1 Dissolution rates of talc as a function of solution composition, pH and temperature Giuseppe D. Saldi a , Stephan J. Köhler b , Nicholas Marty c , and Eric H. Oelkers a, * a Geochimie et Biogéochimie Experimentale, LMTG, Université de Toulouse, CNRS, IRD, OMP. 14 ave Edouard Belin, F-31400 Toulouse, FRANCE b Institute of Applied Geosciences, Graz University of Technology, Rechbauerstraße 12, A-8010 Graz, AUSTRIA c CGS / EOST UMR 7517, 1 rue Blessig, 67084 STRASBOURG Cedex, FRANCE Abstract Steady-state talc dissolution rates, at far-from-equilibrium conditions, were measured as a function of aqueous silica and magnesium activity, pH from 1 to 10.6, and temperature from 25°C to 150°C. All rates were measured in mixed flow reactors and exhibited stoichiometric or close to stoichiometric dissolution. All measured rates at pH > 2 obtained at a fixed ionic strength of 0.02 M can be described to within experimental uncertainty using ( ) RT / E exp a a s r A / BET −           +         = + + + B A A A 4 1 Mg 2 H 2 where r + signifies the BET surface area normalized forward talc steady-state dissolution rate, BET s denotes the BET surface area of talc present in the reactor, A A and B A refer to pre-exponential factors equal to 5.0x10 -9 and 0.8x10 -9 mol cm -2 s -1 , respectively, E A designates an activation energy equal to 45 kJ mol -1 , R represents the gas constant, T denotes absolute temperature, and i a refers to the activity of the subscripted aqueous species. The first term of this rate expression is consistent with talc dissolution rates at acidic pH being controlled by the detachment of partially liberated silica tetrahedral formed talc edge surfaces from the exchange of Mg 2+ for two protons from talc edges. Corresponding atomic force microscopic observations confirms that dissolution proceeds by the removal of T-O-T sheets from talc edges. At pH ≤ 2, the Mg 2+ for proton