The thermal dehydration of hemimorphite (Zn4Si2O7(OH)2·H2O) has been studied by single-crystal x-ray methods. Loss of the molecular water below 550° C. causes only slight change in the rest of the structure. At about 700° C., the hydroxyl water is lost, and there is an oriented conversion to β-Zn2SiO4; below 960° C. there is a second oriented conversion to α-Zn2SiO4 or willemite. γ-Zn2SiO4 does not appear to be formed. Both orientation relationships have been established and the unit cell of β-Zn2SiO4 determined. The structure of β-Zn2SiO4 is probably derived from a distorted tridymite- or cristobalite-like framework in which half of the silicon is replaced by zinc and additional atoms of zinc introduced into suitable interstices. Possible mechanisms for the two oriented conversions are discussed, and are compared with those known or believed to occur in similar reactions of silicates containing other cations.