Four-layer Orthorhombic Polytype (40) In Strontio-joaquinite Group Minerals Found In The Ohmi Region, Central Japan

摘要

Monoclinic (1M) and orthorhombic (2O) polytypes are known to exist in joaquinite group minerals. Strontio-orthojoaquinite [Sr_2Ba_2(Na,Fe~(2+))_2Ti_2Si_8O_(24)(O,OH)·H_2O], a member of the joaquinite group, has been reported previously in the Ohmi-Itoigawa district, Japan. A new polytype (4O), mainly comprising a 2O polytype, was found in mineral grains by transmission electron microscopy (TEM). This four-layer orthorhombic polytype (4O) was also found in mineral grains with the 1M polytype. The TEM observation revealed that the 4O polytype appeared as interstratified bands having a width of ～8 urn that were embedded parallel to the (001) plane in the 2O crystal grains. The unit cell parameters of the 4O polytype are approximately a = 10.6 A, b = 9.8 A, c = 44.2 A,V= 4591.5 A~3, and Z = 8. High-resolution TEM (HRTEM) imaging showed that a unit cell of the 4O polytype can be interpreted as being identical to that of a superstructure having a repeated twinning of two times the size of the 1M unit cell on the (001) plane. The formation conditions of the polytypes were discussed.