowledge, this is the first study directly comparing data derived from natural and laboratory irradiated diamonds, specifically here, the H3, NV0, and GR1 optical centers. © 2015 Elsevier B.V. Source

Pezzotta F.,Natural History Museum | Laurs B.M.,Gemological Institute of America
Elements | Year: 2011

With their multitude of colors, gem tourmalines are among the most popular colored gemstones. Spectacular color-zoned tourmalines are valued as gems and crystal specimens, and some complexly zoned crystals contain nearly the entire spectrum of color variation found in the mineral world. The top-quality "neon" blue-to-green, copper-bearing tourmaline, the Paraíba-type, is one of the highest-priced colored gemstones, with values comparable to those of some diamonds. The wide variety and intensity of colors are related primarily to color-producing ions in the structure and to exposure to natural radiation. Gem tourmalines that form in magmatic, pegmatitic environments are most commonly elbaite and fl uorliddicoatite species, and the rarer gem tourmalines that develop in metamorphic rocks are generally dravite-uvite species. Source

Renfro N.,Gemological Institute of America
Journal of Gemmology | Year: 2015

Episcopic (reflected light) differential interference contrast (DIC) microscopy can reveal information about rough or cut gems that would otherwise be difficult to obtain using standard gemmological optical microscope illumination techniques. Although not widely used in gemmology due to the current expense of the specialized equipment and its limited applications, this type of contrast-enhancing optical microscopy is particularly useful for observing surface features that may help with such tasks as identifying rough gem materials, detecting heat treatment (in stones that have not been repolished) and assessing whether a gemstone was damaged after it was polished. DIC microscopy is valuable for yielding technical information about a gem, as well as for producing informative and often striking images. © 2015 The Gemmological Association of Great Britain. Source

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