Computer Science
Scientific paper
Jan 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44...61p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 44, Issue 1, pp.61-84
Computer Science
18
Scientific paper
The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K -1 mol -1 ; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K -1 mol -1 , respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298-600 K): Cp ° = 66.28 + 55.95 × 10 -3 T - 15.27 × 10 5 T -2 Margarite (298-1000 K): Cp ° = 101.83 + 24.17 × 10 -3 T - 30.24 × 10 5 T -2 Prehnite (298-800 K): Cp ° = 97.04 + 29.99 × 10 -3 T - 25.02 × 10 5 T -2 Zoisite (298-730 K): Cp ° = 98.92 + 36.36 × 10 -3 T - 24.08 × 10 5 T -2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al 2 O 3 -SiO 2 -H 2 O system compare well with experimental results in most cases. However, the reaction zoisite + QUARTZ = anorthite + grossular + H 2 O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.
Essene Eric J.
Perkins Dexter III
Westru Edgar F.
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