Summary: This hydrothermal process has been studied using single crystal X-ray methods. A crystal of tobermorite (Ca5Si6O22H10), treated with water at 380° C. and 250 atmospheres, gives a preferred orientation aggregate ofxonotlite (Ca6Si6O17(OH)2), together probably with unoriented silica. The relationship of orientations of product and starting material was established. The change occurs without intermediate dehydration of the tobermorite to the 9·35Å. state. The structural basis of the relationship is discussed, and it is compared with those already reported for the transformations of tobermorite into β-CaSiO2 and of xonotlite into β-CaSiO3. A simple experimental procedure for the single-crystal study of hydrothermal reactions is described.