Synthesis and stability of Fe₃ ²⁺Fe₂ ³⁺Si₃O₁₂ garnet and phase relations with Fe₃Al₂Si₃O₁₂-Fe₃ ²⁺Fe₂ ³⁺Si₃ O₁₂ solutions

Garnets containing both Fe²⁺ and Fe³⁺ are potentially useful monitors of fO₂ in metamorphic rocks and garnet peridotites of the mantle. However, little is known about the properties or stability of such garnets. End-member Fe₃ ²⁺Fe₂ ³⁺Si₃O₁₂ garnet and garnet along the almandine (Fe₃ ²⁺Al₂Si₃O₁₂)-Fe₃ ²⁺Fe₂ ³⁺Si₃O₁₂ join, where Al and Fe³⁺ mix on the octahedral sites were synthesized. At 1100°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 Δ_fG_1,1373 ^o = -2975.2 ± 3.0 kJ/mol is obtained for Fe₃ ²⁺Fe₂ ³⁺Si₃O₁₂. The spinel becomes increasingly Fe₂SiO₄-rich with increasing pressure. -from Authors