Activity coefficient and polymerization of aqueous silica at 800 degrees C, 12 kbar, from solubility measurements on SiO2-buffering mineral assemblages

The activity coefficient of aqueous silica was determined at 800 degreesC and 12 kbar by measuring the concentration of total dissolved aqueous silica (SiO2(t)) in H2O in equilibrium with four buffering mineral assemblages: quartz (Q), kyanite-corundum (KC), enstatite-forsterite (EF), and forsterite-rutile-geikielite (FRG). SiO2(t) concentrations were determined by reversed equilibration of synthetic minerals with H2O using a weight gain/weight loss technique, yielding molalities of SiO2(t) (m(s)) of Q, 1.413(1); KC, 0.984(43); EF, 0.276(9); FRG, 0.079(1) (1sigma errors). The solubility data can be fitted well according to the homogeneous equilibrium 2 SiO2 monomers (m) = 1 Si2O4 dimer (d) assuming ideal mixing, such that the equilibrium constant (K-md) is K-md=X-d/X-m(2), where X represents the mole fraction of SiO2(t) occurring as the subscripted species. This formulation is valid irrespective of the hydration states of the silica species because of the small silica concentration at these conditions (less than or equal to2.5 mol%). For a standard state of unit activity of the hypothetical pure monomer solution at the P and T of interest, the monomer-dimer model leads to gamma(s)=X-m/X-s and Kmd = (1-gamma(s))/2gamma(s)(2)X(s) where gamma(s) and X-s are the activity coefficient and mole fraction of total silica. Our data yield Kmd = 155(-39)(+25) at 800 degreesC and 12 kbar, in excellent agreement with results from in situ Raman spectroscopy of a quartz-saturated solution at the same P-T. In the system SiO2-H2O, a solution in equilibrium with quartz at 800 degreesC, 12 kbar, contains 2.5 mol% silica, of which 70% occurs in dimers, and gamma(s) is small (0.30). Even at the low concentration of the FRG buffer (0.1 mol%), the activity coefficient is only 0.75 and the dissolved silica is substantially polymerized (25%).