Abstract
Baotite occurs in the Garaoulere orebody, at Pierreftte, France, as an accessory mineral, included in alstonite and celsian, and associated with sphalerite, galena, pyrite, siderite and calcite in hydrothermal veins crosscutting calcareous, rutile-bearing, siltstones. Microprobe analyses revealed high W03 concentrations (up to 6 wt.%) in baotite. The empirical formula of “W-rich” baotite is Ba3.959Ti4(Ti3.169W0.393Fe0.116Al0.073 Cr0.048Nb0.024)3.823 Si4.05O28Cl1.166. The excess of charges due to the presence of W6+ and Nb5+ is compensated by the introduction of M3+ (Fe, Al, Cr) into Ti-octahedra, by the appearance of Al in Si-tetrahedra (for “W-poor” baotite) and by the appearance of vacancies in Ti-octahedra (3Ti4 -> 2W6+ + □ and in Ba-sites (Ti4+ Ba2+ → W6+, □). The unit-cell parameters of “W-rich” baotite are: a = 19.92(2), c = 5.930(8) A. Niobium-rich baotites (Baiyun-Obo,Semenov et al., 1961; Karlstein,Němec, 1987) are characterized by substitutions: Ti4+(VI), Si4+(IV)→Nb5+(VI), Al3+(IV) and 2Ti4+, Ba2+ → 2Nb5+, □.
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