• journal_title：American Mineralogist
• Contributor：Mario Tribaudino ; Andrea Artoni ; Christian Mavris ; Danilo Bersani ; Pier Paolo Lottici ; Daniele Belletti
• Publisher：Mineralogical Society of America
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2008.2528
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：93
• issue：1
• firstpage：88
• section：Articles

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), H₂S also was found, together with CH₄, in the cavities of the structure.

A single-crystal X-ray refinement of the Varano Marchesi sample was done using the Pm3̅n symmetry of β-melanophlogite [a = 13.399(2), R_4σ = 4.7%]. According to the site refinement from X-ray diffraction results, CH₄ occupies 71 and 91% of the 5¹² and 5¹²6² 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.