CESSTIBTANTITE - A GEOLOGIC INTRODUCTION TO THE INVERSE PYROCHLORES

Citation
Ts. Ercit et al., CESSTIBTANTITE - A GEOLOGIC INTRODUCTION TO THE INVERSE PYROCHLORES, Mineralogy and petrology, 48(2-4), 1993, pp. 235-255
Citations number
24
Categorie Soggetti
Mineralogy,Geology
Journal title
ISSN journal
09300708
Volume
48
Issue
2-4
Year of publication
1993
Pages
235 - 255
Database
ISI
SICI code
0930-0708(1993)48:2-4<235:C-AGIT>2.0.ZU;2-M
Abstract
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).