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Topaz [Al2SiO4(F,OH)2]
Structure Orthorhombic
Space Group : Pbnm (No. 62)
a=4.6520, b=8.8010, c=8.4040 Å
a=b=g=90.00
Z=4

Atomic Positional Parameters
Al   8d 0.9030 0.1309 0.0828
Si   4c 0.3972 0.9404 0.2500
O(1) 4c 0.7036 0.0321 0.2500
O(2) 4c 0.4577 0.7560 0.2500
O(3) 8d 0.2102 0.9892 0.0924
F(1) 8d 0.5982 0.2525 0.0561

Reference
Northrup PA, Leinenweber K and Parise JB (1994) American Mineralogist 79 401

Mineral Chemistry
Topaz is found associated with cassiterite, tourmaline, fluorite and muscovite, and occurs in granites, granite pegmatites and rhyolites.

The structure of topaz consists of SiO4 tetrahedra, which link chains of edge-sharing octahedra formed by aluminium coordinated by four oxygens and two fluorines. The fluorines may be partially replaced by OH-.


The CHIME figure shows fluoride ions as green spheres, oxygens as red spheres, silicons as ochre spheres and aluminiums as grey spheres. The unit cell is outlined.