Stability and Occurrence of the Molecule-Containing SiO2 Clathrate Melanophlogite: Metastable Crystallization from a Colloid or Gel?

The mineral melanophlogite is the only known natural SiO2 clathrate. It has been found in a number of localities worldwide in different low-temperature geologic environments. Melanophlogite's thermodynamic stability is not known. Low-temperature hydrothermal laboratory experiments indicate that structure-directing agents and colloid formation are needed for crystallization. The formation of silica-rich colloids/gels and following crystal growth can be observed in glass-ampoule synthesis experiments. In order to better address these issues, the heat capacities of two different molecule-containing melanophlogites of approximate composition 46SiO2·1.80CH4·3.54N2·1.02CO2 from Mt. Hamilton, CA and 46SiO2·3.59CH4·3.10N2·1.31CO2 from Racalmuto, Sicily, along with a heated (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 a natural Racalmuto sample at 1173 K for 24 hr. It has a slightly larger low-temperature heat capacity and standard third-law entropy compared to other low-density SiO2 polymorphs such as various zeosils. The standard third-law entropy of the molecule-free sample is S° = 2216.3 J/(mol·K) for 46SiO2 and the natural Mt. Hamilton and Racalmuto samples give S° = 2805.7 J/(mol·K) and S° = 2956.8 J/(mol· K), respectively. The entropy and Gibbs free energy for molecule-free melanophlogite relative to quartz at 298 K are Δ Strans = 6.7 J/(mol·K) and Δ 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 K to approximately 70 K and then reaches a roughly constant value at 70 K < T < 250 K. The Δ 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 J/(mol·K) 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 and associated phases such as cristobalite and chalcedony, suggest that melanophlogite crystallizes metastabily from gels. The occurrence of melanophlogite, and the lack of other SiO2clathrates/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 clathrate porosils requires more chemically and structurally complex molecules that are not naturally abundant.