Orthorhombic polymorph of rengeite from Ohmi region, central Japan

Abstract

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 A, V = 1748 A3, 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.