SYNTHESIS AND STABILITY OF FE32+FE23+SI3O12 GARNET AND PHASE-RELATIONS WITH FE3AL2SI3O12-FE32+FE23+SI3O12 SOLUTIONS

Garnets containing both Fe2+ and Fe3+ are potentially useful monitors of f(O2) in metamorphic rocks and garnet peridotites of the mantle. However, little is known about the properties or stability of such garnets. We have synthesized end-member Fe32+Fe23+Si3O12 garnet and garnet along the almandine (Fe32+Al2Si3O12)-Fe32+Fe23+Si3O12 join, where Al and Fe3+ mix on the octahedral sites. The experiments were performed with a series of glasses and slags with a fixed ratio of Fe3+/Fe(tot) in a piston cylinder apparatus and a multianvil press using Ag as the capsule material. The extent of Fe3+ substitution for Al is strongly pressure dependent. At 1100-degrees-C, the maximum Fe32+Fe23+Si3O12 content is 13, 43, and 75 mol% at 2.7, 6.5, and 8.0 GPa, respectively. The end-member ferrous-ferric garnet is only stable at pressures above almost-equal-to 9.3 GPa. However, skiagite becomes unstable at higher pressures of even < 13.0 GPa. Mossbauer spectroscopy confirms that Fe2+ and Fe3+ are restricted to the dodecahedral and octahedral sites, respectively. Molar volumes of Fe32+Fe33+Si3O12 and almandine are 121.44 +/- 0.01 cm3 [a0 = 11.7278(6) angstrom] and 115.23 +/- 0.01 cm3, [a0 = 11.5 244(4) angstrom] and vary linearly across the binary join. At 1100-degrees-C, the equilibrium garnet solid solutions coexist with fayalite + spinel + quartz at P < 1.6 GPa, with spinel + Fe2Si2O6 + quartz or coesite at 1.6 < P < 5.0 GPa, and with Si-rich spinel + coesite or stishovite above 5.0 GPa. The coexisting spinel and pyroxene are very Al poor, indicating that Al is strongly partitioned into the garnet. Based upon Al-Fe3+ exchange between garnet and spinel, a DELTA(f)G1,1373o = -2975.2 +/- 3.0 kJ/mol is obtained for Fe32+Fe23+Si3O12, assuming ideal mixing in the garnet. The spinel becomes increasingly Fe2SiO4-rich with increasing pressure such that the Fe3+/Fe(tot) of the end-member Fe32+Fe23+Si3O12 garnet and coexisting spinel are nearly the same.