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-adiabat ic calorimetry and from 420 to 1000 K using differential scanning calo rimetry. These measurements yield CP298 = 342.80 +/- 1.4 J/mol.K and S 298-degrees = 342.60 J/mol.K. Mossbauer characterizations show the alm andine 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 angstrom and V298-degrees = 115.11 +/- 0.01 cm3/mol, somewhat sma ller than previously reported. The low-temperature Cp data indicate a lambda transition at 8.7 K related to an antiferromagnetic-paramagneti c transition with T(N) = 7.5 K. Modeling of the lattice contribution t o the total entropy suggests the presence of entropy in excess of that attributable to the effects of lattice vibrations and the magnetic tr ansition. This probably arises from a low-temperature electronic trans ition (Schottky contribution). Combination of the Cp data with existin g thermodynamic and phase equilibrium data on almandine yields DELTAG( f,298)-degrees = -4938.3 kJ/mol and DELTAH(f,298)-degrees = -5261.3 kJ /mol for almandine when calculated from the elements. The equilibrium almandine = hercynite + favalite + quartz limits the upper TIP for alm andine and is metastably located at ca. 570-degrees-C at P = 1 bar, wi th a dP/dT of + 17 bars/degrees-C. This agrees well with reversed expe riments on almandine stability when they are corrected for magnetite a nd hercynite solid-solutions. In f(O2)-T space, almandine oxidizes nea r QFM by the reactions almandine + O2 = magnetite + sillimanite + quar tz and almandine + O2 = hercynite + magnetite + quartz. With suitable correction for reduced activities of solid phases, these equilibria pr ovide useful oxygen barometers for medium- to high-grade metamorphic r ocks.