Journal
AMERICAN MINERALOGIST
Volume 93, Issue 1, Pages 88-94Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am.2008.2528
Keywords
melanophlogite; single-crystal X-ray diffraction; Raman spectroscopy; phase transition; CH4
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The results of a new single-crystal structure refinement and of a Raman spectroscopy investigation on melanophlogite, a clathrate structure of SiO2, are reported. The studied sample comes from a new finding at Varano Marchesi (Parma, Italy), and occurs in small veins and pockets along fractures in a siliceous marl from a chaotic complex. Melanophlogite is invariably separated from the host rock by a thin crust of opal-CT. Raman spectroscopy was done to investigate the guest molecules that are hosted in the cages of the structure. In the Varano Marchesi melanophlogite, only CH4 is present in the clathrate structure. During a comparative investigation of melanophlogite from different geological setting (Racalmuto, Sicily, Italy), H,S also was found, together with CH,, in the cavities of the structure. A single-crystal X-ray refinement of the Varano Marchesi sample was done using the Pm (3) over barn symmetry of beta-melanophlogite [a = 13.399(2), R-4 sigma = 4.7%]. According to the site refinement from X-ray diffraction results, CH4 occupies 71 and 91% of the 5(12) and 5(12)6(2) site cavities, respectively. The Varano Marchesi melanophlogite formed as a result of low-temperature hydrothermal activity. The mineral growth occurred at the expense of opal, in connection with CH4 flux through the porous material.
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