The heat capacities, Cp, of low albite, analbite, microcline, and high sanidine have been measured between 350 and 1000 К by differential scanning calorimetry. Our data for the alkali feldspars were combined with low-temperature heat-capacity and high-temperature heat-content data taken from the literature to derive the following equations:
Cp(albite)=583.90.09285T+2.272×105T26424T1/2+1.678×106T2(±0.6 percent 2981400 K)Cp(analbite)=671.40.1467T+3.659×105T27974T1/2+3.174×106T2(±0.3 percent 2981400 K)Cp(microcline)=759.50.2171T+6.433×105T29527T1/2+4.764×106T2(±0.6 percent 2981400 K)Cp(high sanidine)=693.40.1717T+4.919×105T28305T1/2+3.462×106T2(±0.5 percent 2981400 K)

where T is in kelvins and Cp is in units of J/(mol · K). Smoothed values of the thermodynamic properties of these feldspar phases derived from the experimental data given in this report are listed in Robie et al. (1979).

Evaluation of the results of several phase equilibrium studies of the reaction quartz + jadeite = analbite yields −3029.870±4.200 and −2851.300±4.200 kJ/mol for the enthalpy and Gibbs free energy of formation, respectively, of jadeite at 298.15 K and 1 bar.

The enthalpy of the low albite–analbite transition determined from calorimetry using Amelia albite for low albite underestimates the magnitude of the enthalpy of the transition because Amelia albite is partially disordered (e.g., Holm and Kleppa, 1968).

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