Abstract
The solubility of datolite, CaBSi04(OH), has been determined experimentally at temperatures from 100 to 250°C and Pv = PH20,Activity products (Q) were calculated for the dissolution reaction after 8-10 time intervals in experiments of up to four months' duration in order to determine Koo..A linear relationship between log Koo.and reciprocal temperature [log Keq= 4.405 + 637.01T (K); R = 0.97] suggests the attainment of equilibrium. The IlG? extrapolated to 25°C, -2318 :t 2 kl/mol, is the same as a calculated calorimetric value (Semenov et aI., 1988) and close to the value of - 2317 kl/mol estimated with the use of the polyhedral model of Chermak and Rimstidt (1989). However, at elevated temperatures f1G? values determined from solubility data diverge from the other two estimations, suggesting that the latter may be more accurate. The IlG? values calculated from the data of Semenov et ai. (1988) have been used to d~termine the temperature dependence of datolite-danburite stability relations and activity diagrams for the system CaO-B203-Si02-H20 at 250°C in an effort to place limits on solution compositions in equilibrium with mineral assemblages in the quaternary system. Higher activities of H3B03.aqand Si02.aq and lower pH are required for the formation of danburite than for datolite, whereas high a Ca2+ favors the crystallization of datolite.
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