Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

S (Solid Earth Sciences) » S-MP Mineralogy & Petrology

[S-MP38] Physics and Chemistry of Minerals

Thu. May 24, 2018 9:00 AM - 10:30 AM A04 (Tokyo Bay Makuhari Hall)

convener:Hiroaki Ohfuji(Geodynamics Research Center, Ehime University), Seiji Kamada(Frontier Research Institute for Interdisciplinary Sciences, Tohoku University), Chairperson:Kayama Masahiro(東北大学)

10:00 AM - 10:15 AM

[SMP38-05] Single-crystal X-ray diffraction study of melanophlogite from Sakhalin, Far East Russia

*Koichi Momma1, Toshiro Nagase2, Robert Jenkins3, Yusuke Miyajima4, Kenichiro Tani1, Akira Ijiri6, Sergei Kasatkin5, Igor Chekryzhov5, Ritsuro Miyawaki1 (1.National Museum of Nature and Science, 2.Tohoku University Museum, 3.Kanazawa University, 4.Kyoto University, 5.Far East Geological Institute, Russian Academy of Sciences , 6.JAMSTEC)

Keywords:melanophlogite, silica clathrate, superstructure

Melanophlogite is one of silica clathrate minerals, which contain various gas molecules in their cage-like framework structures.

We recently discovered new localities of melanophlogite in the South Sakhalin, Far East Russia. In Nevelsk, melanophlogite occurs in calcareous concretion of the Miocene mudstone in association with autochthonous fossils of chemosynthetic fauna such as Conchocele sp. and Acharax sp. Associate minerals are opal and chalcedony, and melanophlogite occurs as aggregates of cubic crystals of sizes up to 0.1 mm.

In “Zamiraylova head” cape, melanophlogite occurs in hydrothermal veins filling fissures of a small basaltic volcanic body. The vein consists of microcrystalline quartz, opal, aragonite, calcite, and pyrite. A lot of cubic pseudomorphs of chalcedony after melanophlogite occur in voids of these veins, while unaltered melanophlogite crystals were only found at the center of thick vein. The same kind of hydrothermal veins are also distributed in a basaltic volcanic body in Kuznetsova cape.

Presence of methane in the samples from “Zamiraylova head” cpae was confirmed by C-H stretching mode of the Raman spectra at ~2900 cm-1. Single-crystal X-ray diffraction experiment at room temperature (23°C) revealed that its crystal structure is consistent with previously reported model, i.e., three-fold twinning of the pseudo-cubic tetragonal structure with space group P42/nbc (#133). At 100°C, all reflections arising from 2x2x1 super-cell disappeared and the structure could be refined using cubic Pm-3n (#223) space group. When it was cooled down to 70°C, additional reflections reappeared at positions slightly shifted from those expected for 2x2x1 super-cell, indicating a possible presence of long-range order or incommensurate phase between alpha-beta transition of melanophlogite.