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

Abstract

The results of a new single-crystal structure refinement and of a Raman spectroscopy investigation on melanophlogite, a clathrate structure of SiO 2 , 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 CH 4 is present in the clathrate structure. During a comparative investigation of melanophlogite from different geological setting (Racalmuto, Sicily, Italy), H 2 S also was found, together with CH 4 , 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, CH 4 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 CH 4 flux through the porous material.