PRECIPITATION OF POORLY CRYSTALLINE ANTIGORITE UNDER HYDROTHERMAL CONDITIONS

Abstract

Magnesium silicate precipitation experiments were carried out in alkaline solutions in the temperature range 39°C–150°C. Titrations were carried out at room temperature where the pH of an aqueous solution containing magnesium and silica was raised to bring about precipitation of a magnesium silicate. The precipitation of the magnesium silicate was rapid. Equilibrium between the solution and the precipitate was attained in a period of less than one hour up to a month at around 90°C, depending on the initial degree of oversaturation. Relative magnesium and silica depletion in the experimental solutions and IR spectra of the precipitate show that the magnesium silicate resembles poorly developed antigorite (p-antigorite). Values for its solubility constant were obtained and an equation describing its solubility in the temperature interval 0°–200°C calculated. The equation is: log Ksp = 9303/T + 3.283, where T is in K, and it is valid for the following reaction:Mg3Si2O5(OH)4+6H+=3Mg2++2H4SiO40+H2O From thermodynamic data, the solubilities of talc, antigorite, chrysotile, sepiolite, and brucite were calculated in the range 0°C–350°C at Psat. The saturation state of selected surface and ground waters with respect to these minerals and p-antigorite were calculated. In the temperature range 3°C–100°C most of the ground waters are within the limit of error at equilibrium for this phase, but waters of higher temperature are undersaturated. Surface waters are generally undersaturated. It is considered that precipitation of p-antigorite is responsible for the magnesium depletion of surface waters when they seep into the bedrock, and the solubility of this phase controls magnesium concentrations in