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

The crystal chemistry of garnet solid solutions on the Fe32+Al2Si3O12 - Fe23+Fe23+Si3O12 (almandine-''skiagite'') and Ca3Fe23+Si3O12 - Fe32+Fe23++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 range 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 show 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 perfect octahedron occurs in a composition of almost-equal-to 35 mol% ''skiagite''. The occupancy of the neighboring dodecahedral sites has the greatest effect on octahedral distortion and vice versa. The Mossbauer hyperfine parameters of Fe2+ remain constant in both solid solutions. The hyperfine parameters of Fe3+ (at room temperature: centre shift=0.32-0.40 mm/sec, quadrupole splitting (QS) almost-equal-to 0.21-0.55 mm/sec) indicate that all Fe3+ is in octahedral coordination. The Fe3+ parameters are nearly constant in almandine'' skiagite'' solid solutions, 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. Fe3+/SIGMAFe area ratios derived from the Mossbauer spectra systematically overestimate Fe3+ contents in both solid solutions series. This is attributable to different recoil-free fractions for Fe on the octahedral and dodecahedral sites. A correction has been derived that yields more accurate Fe3+/SIGMAFe ratios from room temperature and 80 K Mossbauer spectra.