Abstract: Tobermorite [Ca4 (Si6O18H2)Ca·4H2O] is a hydrated calcimn silicate mineral with a layer structure which in some respects resembles that of vermiculite. Its dehydration has been studied using single crystals from Ballyeraigy, N. Ireland. The three most frequently encountered hydration states are characterized by basal spacings (d002) of 14.0, 11.3 and 9.35 Å. Dehydration to the 9.35 Å state is complete by 300°C and is accompanied by a stacking change so that the pseudo-cell (a 5.58, b 3.66, c 18.70 Å) becomes A-face centered. The 9.35 Å structure persists up to 700°C, by which temperature all the water has been expelled, and there is some evidence that interlayer Si-O-Si bonds are formed to an increasing extent as the temperature rises.
At about 800°C the 9.35 Å hydrate changes to β-CaSiO3 twinned in two orientations. The b-axis of the 9.35 Å hydrate becomes b for both orientations of the product, and the (201) planes of the latter are formed parallel to the (101) and (101H-̄) planes of the original material. The mechanism of the change is discussed and is compared with some other transformations occurring under similar conditions. An orientation-determining step is suggested in which the principal effect is a migration of silicon atoms or ions, the calcium-oxygen skeleton remaining relatively undisturbed.