Sorption of atmospheric carbon dioxide and structural changes of Ca and Mgsilicate minerals during grinding - II. Enstatite, akermanite and wollastonite

Processes which occur during dry grinding of the magnesium and calcium sili cate minerals enstatite MgSiO3, akermanite Ca2MgSi2O7 and wollastonite CaSi O3, have been studied with the use of diffuse reflectance Fourier transform infrared spectroscopy (DRIFT), powder X-ray diffraction, Si-29 MAS-NMR spe ctroscopy, CO2-content analysis, and BET surface area measurements. It has been found that the extensive sorption of atmospheric CO2 (carbonization) b y the ground mineral that was reported earlier for diopside CaMgSi2O6 also occurs for the minerals included in this study. The shape and position of t he double peak of the CO32--group in the 1430-1515 cm(-1) region in the FT- IR spectra of ground minerals are similar to that of ground diopside. CO2-c ontent analysis and BET surface area measurements show that carbonate group s are present not only at the surface but also in the bulk of the ground mi nerals. The degree of carbonization of silicate depends on its calcium cont ent. The higher the fraction of Ca in octahedral positions in the silicate: structures, the higher the degree of carbonization. For synthetic wollasto nite CaSiO3, which is a pure calcium silicate, the degree of carbonization is 102.6 mg CaCO3 per gram of sample after 36 h of grinding. The structure of silicate appears to be less important for the degree of carbonization du ring grinding. (C) 2001 Elsevier Science B.V. All rights reserved.