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American Mineralogist; July 2008; v. 93; no. 7; p. 1153-1157; DOI: 10.2138/am.2008.2689
© 2008 Mineralogical Society of America
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Orthorhombic polymorph of rengeite from Ohmi region, central Japan

Hiroki Mashima1, Junji Akai2,*, Yoshihiro Nakamuta3 and Satoshi Matsubara4

1 Graduate School of Science and Technology, Niigata University, 8050, Ikarashi Nino-cho, Nishi-ku, Niigata-si, Niigata, 950-2181, Japan
2 Niigata University, 8050, Ikarashi Nino-cho, Nishi-ku, Niigata-si, Niigata, 950-2181, Japan
3 Kyusyu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka-si, Fukuoka, 812-8581, Japan
4 National Museum of Nature and Science, 3-23-1, Hyakunin-cho, Shinjuku, Tokyo, 169-0073, Japan

Correspondence: * E-mail: akai{at}sc.niigata-u.ac.jp

Rengeite, Sr4ZrTi4(Si2O7)2O8, is a Sr-analog mineral of perrierite and occurs as an accessory mineral in natrolite, pectolite, and itoigawaite veins associated with jadeitic rock from Ohmi-Itoigawa region, Japan. The symmetry of this mineral is reported to be monoclinic. Here, we found an orthorhombic polymorph of rengeite that occurs as micro domains up to 5 µm in width in monoclinic rengeite crystals using electron diffraction and high-resolution TEM analyses. X-ray diffraction analysis using a Gandolfi camera also revealed the presence of the orthorhombic phase in monoclinic rengeite crystals. The unit-cell dimensions of the orthorhombic polymorph are a = 14.0, b = 5.7, c = 21.9 Å, V = 1748 Å3, and Z = 4. Although the space group of orthorhombic polymorph of rengeite has not been determined, the possible space group is Pbca or a similar one from theoretical consideration by Ito (1950). The chemical composition was determined by ATEM-EDS analysis to be SiO2 = 22.5, TiO2 = 30.2, SrO = 39.4, Nb2O5 = 0.2, Fe2O3 = 0.3, and ZrO2 = 7.5, totaling 100.1 wt%, and the ideal chemical formula is Sr4ZrTi4(Si2O7)2O8. Detailed HRTEM imaging revealed that the unit cell of the orthorhombic polymorph of rengeite can be interpreted as a superstructure of monoclinic rengeite, accompanied by repeated twinning on (001) plane. The HRTEM images taken were consistent with the simulated images.

Key Words: Rengeite • perrierite • polymorph • jadeite • HRTEM






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