Zinc Silicates

The orthosilicate, Zn₂SiO₄, occurs as willemite in hexagonal crystals of density 3.89-4.18 and hardness 5.5. In troostite the zinc is partly replaced by manganese. Hydrated or basic zinc silicates also occur as minerals. Calamine or hemimorjphite is often formulated as (Zn.OH)₂SiO₃; it crystallises with a density of 3.45 and a hardness of 4.5-5 in the orthorhombic system.

A solution of sodium silicate, Na₂SiO₃, precipitates an amorphous zinc silicate from a solution of zinc sulphate; it is liable to contamination by zinc hydroxide if the sodium silicate solution is excessively alkaline. Small, clear rhombic crystals of ZnSiO₃ were obtained by igniting the dried precipitate with boric acid and lixiviating the residue with water. They were insoluble in acids and isomorphous with en- statite, MgSiO₂. Ebelmen obtained two mixed crystalline products by heating a mixture of silica and zinc oxide.

Artificial willemite has been prepared by the action of silicon fluoride on zinc oxide, or of zinc fluoride on silica at a white heat. When zinc sulphate was heated with an alkali sulphate and hydrated silica, and the fused mass treated with water, the residue contained birefractive crystals of Zn₂SiO₄ in the form of hexagonal prisms of density 4.25 and hardness 5.5.

The heats of formation of Zn₂SiO₄ and ZnSiO₃ have been given as -23.74 and +2.49 Cal. respectively.