Cation-site partitioning in Ti-rich micas from Black Hill (Australia): A multi-technical approach

The crystal chemistry of Ti-rich trioctahedral micas of plutonic origin, cropping out at Black Hill (South Australia) has been investigated by combining electron microprobc analysis, single crystal X-ray diffraction, Mossbauer spectroscopy and X-ray photoelectron spectroscopy. Chemical analyses have shown the samples taken to be quite homogeneous and Ti-rich (TiO2 approximate to 7 wt.%). Mossbauer investigation yielded Fe2+/Fe3+ approximate to 30. X-ray photoelectron spectroscopy analysis seems to suggest the occurrence of three Ti species: octahedral Ti4+(60%), octahedral Ti3+(26%), and tetrahedral Ti4+( 14%). The analyzed sample belongs to the 1Mpolytype and the relevant crystal data front structure analysis are: a 5.347(l) angstrom, b = 9.261(2) angstrom, c = 10. 195(2) angstrom,beta - 100.29 degrees (1). Anisotropic structure refinement was performed in space group C2/m, and converged at R 2.62, R-w - 2.80. Structural details (the c cell parameter, the off-center shift of the M2 cation towards O4, the bond-length distortions of the cis-M2 octahedron, the interlayer sheet thickness, the projection of K-O4 distance along c*, the difference < K-O >(outer)< K-O >(inner)) support the occurrence of the Ti-oxy substitution (R-VI(2+) + 2(OH)(-) reversible arrow Ti-VI(4+) + 2O(2-) + H-2) in the sample. Analysis of structural distortions as a function of the Ti content revealed that the positions of the oxygens 03 and 04 are displaced in opposite senses along [100]. This produces an enlargement of the M1 site with respect to the M2 site and a shortening of the interlayer distance. This trend seems to be in common with other Ti-rich 1M micas of plutonic origin.