The Effects of F-OH- Substitution on the Crystal Structure of Pegmatitic Topaz



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Pegmatites are intrusive igneous rocks that are characterized by unusually large grain sizes (> 20mm). There are two main geochemical types of granitic pegmatites, LCT and NYF. LCT pegmatites are rich in the elements lithium (Li), cesium (Cs), and tantalum (Ta) and are usually associated with orogenic granites. NYF pegmatites are related to anorogenic granites and are enriched in niobium (Nb), yttrium (Y), and fluorine (F). Topaz is a fluorine-rich mineral that commonly occurs in pegmatites; however, topaz is more common in NYF pegmatites than in LCT pegmatites. The focus of this study is to examine the changes in topaz crystal structure caused by F-OH- substitution and to investigate the structural changes due to temperature in order to determine if there are structural differences between topaz from LCT pegmatites and topaz from NYF pegmatites. In this study, 30 topaz samples from 25 localities were analyzed for unit-cell parameters, fluorine content, trace element content, and heating effects. Cell parameters were calculated from X-ray diffraction data, and fluorine content was obtained by electron microprobe analysis. As consistent with other topaz studies (Alberico et al. 2003), the b unit-cell dimension decreases with increasing F content. Also consistent was a less defined decreasing trend in the a unit-cell dimension as F content increases. No correlation was found between the c cell dimension and any other cell-dimension or compositional variation. Plots of the pegmatitic topaz from this study show distinct and separate linear trends between NYF and LCT pegmatites when comparing the b unit-cell dimension to the F content. Trace element analysis by X-ray fluorescence shows the presence of Ge, Cr, Pb, Ga, W, and Rb but yields no correlation between trace elements and unit-cell variations. Selected samples were chosen for a heat study. Samples were heated at 700oC for 1 hour. After heating, the unit-cell volume of the topaz generally decreased but no trends separating NYF pegmatites from LCT pegmatites were found.



topaz, pegmatite, fluorine