第159回ジオダイナミクスセミナー
　　　Geodynamics Seminar

要　旨
　 Iron sulfide (FeS) has been supposed to be one of the major components of the core of terrestrial planets such as Mars. In the recent decade, in situ X-ray diffraction studies using multi-anvil high pressure apparatus provided some important data on the phase stability relationship of FeS at high pressure and high temperature. The results showed that FeS (toroilite; hexagonal NiAs-type related phase) transforms to FeS II (orthorhombic MnP-type phase) above 3.4 GPa and then to FeS III (monoclinic phase) above 6.7 GPa by room temperature compression. Above room temperature, FeS undergoes two successive phase transitions, from FeS to FeS IV (hexagonal phase) and then to FeS V (NiAs-type phase) with increasing temperature in the pressure range from 0 to 20 GPa. However, no studies have yet been conducted to investigate the phase stability of FeS under higher pressure conditions.

　We studied the phase stability of FeS under high pressure/temperature conditions above 40 GPa and 1500K using laser-heated diamond anvil cell (LHDAC) and found a new polymorphic form of FeS. The structure of the new FeS was examined by in-situ synchrotron X-ray diffraction. The X-ray diffraction peaks of the new phase were reasonably indexed on the basis of orthorhombic cell with space group Pnma and unit cell parameters a = 4.88, b = 3.06 and c = 5.09 at 70 GPa at ambient temperature. The unit cell provides c/b ratio of 1.66, which is apparently smaller than the ratio of NiAs-type hexagonal unit cell (1.73). This indicates that the structure of the new FeS can not be described in the NiAs-type cell dimensions, from which all the known high temperature/pressure FeS phases including an orthorhombic FeS (FeS II) are derived. The new FeS phase was found to be temperature quenchable and remain stable up to at least 120 GPa and 1800K.


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　　　 　　　　　　　　　　　　　　　　　　 E-mail　　takut@sci.ehime-u.ac.jp