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Experimental dissolution of sanidine under hydrothermal conditions: Mechanism and rate

^* Université Paul Sabatier, LMTG, 38 rue des 36 Ponts, 31400 Toulouse, France
^** Université de Poitiers, HydrASA, 40 Avenue Recteur Pineau, 86000 Poitiers, France
^*** TotalElfFina, CSTJF, Avenue Larribeau, 64000 Pau, France

Dissolution rate measurements of sanidine at 100° to 300°C provide new insight into the rate limiting reactions governing feldspar dissolution behavior close to chemical equilibrium. The experiments were conducted under various conditions, but mainly at neutral pH and at saturation with respect to quartz, these conditions being assumed to be representative of diagenetic and hydrothermal environments.

Far from equilibrium, the activity of aqueous silica is found to control the rate at near neutral pH. Our results, when compared to other published studies, suggest a competition between two parallel reactions at the mineral surface: the hydrolysis of the Si and Al network-former oxides. In closed system experiments, the precipitation of secondary minerals such as clays is not observed; we found evidence only for the development of a thick potassium-leached layer at 300°C. The solution composition suggests that a thermodynamic "equilibrium" is essentially attained (constant composition with time) but not with respect to the bulk sanidine, at least at 100°C. By combining solution and surface analyses, we propose that local equilibrium at the reactive sites explains this apparent metastable equilibrium.