Geochimico et Cosmochimica Acto Vol. 46, pp. I I to 22 0 Pcrgamon Press zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ltd. 1982. Printed in U.S.A. The effect of pressure on the solubility of minerals in water and seawater FRANK J. MILLERO Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33 149 (Received March 9, 1981; accepted in revised form August 28, 1981) Abstract-The effect of presure on the solubility of minerals in water and seawater can be estimated from In (K&/K&) + (-AVP + 0.5AKP2)/RT where the volume (Av) and compressibility (AK) changes at atmospheric pressure (P = 0) are given by AV = r(M+, X-) - v[MX(s)] AK = E(M+, X--) - K[MX(s)] Values of the partial molal volume (n and compressibilty (K3 in water and seawater have been tabulated for some ions from 0 to 50°C. The compressibility change is quite large (- 10 X IO-’ cm’ barr’ mol-‘) for the solubility of most minerals. This large compressibility change accounts for the large differences observed between values of AV obtained from linear plots of In KS,, versus P and molal volume data (Macdonald and North, 1974; North, 1974). Calculated values of K&/K$ for the solubility of CaCOI, SrS04 and CaF, in water were found to be in good agreement with direct measurements (Macdonald and North, 1974). Similar calculations for the solubility of minerals in seawater are also in good agreement with direct measurements (Ingle, 1975) providing that the surface of the solid phase is not appreciably altered. lNTRODUCTION IN recent years we have attempted to use molal vol- ume and compressibility data to estimate the effect of pressure on ionic equilibria such as the ionization of acids (Millero, Hoff and Kahn, 1972; Ward and Millero, 1974, 1975; Lo Surdo et al., 1979) the for- mation of ion pairs (Millero, 1971a; Miller0 and Masterton, 1974; Masterton et al., 1974; Miller0 et al., 1974, 1977; Chen and Millero, 1977; Lo Surdo et al., 1979), and the solubility of minerals (Miller0 and Berner, 1972; Millero, 1976) in water and sea- water. The methods of using 0 nd f( data to estimate the effect of pressure on ionic equilibria was first demonstrated by Owen and Brinkley (1941). More recently, Lown, Thirsk and Wynne-Jones (1968) have shown that the effect of pressure on ionic equi- libria to pressures below 1000 b can be estimated by In (K’/p) = -AVP/RT + O.SAKP’/RT (1) where P is the applied pressure, R is the gas constant (83.15 crnem3 bar K-’ mol-’ or 82.06 crnm3 atm K-l), T is the absolute temperature (“K). The molal vol- ume (Av) and compressibility (AK) changes at 1 atm (P = 0) are given by AV = c ri (products) - 2 vi (reactants) (2) AK = 2 Ki (products) - 2 Ki (reactants) (3) (vi and Ki are the partial molal volume and com- pressibility of species i). Until recently (Millero, Ward and Chetirkin, 1977) reliable partial molal compressibility data have not been available for a large number of ions that are common in the marine environment. It was, thus, not possible to make a reliable estimate of AK for ionic equilibria. From an examination of the early tabulations of Owen and Brinkley ( 1941). one can see that K for most ions is about -5 to - 10 X 1O-’ cm3 mol-’ bar-‘. This is equivalent to a change in vof 5 to 10 cm3 mol-’ for a pressure change of 1000 bars. For completely ionic reactions, the AK is small since the Ki of the reactants and products are the same order of magnitude. For the solubility of min- erals, however, the AK can be quite large. For ex- ample, the AK for the solubility of calcite in pure water has been estimated by Owen and Brinkley (1941) to be -15 X 10e3 cm3 mol-’ bar-’ at 25°C. By neglecting this compressibility effect, one can misinterpret the behavior of direct measurements. Macdonald and North (1974) and North (1974) have measured the effect of pressure on the solubility of CaCOr, CaF, and SrS04 in water. The values of AV obtained from a linear fit of In Ksp versus P were found to be higher than the values estimated from 1 atm molal volume data. They interpreted these differences by postulating the formation of a bulk or surface hydrate. As will be shown in this paper, this interpretation is not necessary if one considers the compressibility effect on the AV for ionic equilibria. PARTIAL MOLAL VOLUME AND COMPRESSIBILITY DATA FOR IONS IN WATER A number of tabulations have been made of the partial molal volumes of salts or ions in water at infinite dilution, p (Owen and Brinkley, 1941; Mil- lero, 1969, 197 lb, 1972). Most of the earlier data (Millero, 197 1 b) are confined to simple ions and are reliable only at 25°C. Recent work (Dunn, 1968; Millero, 1970; Miller0 et al., 1972; Miller0 and 11