Magnesium Silicates

The heating curve of magnesia and precipitated silica shows no arrest between 500° C. and 1450° C. Ebelmen obtained Mg₂SiO₄ and MgSiO₃ by heating magnesia and boric acid with appropriate proportions of silica. The artificial crystals of the former were identical with forsterite from Vesuvius, and the latter was obtained both as asbestos-like fibres and as rhombic crystals of enstatite. Enstatite has also been prepared by heating silica with magnesium chloride, and, together with crystals of the orthosilicate, Mg₂SiO₄ (olivine), by heating silica with magnesia and magnesium chloride.6 Enstatite and olivine, structurally similar to the meteoric varieties, were obtained together by heating to redness magnesium, steam, and silicon chloride.

Alkaline silicates precipitate gelatinous magnesium silicate from magnesium salts; the precipitate is apparently MgO.2SiO₂.2H₂O.

The natural magnesium silicates have an alkaline reaction. The orthosilicate, (Mg,Fe)₂SiO₄, occurs as olivine or peridotite in rhombic crystals or compact masses. Its hardness is 6.7 and its density 3.1-3.5. Forsterite, Mg₂SiO₄, has a hardness of 6.7 and a density of about 3.24. The metasilicate, MgSiO₃, occurs as enstatite in fibrous or lamellated masses with a density of 3.19 and a hardness of 5.5. Talc and steatite, Mg₃H₂(SiO₃)₄, are an acid salt of metasilicic acid. When pure they are white, unctuous powders: steatite being less distinctly crystalline than talc. They are used for polishing and other purposes. Serpentine can be represented as a basic orthosilicate, Mg₂(MgHO)H₃(SiO₄)₂. The serpentines vary in colour - green or red (due to iron) predominating. The natural masses of serpentine are much sought after for making ornaments. Meerschaum, 2MgO.3SiO₂.2H₂O, occurs in compact masses which are white or grey. Its density is 0.99-1.28 and its hardness is 2-2.5. Tobacco-pipes, cigarette-holders, etc., are made from it.