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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.

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