The Reaction between Calcium Carbonate and Silica

JSGT_1932_V16_T094_T110

Admixture of quartz grains with calcite grains in both cases of diameter <0·063 mm brings about, in a closed, evacuated system, a development of slight pressure (about 0·2 mm at 500°) at temperatures lower than with calcite alone of 0·1 mm grain diameter; but in the temperature region 500–900° the influence of silica on the course of the pressure curve is scarcely appreciable, although slow chemical reaction occurs. The rate of isothermal decomposition in air of calcite at 700°, 750°, and 800° is very markedly accelerated by quartz increasing progressively with increase from one to three molecular proportions of quartz per one of calcite, although at 800° the rate for CaCO₃:SiO₂ differs little from that for calcite alone. It is shown that the acceleration of decomposition cannot be ascribed to chemical reaction, which is slight but is probably to be associated mainly with promotion of diffusion away of the carbon dioxide. Metallic gold of grain size similar to the quartz and used in equivalent volumes to the quartz produced similar acceleration, but there was also suggestion of catalytic action. Microscopic examination showed that chemical reaction occurred mainly at selected points on the quartz grains, which at an intermediate stage acquired a pitted character, whilst very fine crystalline material, probably wollastonite, also accumulated with time and temperature of heating (up to 1450°). The extent of reaction was ascertained by treating the product successively with cold acetic acid, boiling 5% Na₂CO₃, boiling 20·4% HCl, and boiling 5% Na₂CO₃, and so determining the soluble silica. Slow reaction occurs at comparatively low temperatures. At 610° it was quite appreciable in five weeks. Of the original quartz, 0·3% had been converted into “soluble silica” in 170 minutes at 700°; 4–5% in 60 minutes at 800°, and, in 60 minutes, 19·4% at 1100° and 60·1% at 1400°.

William Maskill, G. H. Whiting & W. E. S. Turner