Barium Silicates

Barium silicate occurs in nature under the form of various silico-aluminates, such as harmotome, brewsterite, and barytafelspar.

A barium orthosilicate, Ba₂SiO₄, melting at 1750° C., has been mentioned, but it is probably not stable in contact with water.

Barium metasilicate, BaSiO₃, may be formed by fusing a mixture of barium chloride and carbonate with silica. It may be obtained as the hydrate, BaSiO₃.H₂O, by precipitation of a soluble barium salt with an alkali silicate. A hexahydrate has also been described, prepared by the action of barium hydroxide solution on sodium silicate or colloidal silica. The heat of reaction of dissolved barium hydroxide with colloidal silica is 8 Cal. It is probably decomposed by water, but the decomposition is prevented by the presence of barium hydroxide in solution. Barium silicate is formed on the walls of glass vessels containing barium hydroxide solution.

The anhydrous metasilicate forms small crystals showing a weak double refraction. The density is 4.435 and melting-point 1604° C. The heat of formation from barium oxide and silica is 26.3 Cal.

The metasilicate is not isomorphous with calcium or strontium metasilicates. With α-calcium metasilicate it forms a double compound, 2CaO.BaO.3SiO₂.

By treatment of hydrated silicic acid, or powdered quartz, with barium hydroxide solution, or by boiling barium metasilicate with water for some time, a number of hydrated acid silicates may be obtained. Crystals of barium disilicate, BaSi₂O₅, have been observed to separate in optical glass in the form of hexagonal plates.

A compound, 2BaO.3SiO₂, which forms a complete series of solid solutions with the disilicates is also known.

The use of barium silicate as a constituent of glass has already been mentioned. Its employment was first suggested by Dobereiner in 1829, and its effect upon the properties of the glass is much the same as that of lead. It gives a higher refractive index and greater brilliancy than lime alone, and it has the additional advantage over lead of being unaffected by the products of combustion in the furnace.

With barium aluminate a cement may be formed of composition 2(SiO₂.2BaO).Al₂O₃.2BaO. It sets hard with water, but, after some time, it splits, becomes soft, and deposits crystals of barium hydroxide.