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

Geiger, C. A., Dachs, E. and Nagashima, M. (2008) Heat capacity and entropy of melanophlogite: Molecule-containing porosils in nature American Mineralogist, 93 (7). pp. 1179-1182. DOI 10.2138/am.2008.2969.

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The heat capacities of two different molecule-containing melanophlogites of approximate composition 46SiO(2).1.80CH(4)center dot 3.54N(2).1.02CO(2) from Mt. Hamilton, California, and 46SiO(2)center dot 3.59CH(4)center dot 3.10N(2)center dot 1.31CO(2) 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 H 73 K for 24 h. The standard third-law entropy of the molecule-free sample is S degrees = 2216.3 +/- 6.6 J/(mol.K) for 46SiO(2) and the natural Mt. Hamilton and Racalmuto samples give S degrees = 2805.7 +/- 8.4 J/(mol.K) and S degrees = 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 S-trans = 6.7 J/(mol.K) and Delta G(trans) = 7.5 kJ/mol, respectively and, thus, it does not have a thermodynamic field of stability in the SiO2 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 similar to 70 K, and it then reaches a roughly constant value at 70 K < T < 250 K. The Delta S-rxn at 298 K for 46SiO(2)(melan.) + xCH(4)(gas) + yCO(2)(gas) + zN(2)(gas) = 46SiO(2)center dot(xCH(4))(12)center dot(YCO2, zN(2))(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, N-2, and CH4, whereas the crystallization of other porosils requires more chemically and structurally complex molecules that are not naturally abundant.

Document Type: Article
Keywords: melanophlogite heat capacity entropy clathrasils microporous minerals clathrate thermodynamic properties crystal-structure x-ray silica temperature sieves sio2
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.2138/am.2008.2969
ISSN: 0003-004X
Date Deposited: 12 Jan 2012 05:56
Last Modified: 08 Oct 2012 10:14
URI: http://eprints.uni-kiel.de/id/eprint/15839

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