Abstract

Low-temperature heat capacities for sillimanite, fibrolite, and both fine-grained and coarse-grained quartz have been measured. Superambient heat capacities have been measured for four sillimanite, two andalusite, one kyanite, and two fibrolite samples. The following equations are recommended for the temperature dependence of the heat capacities of kyanite, andalusite, sillimanite, and fibrolite:  
CP0(kyanite)=279.4357.124×103T2.28936×103T0.52.0556×106T2(valid 298.152000 K, average deviation 0.17%)CP0(andalusite)=277.3066.588×103T2.26560×103T0.51.9141×106T2(valid 298.152000 K, average deviation 0.24%)CP0(sillimanite)=280.1906.900×103T2.39937×103T0.51.37573×106T2(valid 298.152000 K, average deviation 0.52%)CP0(fibrolite)=417.59670.2335×103T5.00119×103T0.51.45450×106T2(valid 298.15800 K, average deviation 0.25%)CP0(fibrolite)=306.1089.99098×103T3.48957×103T0.57.81586×106T2(valid 8002000 K, average deviation 0.16%)
The heat capacity functions have been combined with thermal expansion (fibrolite and sillimanite reported here), enthalpy of solution, and phase equilibrium data in order to construct a phase diagram for the Al2SiO5 polymorphs. The recommended thermodynamic properties of entropy and enthalpy of formation at 298.15 K consistent with the phase diagram are 95.40 ± 0.52 J/mol·K and −2586.1 ± 3 kJ/mol for sillimanite,95.43 ± 0.55 J/mol·K and −2586.1 ± 3 kJ/mol for fibrolite, 82.80 ± 0.50 J/mol·K and −2593.8 ± 3 kJ/mol for kyanite, and 91.39 ± 0.52 J/mol.K and −2589.9 ± 3 kJ/mol for andalusite. These values have been derived by minimizing the differences between the experimentally measured and calculated properties. The triple point was located at 3.87 ± 0.3 kbar and 784 ± 20 K.

Anomalous thermal response of a fibrolite sample near room temperature is interpreted as a slow structural change in fibrolite. The heat effect would appear small for the completed transition; however, failure of fibrolite to transform could result in larger measured heat capacities for the sample at temperatures above room temperature than those measured for the transformed sample. This may explain why the heat capacities reported by Salje (1986) for fibrolite are about 2% larger than those reported here.

Lack ofa demonstrated excess heat capacity for novaculite at low temperatures suggests that aggregates of cemented or intergrown small crystals do not have the same degree of surface lattice distortion and degrees of freedom for surface atoms and consequently have smaller surface energies than powders of the same mineral. The chemically similar qvartz grains and quartz cement of the novaculite represent one limit on the excess properties that fine-grained geologic materials may exhibit.

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