FLUORINE-RICH HIBSCHITE FROM SILICOCARBONATITE, AFRIKANDA COMPLEX, RUSSIA: CRYSTAL CHEMISTRY AND CONDITIONS OF CRYSTALLIZATION

Calcite - amphibole - diopside silicocarbonatite from the Afrikanda complex, in the Kola Peninsula, northwestern Russia, contains abundant crystals of F-rich hibschite associated with titanite, chlorite, calciocatapleiite and calcite. The crystals range from 10 to 50 mm across and consist of an oscillatory-zoned core and a uniform rim arising from variations in Al and Fe contents across the crystal. The hibschite contains from 4.2 to 6.0 wt.% F, which is unparalleled by any F-bearing silicate garnet described to date. The compositional variation of the Afrikanda hibschite can be described by the formula Grs(57-63)Kt(21-27)Fgr(8-11)Adr(0-13), where Grs, Kt and Adr stand for the grossular, katoite and andradite, respectively, and Fgr denotes the hypothetical end-member Ca3Al2F12. The average Fgr content is 10 mol.%. The crystal structure of F-rich hibschite was refined in space group Ia (3) over bard to R-1 = 2.8%. In agreement with the chemical data, the refinement shows that about one-third of the tetrahedrally coordinated positions (Z) are vacant. These vacancies are coordinated by (OH)(-) and F- anions, which results in expansion of the unit cell [a = 12.037(3)angstrom] relative to that of grossular. The size of the Z-centered tetrahedron is sensitive to the substitution of (OH)(-) by F-, as indicated by the smaller center-to-corner and corner-to-corner distances measured for the Afrikanda hibschite relative to garnets with a similar Si content along the grossular - katoite join. The F- rich hibschite crystallized in the final stages of silicocarbonatite evolution from a low-temperature (200-250 degrees C) deuteric fluid with log a(H+)(aq) approximate to log a(F-)(aq) approximate to -5.