Monocalcium Silicate, CaSiO₃

Monocalcium Silicate or Calcium Meta-silicate, CaSiO₃, is found in nature as the mineral wollastonite, generally in calcareous rocks in volcanic regions. There is also a double calcium magnesium silicate, diopside, CaSiO₃.MgSiO₃, and a hydrated form, plombierite, CaSiO₃.H₂O. Calcium metasilicate, in addition, takes part in the formation of a number of poly basic silicates. It is also produced during the crystallisation or devitrification of certain acid slags or of ordinary glass.

It can be prepared by heating together lime and silica with, or without, calcium chloride, or a mixture of calcium and sodium chlorides, or in the wet way by the action of potassium silicate on calcium acetate. Lime and precipitated silica begin to react at 1011° C., but the reaction is more vigorous above 1400° C.

The natural compound has not the same optical characters as the artificial. Both crystallise in the monoclinic system, but the artificial compound is pseudo-hexagonal, and the name pseudo-wollastonite is given to it. The two are also distinguished as α- (artificial) and β-metasilicate.

Wollastonite is stable below about 1200° C., and pseudo-wollastonite above. The transformation of the former into the latter is easily observed, but the reverse change is never obtained by simple cooling. Certain substances, however, catalyse the transition from the α- to the β-form, probably by acting as a solvent, for example, excess of calcium oxide or silica, calciym vanadate, or calcium fluoride. The transition apparently produces no thermal effect.

The density of wollastonite is 2.919, and of pseudo-wollastonite 2.914. Values ranging from 1500°-1540° C. have been given for the melting-point of pseudo-wollastonite by different investigators, but, according to Doelter, these are all too high, due to a lag in the determinations, and the true value lies between 1310° and 1380° C.7 Leitmeier also obtained a low value and found that it varies with the size of the particles. The melting of fine grains begins at 1350° C., the whole becoming liquid at 1410° C., whilst coarse grains begin to melt at 1420° C.

Doelter found a metastable melting-point for wollastonite between 1240° and 1320° C.

The molecular heat of formation of calcium metasilicate from silica and calcium oxide is 16.1 Cal. The specific heat of wollastonite is 0.178.

By preparation in the wet way a monohydrate, CaSiO₃.H₂O, has been isolated, and the anhydrous salt also hydrates when treated with lime water for some weeks, but it loses its water of hydration on drying at 100° C. A hydrate, CaSiO₃.2.5H₂O, is mentioned by Le Chatelier.

A study of the freezing-point curves of mixtures of sodium and calcium metasilicates indicates the existence of two compounds, 2Na₂SiO₃.3CaSiO₃ and 3Na₂SiO₃.2CaSiO₃. With barium silicate a compound, 2CaO.BaO.3SiO₂, is formed. Freezing-point curves have also been given for mixtures of calcium metasilicate with calcium fluoride, chloride, and sulphide, and with ferrous, magnesium, aluminium, manganese, lithium, and titanium silicates.