Studies in the system MgO-SiO2-CO2-H2O(I): The activity-product constant of chrysotile

Chrysotile dissolves congruently in water according to the reaction: Mg₃Si₂O₆(OH)_4c + 5H₂O_l = 3Mg_aq²⁺ + 6OH_aq⁻ + 2H₄SiO_4aq. Experimental determination of the activity-product constant of chrysotile, K_chr = [Mg²⁺]³[OH⁻]⁶[H₄SiO_4aq]², at 90°C, yields the value of K_chr = 10^−49.2 ± 10^0.5. A synthetic sample and a natural sample from New Idria, California, were used in the determination.

Values of K_chr were calculated for temperatures ranging from 0°C to 200°C, using the thermochemical data of Kinget al. (1967) for chrysotile and antigorite, various solubility data for silica, and ionic partial molal heat capacities estimated by the method of criss and Cobble (1964a). K_chr is 10^−54.1 at 0°C, rises to a maximum value of 10^−48.5 at approximately 135°C, and is 10^−49.1 at 200°C (all values for the three-phase system, chrysotile plus solution plus vapor). The calculated 90°C value is 10^−49.1, in excellent agreement with the experimental value; for 25°C, the calculated value is 10^−50.8.