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 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 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 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.

NYF Pegmatites

- Related to subaluminous to metaluminous A-type or I-type granites

- Nb, Ta, Ti, Y, Sc, REE, Zr, U, Th, F

LCT Pegmatites

- Related to peraluminous S-type or I-type granites

- Li, Rb, Cs, Be, Sn, Ta, Nb, B, P, F

Topaz Structure and F- OH- Substitution

Topaz, denoted by the chemical formula Al2SiO4(F,OH)2, is a nesosilicate. In this class of silicates, the tetrahedra are not bonded to other tetrahedra in the structure. Aluminum octahedra link together in long chains (edge sharing) in the direction of the c axis. The slight differences in the lengths of the shared edges give the octahedral chain its contorted shape. OH- will substitute for F in two sites on the aluminum octahedron. Topaz is usually considered to have orthorhombic symmetry but there is evidence that the symmetry of OH--rich topaz reduces to triclinic symmetry.

Fluorine Content and Cell Dimensions

For this study, cell dimensions were calculated and refined using the Rietveld method. The average error in the cell dimensions are 0.0003 Å. The average error in the cell volumes is 0.03 Å3. Chemical data was acquired by electron microprobe analysis. Topaz compositions ranged from Al2.01Si0.99O4(F1.81OH0.19) to Al2.02Si0.99O4(F1.96OH0.04) with an F/(F+OH) range of 0.88 to 0.98.

Previous studies have examined the relationship of F content to topaz cell dimensions. Ribbe & Rosenberg (1971) included pegmatitic topaz in their data set. All other data consisted of synthetic topaz or natural samples from rhyolite and aplite. The plot to the right supports earlier finds that the b cell dimension in topaz decreases with increasing F content. The pegmatite data from this study is denoted by green circles.

Data from this study shows that within the small range of pegmatitic topaz, LCT and NYF pegmatitic topaz are separated into distinct trends. There is also a decreasing trend when the a unit-cell dimension is plotted against wt% F. This trend is more scattered than the b dimension plot and there is no distinction between LCT and NYF pegmatites. No trends were found when the c unit-cell dimension was plotted against other cell parameters (a, b, volume) or compositional data.

After heating, the unit-cell volume of topaz generally decreased. The rate of decrease, however, was inconsistent within localities and pegmatite types. The amount of F present did not appear to affect the rate of decrease of the unit-cell volume.

Trace elements have little influence on the variations of the unit-cell. X-ray fluorescence showed the presence of Ge, Cr, Pb, Ga, W, and Rb but yielded no correlation between trace elements and unit cell variations.

Conclusions

1) For all topaz, the b unit-cell dimension decreases with increasing F content.

2) In pegmatitic topaz, the b unit-cell dimension is higher for a given wt% F in LCT pegmatites than in NYF pegmatites and can be used to distinguish between the two types of pegmatites.

3) F- OH- substitution, trace elements, and heating are not responsible for the major fluctuations in the c unit-cell direction.

Acknowledgements

I would like to express my gratitude for the support from the Smithsonian Women’s Committee, Dr. Michael Wise, Cathe Brown, and Dr. Amelia Logan

References

Alberico, A., Ferrando, S., Ivaldi, G. & Ferrais, G. (2003): X-ray single crystal structure refinement of an OH-rich topaz from Sulu UHP terrane (Eastern China) – structural foundation of the correlation between cell parameters and fluorine content. European Journal of Mineralogy, 15, 875-881.