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/molK). 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.

The new data for the alkali feldspars have been combined with recent data for plagioclase feldspars to derive an expression for the two-feldspar thermometer that is consistent with present knowledge of the thermodynamics of these solid solutions.
TK=(XOrAF)2(18810+17030XAbAF+0.364P)(XAnPl)2(2823039520XAbPl)10.3(XOrAF)2+8.31431n{(XAbPl)2(2XAbPl)XAbAF}
where the mole fractions refer to the ternary system and P is in bars. Temperatures calculated from this expression tend to be higher than those calculated from previous formulations.
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