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Heat capacity and entropy of melanophlogite: Molecule-containing porosils in nature by C. A. Geiger; E. Dachs; M. Nagashima is a Earth and Planetary Sciences article available to read on EtoBox.

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The heat capacities of two different molecule-containing melanophlogites of approximate composition 46SiO 2 •1.80CH 4 •3.54N 2 •1.02CO 2 from Mt. Hamilton, California, and 46SiO 2 •3.59CH 4 •3.10N 2 •1.31CO 2 from Racalmuto, Sicily, along with a heat-treated (molecule-free) sample of composition SiO 2 , were studied between 5 and 300 K using heat-pulse microcalorimetry. The molecule-free sample was obtained by heating natural Racalmuto crystals at 1173 K for 24 h. The standard third-law entropy of the molecule-free sample is S° = 2216.3 ± 6.6 J/(mol•K) for 46SiO 2 and the natural Mt. Hamilton and Racalmuto samples give S° = 2805.7 ± 8.4 J/(mol•K) and S° = 2956.8 ± 8.9 J/(mol•K), respectively. The entropy and Gibbs free energy for molecule-free melanophlogite relative to quartz at 298 K are ∆S trans = 6.7 J/(mol•K) and ∆G trans = 7.5 kJ/mol, respectively and, thus, it does not have a thermodynamic field of stability in the SiO 2 system. The difference in C P values between molecule-containing and molecule-free melanophlogite is characterized by an increase in C P from 0 to ~70 K, and it then reaches a roughly constant value at 70 K < T < 250 K. The ∆S rxn at 298 K for 46SiO 2 (melan

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Author

C. A. Geiger; E. Dachs; M. Nagashima

Publisher

Mineralogical Society of America

Published

2008

Language

EN

Field

Earth and Planetary Sciences (Physical Sciences)