Synthesis and stability of the garnet calderite in the system Fe-Mn-Si-O

The rare garnet end member calderite, Mn ₃ ²⁺ Fe ₂ ³⁺ Si₃O₁₂, has been synthesized, under the oxygen fugacity of the hematite/magnetite buffer, at pressures not lower than 22 kbar. The synthetic crystals are generally zoned and may contain up to 3 mol% of the Fe²⁺-(=skiagite-) or 10 mol% of the Mn³⁺-(=blythite-) end members, but, under equilibrium conditions, the stable garnets have a restricted compositional range with about 1–4 mol% skiagite. Mn³⁺ represents only a residue of the Mn₂O₃ used in the starting material. At temperatures above 720° C (at 24 kbar) to 850° C (at 30 kbar) these garnets break down into coexisting pairs of magnetite-jacobsite and pyroxmangite-FeSiO₃ solid solutions. No indication for divariancy of this breakdown reaction could be established so that the observed coexistence of garnet and breakdown products over a PT interval must be due to disequilibrium. Although extrapolations of the high-pressure stability data towards lower pressures are hazardous, it is clear that nearly pure calderite garnets can only form in metamorphic environments characterized by geothermal gradients not exceeding some 10°–15° C/km, that is in subduction zone metamorphism. A low-pressure end of the calderite stability is likely because, at temperatures below 250°–300° C, pyroxmangite probably becomes unstable and hydrous Mn²⁺-silicates appear among the low-temperature breakdown products of calderite. Since the upper temperature stability limits of the common garnet end members spessartine and andradite lie some 800° C above that for calderite, solid solutions with these components will drastically stabilize the garnet phase towards both higher temperatures and lower pressures. This explains why garnets containing around 70 mol% calderite can be formed in amphibolitefacies metamorphism.