Abstract: Clinotobermorite, Ca₅Si₆(O,OH)₁₈·5H₂O, has been found as a vein-forming mineral in gehlenite- spurrite skarns at Fuka, Okayama Prefecture. It is associated with tobermorite, plombierite, apophyllite, and calcite. The clinotobermorite is colourless or white and occurs as tabular or acicular crystals. It is monoclinic with the space group Cc or C2/c. The unit cell dimensions are a 11.331, b 7.353, c 22.67 Å, β 96.59° Microtwinning and stacking disorder on (001) are observed. On heating the clinotobermorite at 300°C the 002 spacing is reduced from 11.3 to 9.3 Å. Its refractive indices are α 1.575, β 1.580, γ 1.585, and the density 2.58 g/cm³ (meas.), 2.69 g/cm³ (calc). The Moh's hardness is 4.5. Calculation of the analytical data on the basis of six tetrahedral cations shows that this mineral has a simplified chemical formula Ca_5.3Si₆(O,OH,F)₁₈·5H₂O. The chemical composition and the unit cell are closely related to those of tobermorite. It is most likely that clinotobermorite is a low-temperature polymorph of tobermorite.