Heat capacity and phase equilibria of almandine, Fe3Al2Si3O12

The heat capacity of a synthetic almandine, Fe₃Al₂Si₃O₁₂, was measured from 6 to 350 K using equilibrium, intermittent-heating quasi-adiabatic calorimetry and from 420 to 1000 K using differential scanning calorimetry. These measurements yield Cp₂₉₈ = 342.80 ± 1.4 J/mol · K and S₂₉₈^o = 342.60 J/mol · K. Mössbauer characterizations show the almandine to contain less than 2 ± 1% of the total iron as Fe³⁺. X-ray diffraction studies of this synthetic almandine yield a = 11.521 ± 0.001 Å and V₂₉₈^o = 115.11 +- 0.01 cm³/mol, somewhat smaller than previously reported. The low-temperature Cp data indicate a lambda transition at 8.7 K related to an antiferromagnetic-paramagnetic transition with T_N = 7.5 K. Modeling of the lattice contribution to the total entropy suggests the presence of entropy in excess of that attributable to the effects of lattice vibrations and the magnetic transition. This probably arises from a low-temperature electronic transition (Schottky contribution).

Combination of the Cp data with existing thermodynamic and phase equilibrium data on almandine yields ΔG_f,298^o = −4938.3 kJ/mol and ΔH_f,298^o= —5261.3 kJ/mol for almandine when calculated from the elements. The equilibrium almandine = hercynite + fayalite + quartz limits the upper TP">TP for almandine and is metastably located at ca. 570°C at P = 1 bar, with a dPdT">dPdT of +17 bars/°C. This agrees well with reversed experiments on almandine stability when they are corrected for magnetite and hercynite solid-solutions. In ‖_O2-T space, almandine oxidizes near QFM by the reactions almandine + O₂ = magnetite + sillimanite + quartzandalmandine + 02 = hercynite + magnetite + quartz. With suitable correction for reduced activities of solid phases, these equilibria provide useful oxygen barometers for medium- to high-grade metamorphic rocks.