The natural color of some topaz crystals can be altered to an orangey pink when heat treated. Some natural pink topaz crystals exist, however in these cases the pink coloring is a result of chromium replacing the aluminum atoms in its structure. Other color varieties of topaz occur, including blue, yellow, orange, and brown; all contain color centers\u2014crystallographic defects in which anionic vacancy in a crystal is filled by one or more unpaired electrons. The more common blue color of gem topaz is primarily unnatural, and is produced by irradiating and then heating colorless crystals.<\/p>\n
Polarized Light Microscopy<\/h2>\n
Crushed topaz consists of transparent, sharply angular flakes and chips (001000) devoid of color. The edges and faces are uneven due to conchoidal fracture. Topaz is orthorhombic and has indices of 1.606-1.629 (\u03b1), 1.609-1.631(\u03b2), 1.616-1.638 (\u03b3); (+) 0.009-0.011. It resembles quartz, but has much lower contrast in Meltmount refractive index medium (1.6620). Topaz does not normally present pleochroism, but thick fragments may present shades of yellow to pink pleochroism. The specific gravity is 3.5, and the hardness is 8. Topaz has perfect single basal cleavage (001), and this tends to control orientation of crushed grain fragments.<\/p>\n In crossed polarized light, topaz exhibits low to moderate retardation colors, with the possibility of higher order colors being visible in thicker grains. Upon rotating the stage, topaz shows complete extinction. Conoscopic analysis of topaz reveals a biaxial interference figure with a 2V angle from 45 \u2013 60\u00b0. Topaz shows a positive optic sign when the red one compensator is introduced during conoscopic analysis.<\/p>\n When placed in narrow band UV light (360 \u2013 370 nm), there is no observed fluorescence.<\/p>\n Click on any picture to view the full size image.<\/p>\n Measuring n\u03b2<\/sub><\/strong><\/p>\n For any biaxial mineral, n\u03b2<\/sub> can be determined using optic axis and pendulum figures (off center optic axis).<\/p>\n Additionally, Bxa\/Bxo figures can be used, but more patience and skill are required to find well-centered figures.<\/p>\n Measuring n\u03b1<\/sub> and n\u03bb<\/sub><\/strong><\/p>\n n\u03b1<\/sub> and n\u03bb<\/sub> of a grain can be measured using an optic normal (flash figure). Additionally, these refractive indices can be measured from a Bxa figure; however, the optic normal (flash figure) method is most commonly used throughout this procedure. Determining the optic sign is necessary before measuring both refractive indices.<\/p>\n Nesse, W. D.<\/strong> (2004). Introduction to optical mineralogy<\/em> (3rd ed.). Oxford University Press, NY.<\/p>\n Karen Hurrell; Mary L. Johnson<\/strong> (2016). Gemstones: A Complete Color Reference for Precious and Semiprecious Stones of the World<\/em>. Book Sales. p. 169.\u00a0Full text available online<\/a>.<\/p>\n Due to the nonconductive nature of topaz, Al2<\/sub>SiO4<\/sub>(F,OH)2<\/sub>, a 2.0 nm gold-palladium coating was applied to the grains to provide a conductive coating. Without the use of a gold-palladium coating, analysis of topaz would be limited to low vacuum conditions in order to reduce charging.<\/p>\n SEM images highlight the characteristic conchoidal fractures found on comminuted topaz grains (S-101, 102). S-101 displays the irregular shape that crushed topaz grains usually form, with very few straight edges. Topaz has perfect basal cleavage and this tends to control orientation of crushed grain fragments, though this is not demonstrated in the images.<\/p>\n Click on any picture to view the full size image.<\/p>\n Energy dispersive X-ray spectrometry was used to determine the elemental composition of topaz. Due to the homogeneity of the sample, variation between areas yield similar elemental results. The summary table of the elemental weight percentages can be found below. The presence of gold in each spectrum is due to the ~2.0 nm gold-palladium coating applied to the sample, but a deconvolution function was applied to the quantitative data to remove gold and accurately report the weight percentages of the sample. The major carbon component may be due to the carbon tab on which the topaz was mounted.<\/p>\n Trace elements may vary depending on impurities in source material.<\/p>\n Click on the image to view an enlargement.<\/p>\n Topaz shows very strong, sharp peaks in its FTIR transmission spectrum. The most recognizable peaks include the strong peak at ~870 cm-1<\/sup>, which corresponds to a shifted Si-O-H bonding peak. Additionally, the fluorine presence is indicated by the sharp C-F bonding peaks at 1164 and 1006 cm-1<\/sup>. The peak at 3650 cm-1<\/sup> is related to the structurally hindered O-H stretch.<\/p>\n Topaz is a relatively good Raman scatterer, showing sharp bands across the spectrum. The peaks around 1000 cm-1<\/sup> correspond to Si-O-H stretching. Peaks below 500 cm-1<\/sup> are attributed to the crystal lattice structure.<\/p>\n![\"Topaz](\"http:\/\/www.mccrone.com\/wp-content\/uploads\/2018\/06\/Topaz-PLM-Table.jpg\")
<\/a><\/p>\nFluorescence Microscopy<\/h5>\n
<\/a>\n\t\t\t<\/div>\n\t\t\t\t<\/a>\n\t\t\t<\/div>\n\t\t\t\t<\/a>\n\t\t\t<\/div>\n\t\t\t\t<\/a>\n\t\t\t<\/div>\n\t\t\t\t<\/a>\n\t\t\t<\/div>\n\t\t\t\t<\/a>\n\t\t\t<\/div>\n\t\t\t\tTutorial: Selecting Biaxial Mineral Grains for Interference Figures and Refractive Index Measurement<\/strong><\/h3>\n
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PLM REFERENCES<\/strong><\/h5>\n
SEM Characterization<\/h2>\n
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Energy Dispersive X-Ray Spectrometry (EDS)<\/strong><\/h2>\n
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\nEDS System: Oxford AZTEC v. 3.3
\nAccelerating Voltage: 15kV
\nAnalysis Range: 1-10kV<\/li>\n<\/ul>\nFourier Transform Infrared Spectroscopy<\/h2>\n
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Raman Spectroscopy<\/h2>\n
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