A CRYSTALLOGRAPHIC AND MOSSBAUER-SPECTROSCOPY STUDY OF FE3(2-FE3(2+)FE2(3+)SI3O12, (ALMANDINE-SKIAGITE) AND CA-3 FE2(3+)SI3O12-FE3(2+)FE2(3+)SI3O12 (ANDRADITE-SKIAGITE) GARNET SOLID-SOLUTIONS()AL2SI3O12)

The crystal chemistry of garnet solid solutions on the Fe32+Al2Si3O12 - Fe23+Fe23+Si3O12 (almandine-''skiagite'') and Ca3Fe23+Si3O12 - Fe32Fe23++Si3O12 (andradite-''skiagite'') joins have been investigated by single-crystal X-ray structure refinements and Mossbauer spectroscopy. Together, these two solid solution series encompass the complete rang e in Fe3+/SIGMAFe from 0.0 to 1.0. All garnets are isotropic and were refined in the Ia3dBAR space group. Small excess volumes of mixing are observed in andradite-''skiagite'' solid solutions (W(v) = 1.0 +/- 0. 2 cm3 mol-1) and along the almandine-''skiagite'' join (W(v) = -0. 77 +/- 0.17 cm3 mol-1). The octahedral (Al, Fe3+) -0 bond lengths show a much greater variation across the almandine-skiagite join compared to the andradite-skiagite garnets. The dodecahedral (X)-O bond lengths sh ow the opposite behaviour. In andradite-''skiagite'' solid solutions, the octahedral site passes from being flattened to elongated parallel to the 3BAR axis of symmetry with increasing ''skiagite'' content. A p erfect octahedron occurs in a composition of almost-equal-to 35 mol% ' 'skiagite''. The occupancy of the neighboring dodecahedral sites has t he greatest effect on octahedral distortion and vice versa. The Mossba uer hyperfine parameters of Fe2+ remain constant in both solid solutio ns. The hyperfine parameters of Fe3+ (at room temperature: centre shif t=0.32-0.40 mm/sec, quadrupole splitting (QS) almost-equal-to 0.21-0.5 5 mm/sec) indicate that all Fe3+ is in octahedral coordination. The Fe 3+ parameters are nearly constant in almandine'' skiagite'' solid solu tions, but vary significantly across the andradite-''skiagite'' join. The structural unit that contributes to the electric field gradient of the octahedral site is different from that of the coordinating oxygen polyhedron, probably involving the neighboring dodecahedral sites. Fe 3+/SIGMAFe area ratios derived from the Mossbauer spectra systematical ly overestimate Fe3+ contents in both solid solutions series. This is attributable to different recoil-free fractions for Fe on the octahedr al and dodecahedral sites. A correction has been derived that yields m ore accurate Fe3+/SIGMAFe ratios from room temperature and 80 K Mossba uer spectra.