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

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 Pm3n symmetry of -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.