year of publication
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 298.15K is well described by the symmetric expression SXS298 = XAbXOr(10.3 ± 0.3 J/mol·K). For practical calculations, the entropy and enthalpy of mixing can be regarded as temperature-independent at above room temperature.
The excessentropy 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 feldsparsto derive an expression for the two-feldspar thermometer that is consistent with present knowledge of the thermodynamics of these solid solutions.
(eq. abbrev. here)
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.