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Single-crystal X-ray and Raman investigation on melanophlogite from Varano Marchesi (Parma, Italy)

¹ Dipartimento di Scienze della Terra, Università di Parma, Viale Usberti, 157/A, I-43100, Parma, Italy
² Dipartimento di Fisica, Università di Parma, Viale Usberti, 7/A, I-43100, Parma, Italy
³ Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Università di Parma, Viale Usberti, 17/A, I-43100, Parma, Italy

Correspondence: ^* E-mail: mario.tribaudino{at}unipr.it

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 Pmn symmetry of β-melanophlogite [a = 13.399(2), R₄ = 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.

Key Words: Melanophlogite • single-crystal X-ray diffraction • Raman spectroscopy • phase transition • CH₄

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