The heat capacities of glasses near the feldspar compositions KAlSi3O8, NaAlSi3O8, and CaAl2Si2O8, and of crystalline anorthite were measured between 12 and 380 K by means of an adiabatic calorimeter. Difference plots, Cpo(glass) – Cpo(crystals), show pronounced maxima at 30 K for KAlSi3O8, at 50 K for NaAlSi3O8, and at 35 K for CaAl2Si2O8, similar to maxima previously observed for other inorganic glass-crystal pairs (e.g., As2O3 and SiO2). The entropy changes, S2980S0o, for KAlSi3O8, NaAlSi3O8, and CaAl2Si2O8 glasses and for anorthite based on our measurements are 224.3 ± 0.3, 213.8 ± 0.3, 198.7 ± 0.3, and 199.3 ± 0.3 J/(mol·K), respectively.

Approximate values for the zero-point entropies, S0o, of NaAlSi3O8, KAlSi3O8 and CaAl2 Si2O8 glasses were calculated from our heat-capacity data combined with (1) the high-temperature HToH298o data for these glasses and their crystals; (2) the low-temperature heat capacities of analbite and high sanidine; and (3) the enthalpy changes, obtained by HF(aq) solution calorimetry, for the transformation crystals → glass, ΔH323o.

Our calculated values for the entropies of KAlSi3O8 glass, NaAlSi3O8 glass, and CaAl2Si2O8 glass at zero Kelvin, that is, the residual or zero-point entropies, are 37.3 ± 2.5, 38.1 ± 1.5, and 38.6 ± 2.2 J/(mol·K), respectively. The entropies of fusion at the melting points of high sanidine, analbite, and anorthite are 33.8, 42.6, and 44.3 J/(mol·K), respectively. The Debye temperature, θD, of CaAl2Si2O8 glass is 210 ± 10 K, and that of anorthite is 227 ± 15 K.

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