Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Abstract Figures Only Full Text (PDF) Supplementary Data Info Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Geiger, C. A. Articles by Nagashima, M. GeoRef GeoRef Citation

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

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

	ABSTRACT

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 
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 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 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 ~70 K, and it then reaches a roughly constant value at 70 K < T < 250 K. The S^rxn 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

	INTRODUCTION

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 
The SiO₂ group of phases shows great variation in crystal structure in both nature and in the laboratory. It includes several low-density SiO₂ porosils, which are further divided into the clathrasils and zeosils (Liebau 1988). The mineral melanophlogite is the only known natural molecule-containing porosil/clath-rasil. It was first described by Lasaulx (1876) in Sicilian sulfur deposits, but has since been found in other localities worldwide in various low-temperature geologic environments (Zák 1972; Cooper and Dunning 1972; Grasselini Troysi and Orlandi 1972; Kropatsheva and Makarov 1975; Dunning and Cooper 2002; Tribaudino et al. 2008). Melanophlogite has the ideal formula 46SiO₂ · 2M12 · 6M14, where the superscripts are the coordination numbers of the guest molecules M.

Melanophlogite, whose high-temperature crystal-structure was refined by Gies (1983), Nakagawa et al. (2005), and Tribaudino et al. (2008) and low-temperature modification by Nakagawa et al. (2001, 2005), has a microporous framework built-up of corner-sharing SiO₄ tetrahedra. It is isostructural with gas hydrate I (Kamb 1965; Zák 1972). Both structures are classified as clathrates and they contain two different polyhedral cages (Fig. 1¹), labeled by the notation [5¹²] and [5¹²6²], where the superscripts give the number of pentagonal and hexagonal faces making up the cage polyhedron. The cages in melanophlogite can enclathrate simple molecular guest species such as CH₄, N₂, H₂S, and CO₂ but not H₂O (Gies et al. 1982; Kortus et al. 2000; Kolesov and Geiger 2003; Tribaudino et al. 2008) and is, thus, hydrophobic. Their exact role in the crystallization behavior and the geochemical conditions under which melanophlogite forms in nature have not been fully addressed.

The phase relations for both guest-containing and guest-free melanophlogite are unknown. The enthalpy for guest-free melanophlogite relative to quartz at 298 K was determined (Navrotsky et al. 2003). They discussed the enthalpy systematics for melanophlogite, as well as for several low-density SiO₂ polymorphs such as the zeosils. All studied molecule-free porosils are enthalpically unstable compared to quartz (Navrotsky et al. 2003). However, there are no data on the heat capacity of melanophlogite and its standard third-law entropy. This prohibits an assessment of its thermodynamic stability via its Gibbs free energy and, thus, a better analysis regarding its occurrence in nature. To do this, we have measured the heat capacity of two natural molecule-containing melanophlogites and a degassed sample containing no molecules. Finally, we discuss the occurrence of melanophlogite, as well as possible reasons for the absence of other clathrasils and zeosils in nature.

	EXPERIMENTAL METHODS

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 
Samples
The melanophlogite crystals that were investigated are from Mt. Hamilton, California, and Racalmuto, Sicily (Smithsonian Institute label NMNH C1279). The first occurrence is discussed in Cooper and Dunning (1972) and Dunning and Cooper (2002). Gies (1983) determined an average cage occupation of 90% CH₄ in the small [5¹²] cage and 59% N₂ and 17% CO₂ in the large [5¹²6²] cage from an X-ray study of a crystal from Mt. Hamilton. Kolesov and Geiger (2003) argued from their Raman spectra that CH₄ and CO₂ molecules in these crystals are found in both cages and that they could be compositionally inhomogeneous and zoned with respect to the various guest species, as shown first by Gies et al. (1982) using mass spectrometry. The occurrence of the Racalmuto crystals is described in Lasaulx (1876) and Skinner and Appleman (1963). For it, Gies et al. (1982) presented a composition of 46SiO₂ · 3.59CH₄ · 3.10N₂ · 1.31CO₂ assuming full cage occupancies. H₂S may also occur in some Racalmuto crystals (Tribaudino et al. 2008).

For our C_P study, small melanophlogite crystals were handpicked and separated under a binocular microscope from their underlying matrix. X-ray diffraction, both powder and single crystal, showed that much of the Mt. Hamilton sample consists of quartz, even for crystals with a cubic morphology, as described by Dunning and Cooper (2002). For the C_P measurements "pure" melanophlogite samples, based on slow-scan powder X-ray patterns, were obtained and gently ground to a powder.

Following completion of the C_P measurements on both natural samples, the Mt. Hamilton sample was heated at 50 K/h to 1223 K and held for 10 h. The Racalmuto sample was heated at 50 K/h to 1173 K and held there for 24 h. This was done using a four-zone tube furnace with a temperature control of T 1 K, which was further checked by use of an external Ni-NiCr thermocouple. Gies et al. (1982) and Navrotsky et al. (2003) note that these temperatures should allow for the degassing of melanophlogite, but should not cause recrystallization to another phase. Skinner and Appleman (1963) observed a breakdown to cristobalite at T 1173 K, but not at 1073 K. Slow-scan powder X-ray measurements for 20 h between 10 and 60° 2 were made on both heat-treated samples. Both heated samples were studied further by heat-pulse calorimetry.

Low-temperature calorimetry
Low-temperature heat capacities were measured with a commercially designed heat-pulse calorimeter [i.e., heat capacity option of the Physical Properties Measurement System (PPMS), produced by Quantum Design]. The measurements were performed at temperatures between 5 and 300 K on small milligram-sized samples (about 13 to 15 mg) sealed in an Al-container and covered by a lid. C_P values below 5 K were estimated by a linear extrapolation to 0 K in a plot of C_P/T vs. T². The C_P data were collected at 60 different temperatures on cooling with a logarithmic or linear spacing, with C_P determined three times at each temperature. Details behind the heat-pulse calorimetric method are given in Dachs and Bertoldi (2005) and Dachs and Geiger (2006).

	RESULTS

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 
The heat-treated samples were dark in color as reported by Zák (1972). The X-ray pattern of the heat-treated Mt. Hamilton melanophlogite showed substantial amounts of cristobalite. That of the Racalmuto sample showed a small amount (less than a couple of percent) of what appears to be a synthetic SiO₂ zeosil phase or possibly cristobalite. No reflections could be indexed to graphite in either sample. Zák (1972) reports in his study of a heat-treated melanophlogite that no graphite could be observed at 1600x magnification under the microscope. The powder IR spectrum of the Mt. Hamilton sample showed no measurable amounts of CH₄ or CO₂.

The raw experimental C_P data for the two natural melanophlogites of approximate composition 46SiO₂ · 1.80CH₄ · 3.54N₂ · 1.02CO₂ and 46SiO₂ · 3.59CH₄ · 3.10N₂ · 1.31CO₂ and the degassed samples of composition 46SiO₂ are given in Table 1a¹. No correction for the presence of the minor SiO₂ phase in the heat-treated Racalmuto sample or for cristobalite in the Mt. Hamilton sample was made. The C_P data were fitted to polynomials of the general form C_P = k_o + k₁T^–0.5 + k₂T^–2 + k₃T^–3 + k₄T + k₅T² + k₆T³. For this purpose, a data set was divided into three temperature regions, whereby each was fit individually, but such that a certain overlap of data were present. From this, smoothed molar thermodynamic functions for all phases were obtained (Tables 1b, 1c, and 1d¹). Their heat capacity behavior is shown in Figure 2. The Racalmuto sample probably shows the beginning of a phase transition at T 280 K with increasing temperature, while the Mt. Hamilton sample and the heat-treated melanophlogites show no such behavior. The standard third-law entropy, S°, was determined by analytically solving the integral:

View larger version (13K): [in this window] [in a new window]   FIGURE 2. Heat capacity of natural guest-containing and heat-treated melanophlogite from Mt. Hamilton, California (filled and open squares) and Racalmuto, Sicily (filled and open circles) from 0 to 300 K using smoothed data. The cross and plus symbols show the difference in CP between the natural Mt. Hamilton and Racalmuto samples and their heat-treated analogs, respectively.

(1)

The uncertainty in S° was calculated as described in Dachs and Geiger (2006) and the entropy at T = 0 K was assumed to be zero. We obtained third-law entropies values S° = 2805.7 ± 8.4 J/(mol · K) and the S° = 2956.8 ± 8.9 J/(mol · K) for the natural Mt. Hamilton and Racalmuto melanophlogites, respectively, and S° = 2216.3 ± 6.6 J/(mol · K) for the guest-free Racalmuto sample (the S° value for the heated Mt. Hamilton sample cannot be interpreted, due the presence of substantial cristobalite, and will not be considered further).

	DISCUSSION

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 
Crystal-chemical and thermodynamic properties
Melanophlogite is tetragonal with space group P4₂/nbc at room temperature (Zák 1972) and it undergoes a phase transition around 338 K to a cubic phase of space group Pm. This depends on the composition of the enclathrated guest molecules in both natural and synthetic samples (Gies 1983). The molecules, which are partitioned between the two cages (Fig. 1¹) in manner that is not precisely understood, are quasi free and show weak interactions with the neutral SiO₂ framework or with each other, as shown by single-crystal Raman spectroscopy (Kolesov and Geiger 2003). In addition to melanophlogite, other clathrasils have been synthesized in the laboratory (Liebau 1988; Higgins 1994; Gies et al. 1998). We are not aware of any C_P studies that have been made on any clathrasil and, thus, their P-T relations are not known.

We begin our analysis of the C_P data of molecule-free melanophlogite with a comparison to the C_P behavior of various SiO₂ phases, especially the structurally related zeosils (Liebau 1988; Higgins 1994; Gies et al. 1998). The latter SiO₂ phases are, unlike clathrasils, more related to the zeolite group of phases, because they have infinite structural channels in which molecules can diffuse. In terms of C_P behavior, molecule-free melanophlogite has a slightly larger low-temperature heat capacity (Fig. 3) than other low-density SiO₂ phases (Table 2¹). Indeed, its C_P between 0 and 240 K is the largest of all the SiO₂ phases plotted in Figure 3. Thus, clathrasil phases may behave in a similar and unusual fashion as zeosils (Boerio-Goates et al. 2002), because both have higher C_P values in the low-temperature region (excepting zeolite beta) compared to silica glass.

View larger version (15K): [in this window] [in a new window]   FIGURE 3. Heat capacities for various SiO2 phases (all are guest free) relative to quartz from 0 to 300 K, i.e., CP = CP[SiO2 (phase) – CP (quartz)]. The data for quartz, cristobalite (open circles) and silica glass (squares with crosses) are from Westrum (1954), for the zeosil ZSM-23 (open squares), silica faujasite (inverted triangle) and zeolite beta (normal triangle) from Boerio-Goates et al. (2002), silicalite (ZSM-5 = diamonds) from Johnson et al. (1987) and melanophlogite (solid squares) from this study.

The C_P and S° values of guest-containing melanophlogites from Mt. Hamilton and Racalmuto are larger than its guest-free counterpart (Fig. 2), obviously reflecting the presence of the enclathrated CH₄, N₂, and CO₂ molecules. In terms of enclathration energetics, the thermodynamic properties for the reaction:

(2)

need to be determined. The heat capacity and entropy of reaction, C_P^rxn and S^rxn, at 298 K can be calculated using results from this work. We stress, though, that a precise determination is difficult, because the concentrations and distributions of the various molecules in both natural melanophlogites are not known quantitatively (i.e., crystals are sometimes inhomogeneous in composition and partial cage occupancies can occur). With respect to cage occupancies, those for the Mt. Hamilton sample have been determined by Gies (1983), whereas for the Racalmuto sample full cage occupancies were assumed (Gies et al. 1982). With this caveat, using the heat capacity and entropy values for the natural samples and the degassed melanophlogite, along with thermodynamic properties for ideal gaseous CH₄, N₂, and CO₂ (Chase 1998), we obtain C_P^rxn = 80.1 J/(mol · K) and S^rxn = –642.2 J/(mol · K) for the Mt. Hamilton sample and C_P^rxn = 202.2 J/(mol · K) and S^rxn = –802.2 J/(mol · K) for the Racalmuto sample at 298 K and 1 bar. Measurements to determine H^rxn are needed to obtain G^rxn. In addition, any configurational entropy contribution needs to be determined and this will require further spectroscopic study of the enclathrated molecules.

Kolesov and Geiger (2003) showed that the internal vibrations of the enclathrated CO₂, N₂, and CH₄ molecules in melanophlogite are similar in wave number to those that these molecules posses in the free state. This implies that the molecules have only weak interactions with the SiO₂ framework. In terms of P-T stability of molecule-containing melanophlogite, the enthalpies of mixing will have to be negative to counteract the entropy decrease associated with enclathration energetics (Eq. 2). Figure 2 shows the difference in C_P values between molecule-containing and molecule-free melanophlogites. Here, the difference in C_P values rise from 0 to ~70 K and then reach a roughly constant value at 70 < T < 250 K. C_P behavior at T < 70 K will be strongly controlled by the nature of the lowest energy external librational and translational modes of the CO₂, N₂, and CH₄ molecules. They have yet to be determined (cf. the case for quasi-free molecular H₂O in microporous Mg-cordierite, Paukov et al. 2007).

Occurrence of melanophlogite in nature
Melanophlogite is the only porosil that has been identified on Earth, although others can be synthesized in the laboratory (Liebau 1988; Higgins 1994; Gies et al. 1998). Little mineralogical and geochemical research has been undertaken in this area. Gies et al. (1998) discuss the crystallization behavior of various porosils in the laboratory and their analysis is useful in understanding possible natural processes. Of central importance is the role of molecular structure-directing agents (SDAs) in porosil formation.

Melanophlogite often occurs with cristobalite and chalcedony or opal in low-temperature geologic environments (Zák 1972; Cooper and Dunning 1972; Tribaudino et al. 2008). Cooper and Dunning (1972, p. 1496) state that at Mt. Hamilton there were "interchanging and overlapping periods of crystallization of cristobalite and melanophlogite." This is consistent with the observation that cristobalite crystallizes from aqueous solution before melanophlogite in synthesis experiments (Gies et al. 1982). At Racalmuto, melanophlogite occurs with opal and quartz and is often nucleated around these phases (Skinner and Appleman 1963). A thermodynamic analysis and the field observations are consistent and suggest a metastable crystallization process. However, melanophlogite may crystallize in nature more often than realized, because it remains unrecognized or is later pseudomorphed by other SiO₂ phases.

Except for melanophlogite, no other clathrasils or zeosils have been found in nature. It appears, at least for molecular-free species, that their thermodynamic stabilities are similar. Gies et al. (1998) note that the formation of synthetic porosils is a kinetically controlled process and write, "spherical molecules favor cage-like voids, whereas chain-like molecules lead to 1-D channels." Thus, the simplest explanation for the occurrence of guest-containing melanophlogite and the absence of other porosils in nature is related to bulk composition, i.e., the presence or absence of SDAs. Gies et al. (1982, 1998) showed that the presence of various molecules other than CH₄, N₂, and CO₂, such as aliphatic and cyclic amines, is a prerequisite for the crystallization of most clathrasils. Many of these molecules are either not found in nature or only occur in small concentrations. Melanophlogite seems to be different and perhaps unique as a natural clathrasil (if another clathrasil will be found in nature, it would probably be dodecasil 3C), because it can crystallize in the presence of common naturally occurring molecular SDAs such as CO₂. As with most clathrasils, the case for synthetic zeosils is similar. Their formation requires even larger and naturally uncommon SDAs (see Gies et al. 1998). The zeolite, mutinaite, of composition (Na_2.76K_0.11Mg_0.21Ca_3.78)_6.86(Al_11.20Si_84.91)_96.11 O₁₉₂. · 60H₂O, is a natural counterpart of the zeosil ZSM-5 and it has been found at one locality (Galli et al. 1997). In its case, H₂O molecules and extra framework cations are present and they may act as SDAs and allow crystallization as described by Breck (1974).

	ACKNOWLEDGMENTS

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 
We thank G. Dunning and J. Post, and P.W. Pohwat and P. Dunn of the Smithsonian Institute in Washington for donating melanophlogite samples from Mt. Hamilton and Racalmuto (catalog number NMNH C1279), respectively, for study. We also thank F. Liebau and H. Gies for helpful discussions on porosils and A. Hofmeister and A. Navrotsky for their constructive reviews of the manuscript.

	Footnotes

 
MANUSCRIPT HANDLED BY BRYAN CHAKOUMAKOS

¹ Deposit item AM-08-036, Figure 1, Tables 1a, 1b, 1c, and 1d (raw experimental C_P data and thermodynamic functions) and Table 2 (thermodynamic properties for various SiO₂ phases). Deposit items are available two ways: For a paper copy contact the Business Office of the Mineralogical Society of America (see inside front cover of recent issue) for price information. For an electronic copy visit the MSA web site at http://www.minsocam.org, go to the American Mineralogist Contents, find the table of contents for the specific volume/issue wanted, and then click on the deposit link there.

MANUSCRIPT RECEIVED March 3, 2008; MANUSCRIPT ACCEPTED March 28, 2008

	REFERENCES CITED

Top Abstract Introduction Experimental methods Results Discussion Acknowledgments References cited

 

Boerio-Goates, J., Stevens, R., Hom, B.K., Woodfield, B.F., Piccione, P.M., Davies, M.E., and Navrotsky, A. (2002) Heat capacities, third-law entropies and thermodynamic functions of SiO₂ molecular sieves from T = 0 to 400 K. Journal of Chemical Thermodynamics, 34, 205–227.[CrossRef][ISI]

Breck, D.W. (1974) Zeolite Molecultar Sieves, 771 p. John Wiley and Sons, New York.

Chase, Jr., M.W. (1998) NIST-JANAF Thermochemical Tables, 4th edition. Journal of Physical and Chemical Reference Data, Monograph No. 9, Parts I and II.

Cooper, Jr., J.F. and Dunning, G.E. (1972) Melanophlogite from Mount Hamilton, Santa Clara County, California. American Mineralogist, 57, 1494–1504.[ISI][GeoRef]

Dachs, E. and Bertoldi, C. (2005) Precision and accuracy of the heat-pulse calorimetric technique: Low-temperature heat capacities of milligram-sized synthetic mineral samples. European Journal of Mineralogy, 17, 251–261.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]

Dachs, E. and Geiger, C.A. (2006) Heat capacities and entropies of mixing of pyropegrossular (Mg₃Al₂Si₃O₁₂-Ca₃Al₂Si₃O₁₂) garnet solid solutions: A low temperature calorimetric and a thermodynamic investigation. American Mineralogist, 91, 894–906.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]

Dunning, G.E. and Cooper, Jr., J.F. (2002) Pseudomorphic melanophlogites from California. The Mineralogical Record, 33, 237–242.[GeoRef]

Galli, E., Vezzalini, G., Quarteri, S., Alberti, A., and Franzini, M. (1997) Mutinaite, a new zeolite from Antarctica: The natural counterpart of ZSM-5. Zeolites, 19, 318–322.[CrossRef][ISI][GeoRef]

Gies, H. (1983) Studies on clathrasils. III. Crystal structure of melanophlogite, a natural clathrate compound of silica. Zeitschrift fur Kristallographie, 164, 247–257.[ISI][GeoRef]

Gies, H., Gerke, H., and Liebau, F. (1982) Chemical composition and synthesis of melanophlogite, a clathrate compound of silica. Neues Jahrbuch fur Mineralogie, Monatshefte, 3, 119–124.

Gies, H., Marler, B., and Werthmann, U. (1998) Synthesis of porosils: Crystalline nanoporous silicas with cage- and channel-like void structures. Molecular Sieves, 1, p. 35–64. Springer-Verlag, Berlin.

Grasselini Troysi, M. and Orlandi, P. (1972) Sulla melanoflogite del Fortullino (Liverno). Atti della Società Toscana di Scienze Naturali, Memorie, Serie A, 79, 245–250.

Higgins, J.B. (1994) Silica zeolites and clathrasils. In P.J. Heaney, C.T Prewitt, and G.V. Gibbs, Eds., Silica: Physical Behavior, Geochemistry, and Materials Applications, 29, p. 507–543. Reviews in Mineralogy, Mineralogical Society of America, Chantilly, Virginia.

Johnson, G.K., Tasker, I.R., Howell, D.A., and Smith, J.V. (1987) Thermodynamic properties of silicalite SiO₂. Journal of Chemical Thermodynamics, 19, 617–632.[CrossRef][ISI]

Kamb, B. (1965) A clathrate crystalline form of silica. Science, 148, 232–234.[Abstract/Free Full Text][CrossRef][ISI][Medline]

Kolesov, B.A. and Geiger C.A. (2003) Molecules in the SiO₂-clathrate melanophlogite: A single-crystal Raman study. American Mineralogist, 88, 1364–1368.[Abstract/Free Full Text][ISI][GeoRef]

Kortus, J., Irmer, G., Monecke, J., and Pederson, M.R. (2000) Influence of cage structures on the vibrational modes and Raman activity of methane. Modeling and Simulation in Materials Science and Engineering, 8, 403–411.[CrossRef]

Kropatsheva, S.K. and Makarov, J.J. (1975) First occurrence of melanophlogite in the USSR. Doklady Akademii Nauk USSR, 224, 905–908.

Lasaulx, A.V. (1876) Mineralogischkrystallographische Notizen. VII. Melanophlogit, ein neues Mineral. Neues Jahrbuch fur Mineralogie, 1876, 250–257.

Liebau, F. (1988) Synthesis of porous tectosilicates: Parameters controlling the pore geometry. In E.R. Corey, J.y. Corey, and P.P. Gasper, Eds., Silicon Chemistry, p. 307–323. Ellis Horwood, Chichester.

Nakagawa, T., Kihara, K., and Harada, K. (2001) The crystal structure of low melanophlogite. American Mineralogist, 86, 1506–1512.[Abstract/Free Full Text][ISI][GeoRef]

Nakagawa, T., Kihara, K., and Fujinami, S. (2005) X-ray studies of structural changes in melanophlogite with varying temperature. Journal of Mineralogical and Petrological Sciences, 100, 247–259.[CrossRef][ISI][GeoRef]

Navrotsky, A., Xu, H., Moloy, E.C., and Welch, M.D. (2003) Thermochemistry of guest-free melanophlogite. American Mineralogist, 88, 1612–1614.[Abstract/Free Full Text][ISI][GeoRef]

Paukov, I.E., Kovalevskaya, yu.A., Rahmoun, N.-S., and Geiger, C.A. (2007) The heat capacity of hydrous cordierite at low temperatures: The thermodynamic behavior of the H₂O molecule in hydrous micro and nanoporous silicates. American Mineralogist, 92, 388–396.[Abstract/Free Full Text][CrossRef][GeoRef]

Piccione, P.M., Woodfield, B.F., Boerio-Goates, J., Navrotsky, A., and Davies, M.E. (2001) Entropy of pure-silica molecular sieves. Journal of Physical Chemistry B, 105, 6025–6030.

Robie, R.A. and Hemingway, B.S. (1995) Thermodynamic properties of minerals and related substances at 298.15 K and 1 Bar (10⁵ Pascals) pressure and at higher temperatures. U.S. Geological Survey Bulletin 2131, 461 p.

Skinner, B.J. and Appleman, D.E. (1963) Melanophlogite, a cubic polymorph of silica. American Mineralogist, 48, 854–867.[ISI][GeoRef]

Tribaudino, M., Artoni, A., Mavris, C., Bersani, D., Lottici, P.P., and Belletti, D. (2008) Single-crystal X-ray and Raman investigation on melanophlogite from Varano Marchesi (Parma, Italy). American Mineralogist, 93, 88–94.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]

Westrum, Jr., E.F. (1954) Determination of the low-temperature heat capacity of vitreous silicon dioxide, or quartz, and of cristobalite. Unpublished Report for the Owens Illinois Glass Company, 1–26.

Zák, L. (1972) A contribution to the crystal chemistry of melanophlogite. American Mineralogist, 57, 779–796.[ISI][GeoRef]

This Article Abstract Figures Only Full Text (PDF) Supplementary Data Info Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Geiger, C. A. Articles by Nagashima, M. GeoRef GeoRef Citation