Calorimetric investigation of the excess entropy of mixing in analbite-sanidine solid solutions; lack of evidence for Na,K short-range order and implications for two-feldspar thermometry

Heat capacities (5-380 K) have been measured by adiabatic calorimetry for five highly disordered alkali feldspars (Ab₉₉Or₁, Ab₈₅Or₁₅, Ab₅₅Or₄₅, Ab₂₅Or₇₅, and Ab₁Or₉₉). Positive heat capacity deviations from a linear combination of the end-member heat capacities, which are present mostly at very low temperatures, result in an excess entropy for intermediate compositions. The excess entropy at 298.15 K is well described by the symmetric expression $S298XS=XAbXOr(10.3±0.3 J/mol⋅K)$⁠. For practical calculations, the entropy and enthalpy of mixing can be regarded as temperature-independent above room temperature.

The excess entropy and volume of mixing have been combined with solvus determinations to obtain a calculated enthalpy of mixing. Because the measured enthalpies of mixing are essentially coincident with those calculated from the solvus determinations, no short-range order for the alkali site could be inferred.