Activity coefficient and polymerization of aqueous silica at 800 [degrees]C, 12 kbar, from solubility measurements on Si[O.sub.2]-buffering mineral assemblages

The activity coefficient of aqueous silica was determined at 800 [degrees]C and 12 kbar by measuring the concentration of total dissolved aqueous silica (Si[O.sub.2(t)]) in [H.sub.2]O in equilibrium with four buffering mineral assemblages: quartz (Q), kyanite--corundum (KC), enstatite-forsterite (EF), and forsterite-rutile-geikielite (FRG). Si[O.sub.2(t)] concentrations were determined by reversed equilibration of synthetic minerals with [H.sub.2]O using a weight gain/weight loss technique, yielding molalities of Si[O.sub.2(t)] ([m.sub.s]) of Q, 1.413(1); KC, 0.984(43); EF, 0.276(9); FRG, 0.079(1) (1[sigma] errors). The solubility data can be fitted well according to the homogeneous equilibrium 2 Si[O.sub.2] monomers (m) = 1 [Si.sub.2][O.sub.4] dimer (d) assuming ideal mixing, such that the equilibrium constant ([K.sub.md]) is [K.sub.md] = [X.sub.d]/[X.sub.m.sup.2], where X represents the mole fraction of Si[O.sub.2](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 to] 2.5 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].sub.s] = [X.sub.m]/[X.sub.s] and [K.sub.md] = (1 - [[gamma].sub.s])/2[[gamma].sup.2.sub.s][X.sub.s] where [[gamma].sub.s] and [X.sub.s] are the activity coefficient and mole fraction of total silica. Our data yield [K.sub.md] = [155.sup.+25.sub.-39] at 800 [degrees]C 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 Si[O.sub.2]-[H.sub.2]O, a solution in equilibrium with quartz at 800 [degrees]C, 12 kbar, contains 2.5 mol% silica, of which 70% occurs in dimers, and [[gamma].sub.s] is small (0.30). Even at the low concentration of the FRG buffer (0.1 tool%), the activity coefficient is only 0.75 and the dissolved silica is substantially polymerized (25%).