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

Abstract

The activity coefficient of aqueous silica was determined at 800 °C and 12 kbar by measuring the concentration of total dissolved aqueous silica (SiO_2(t)) in H₂O in equilibrium with four buffering mineral assemblages: quartz (Q), kyanite–corundum (KC), enstatite–forsterite (EF), and forsterite–rutile–geikielite (FRG). SiO_2(t) concentrations were determined by reversed equilibration of synthetic minerals with H₂O using a weight gain/weight loss technique, yielding molalities of SiO_2(t) (m _s) of Q, 1.413(1); KC, 0.984(43); EF, 0.276(9); FRG, 0.079(1) (1σ 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 SiO_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 (≤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 γ _s =X _m /X _s and

$$ {K_{{md}} = {{{\left( {1 - \gamma _{s} } \right)}} \over {2\gamma ^{2}_{s} X_{s} }}} $$

where γ _s and X _s are the activity coefficient and mole fraction of total silica. Our data yield ${K_{{md}} = 155^{{ + 25}}_{{ - 39}} }$ at 800 °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 SiO₂–H₂O, a solution in equilibrium with quartz at 800 °C, 12 kbar, contains 2.5 mol% silica, of which 70% occurs in dimers, and γ _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%).