CESSTIBTANTITE - A GEOLOGIC INTRODUCTION TO THE INVERSE PYROCHLORES

The crystal structure of cesstibtantite has been solved from diffracto meter data collected on samples from Leshaia, Russia and the Tanco peg matite, Manitoba. Cesstibtantite from the Leshaia pegmatite (type loca lity) has a 10.515(2) angstrom, space group Fd3m, composition 0.02Bi0. 01)SIGMA0.91(Ta1.88Nb0.12)SIGMA2(O5.69[OH, F]0.31)SIGMA6(OH,F)0.69Z 8; its structure was refined to R 3.8, wR 4.3% using 96 observed (F > 3s igma[F(o)]) reflections (MoKalpha). Cesstibtantite from the Tanco pegm atite has a 10.496(1) angstrom, space group Fd3m, composition 0.06Bi0. 02)SIGMA1.06(Ta1.95Nb0.05)SIGMA2(O5.78[OH, F]0.22)SIGMA6(OH,F)0.55, Z 8; its structure was refined to R 3.9 wR 3.7% using 104 observed refle ctions. Cesstibtantite differs from the normal pyrochlores in that it contains significant amounts of very large cations such as Cs. As thes e cations are too large (VIII[r] > 1.60 angstrom) for the conventional [8]-coordinated A site, they occupy the [18]-coordinated phi site, wh ich normally contains mono-valent anions. Natural cesstibtantite sampl es are non-ideal in that both Cs and mono-valent anions occur at the p hi site; thus cesstibtantite is intermediate to the normal pyrochlores (with only monovalent anions at the phi site) and the inverse pyrochl ores (with only large cations at the phi site).