Thaumasite From Sulitelma, and from all known occurrences, seems to have been formed at a very low temperature, at a post-zeolitic stage of mineralization. This is in accordance with the fact that the bulk of crystal water is given off between about 95° and 120° at ordinary pressure. A new analysis gives the empirical form ula CaSiO3 • CaCO3 • CaSO4 · 14 1/2 H2O, instead of the generally accepted form ula with 15 H2O. As one water is more firmly held than the other 13 1/2, and as the number of molecules in the unit cell is known to be 2, the formula Ca6Si2C2S2O18(OH)1· 27 H2O is given. The first assumption as to the crystal structure may be a closed "metasilicate" ring-linkage of the Z6O18 type, the mineral being then placed structurally with beryl.