Monoclinic tobermorite from Fuka, Bitchu-cho, Okayama prefecture, Japan.

Unusual monoclinic 11 Å tobermorite was found along with usual orthorhombic 11 Å tobermorite. Monoclinic tobermorite occurs in gehlenite-spurrite skarns at Fuka, Okayama Prefecture. It occurs as a vein-forming mineral in contaminated rock (hybrid) and in retrograde garnet. It is associated with orthorhombic 11 Å tobermorite, 14 Å tobermorite, calcite and appophyllite. It is found as colorless or white crystals up to 3mm. The crystals are monoclinic, space group Cc or C2/c, with the unit cell dimensions: a 11.331, b 7.353, c 22.67 Å, β 96.59°. Microtwinning on (011) is always observed. The 002 spacing is reduced to 9.3 Å by dehydration at 300°C . Refractive indices are α 1.575, β 1.580, γ 1.585. Chemical analyses give a formula Ca5.3Si6(O, OH, F)18•5.1H2O. The chemical composition and the unit cell are closely related to those of orthorhombic tobermorite. Monoclinic tobermorite may be a low-temperature form of orthorhombic tobermorite.