F-OH- SUBSTITUTION AND THERMAL EFFECTS ON THE CRYSTAL STRUCTURE OF PEGMATITIC TOPAZ

Pegmatites are intrusive igneous rocks that are characterized by unusually large grain sizes (> 20mm). There are two geochemical types of granitic pegmatites, LCT and NYF. LCT pegmatites are rich in lithium (Li), cesium (Cs), and tantalum (Ta) and are usually associated with orogenic events. NYF pegmatites are related to anorogenic granites and are enriched in niobium (N), 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 project was 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, 32 topaz samples from 26 localities were analyzed for unit-cell parameters, fluorine content, trace element content, and heating effects. Cell parameters were obtained by using X-ray diffraction, and fluorine content was obtained by electron microprobe analysis. The b unit-cell dimension decreases with increasing F content. There is 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 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 700^oC 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.