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American Mineralogist; July 2008; v. 93; no. 7; p. 1179-1182; DOI: 10.2138/am.2008.2969
© 2008 Mineralogical Society of America
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Letter

Heat capacity and entropy of melanophlogite: Molecule-containing porosils in nature

Charles A. Geiger1,*, Edgar Dachs2 and Mariko Nagashima1

1 Institut für Geowissenschaften, Abteilung Mineralogie, Christian-Albrechts-Universität zu Kiel, D-24098 Kiel, Germany
2 Fachbereich Materialforschung und Physik, Abteilung Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria

Correspondence: * E-mail: chg{at}min.uni-kiel.de

The heat capacities of two different molecule-containing melanophlogites of approximate composition 46SiO2 · 1.80CH4 · 3.54N2 · 1.02CO2 from Mt. Hamilton, California, and 46SiO2 · 3.59CH4 · 3.10N2 · 1.31CO2 from Racalmuto, Sicily, along with a heat-treated (molecule-free) sample of composition SiO2, 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 46SiO2 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 {Delta} Strans = 6.7 J/(mol · K) and {Delta}Gtrans = 7.5 kJ/mol, respectively and, thus, it does not have a thermodynamic field of stability in the SiO2 system. The difference in CP values between molecule-containing and molecule-free melanophlogite is characterized by an increase in CP from 0 to ~70 K, and it then reaches a roughly constant value at 70 K < T < 250 K. The {Delta}Srxn at 298 K for 46SiO2(melan.) + xCH4(gas) + yCO2(gas) + zN2(gas) = 46SiO2 · (xCH4)12 · (yCO2, zN2)14 is estimated to be about –642 and –802 J/(mol · K) for the Mt. Hamilton and Racalmuto samples, respectively. The thermodynamic data, as well as published results on the occurrence of natural molecule-containing samples suggest that melanophlogite crystallizes metastabily. The occurrence of melanophlogite and the lack of other porosils in nature are probably due to the essential role of molecular structure-directing agents. For melanophlogite they can be CO2, N2, and CH4, whereas the crystallization of other porosils requires more chemically and structurally complex molecules that are not naturally abundant.

Key Words: Melanophlogite • heat capacity • entropy • clathrasils • microporous minerals • clathrate






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