Summary: A new analysis gives the formula Ca₄Si₃O₁₂F₂H₆; the specimen was from Alter Berg, Leitmeritz, Bohemia, and also contained apophyllite. The unit cell is triclinie, but all reflections can be indexed on a geometrically monoclinic cell which is c-face centred and has a 16·2, b 9·34, c 13·2 Å., β 113°, Z = 6, principal cleavage (001). This cell could also be described as having a hexagonal base of side 9·34 Å., but with c tilted through 23° from the vertical in the plane of one of the sides. A dehydration curve showed a step at 300·400°C., corresponding to loss of two-thirds of the water. The remaining water is lost between 400° and 940° C., together with some fluorine, the latter probably as HF. Crystals heated at 450–600° C. retain the hexagonal-based cell, but d₀₀₁ shrinks from 12·1 to 11·1 Å, and there is evidence of much stacking disorder of layers normal to c. Single crystals heated at 700° C. show ordered transformation, giving cuspidine twinned in three orientations as the only recognizable product. At 940°C. a mixture is obtained, mainly β-CaSiO₃ with some β-Ca₂SiO₄.