EXSOLUTION IN NIOBIAN RUTILE FROM THE PEGMATITE DEPOSIT AT GREENBUSHES, AUSTRALIA

Niobian rutile (Ti 4+ 0.63 Nb 5+ 0.14 Fe 2+ 0.11 Ta 5+ 0.08 Fe 3+ 0.02 W 6+ 0.01 Sn 4+ 0.003)� 0.99 O2 from the Greenbushes granitic pegmatite, Western Australia, contains lamellae (~5 � 50 � m in size) of disordered ferrocolumbite (Fe 2+ 0.80 Mn 2+ 0.12 Ti 4+ 0.07)� 0.99 (Nb 5+ 1.48 Ta 5+ 0.28 Ti 4+ 0.13 W 6+ 0.09 Sn 4+ 0.01)� 1.99 O6. The lamellae apparently formed by exsolution from the original rutile-like phase, which is supported by several features, most notably by the fixed structural orientation of lamellae with respect to the host phase. Singlecrystal X-ray diffraction shows that ferrocolumbite lamellae are present in four orientations, with the reciprocal axis cfcl* of each approximately parallel to one of the rutile directions [101]*, [011]*, [ ‐ [0 ‐ 11]*. The ferrocolumbite a* axes are strictly parallel to the ±a1* and ±a2* axes of rutile, whereas bfcl* is not parallel to any rational reciprocal direction of rutile. This may be the first documented case where the mutual crystallographic orientation between host rutile and a precipitate of disordered ferrocolumbite has been determined. Weak reflections corresponding to a ferrocolumbite supercell with a triple a indicate the presence of ordered ferrocolumbite in what appears to be a parallel orientation with the disordered ferrocolumbite.