INTERACTIONS BETWEEN DISSOLVED SILICA AND CARBONATE MINERALS - AN EXPERIMENTAL-STUDY AT 25-50-DEGREES-C

Replacement of carbonate minerals by silica polymorphs in marine and m eteoric environments is a common early diagenetic feature of sedimenta ry rocks. Although relatively few experimental investigations have bee n made of interactions between carbonate minerals and dissolved SiO2 a t moderate temperatures, available data suggest that silica uptake ont o carbonate surfaces can be significant, We present results of experim ents conducted between 25 degrees and 50 degrees C of the uptake of aq ueous SiO2 from artificial seawater (Mg-Na-Cl-Ca-HCO3, I = 0.7) and me teoric-type (CaCl2-Na2CO3, I < 0.1) solutions onto well-characterized Ca-Mg-carbonates. Silica concentrations ranged from 1.5 to 3.0 mM, a r easonable upper limit for natural diagenetic waters, Dissolved silica- carbonate mineral interactions were investigated under three experimen tal conditions: (1) in solutions near equilibrium with respect to calc ite; (2) in calcite-seeded precipitation experiments at fixed degrees of calcite supersaturation; and (3) during closed-system, dissolution- precipitation of metastable carbonate minerals (aragonite and 18 mole% Mg-calcite) to low-Mg calcite (LMC). In artificial seawater (ASW) mai ntained near equilibrium with respect to calcite at various P-CO2-valu es, silica uptake from solution onto calcite surfaces (similar to 1-15 mu mol Si/gram seed) was irreversible and strongly time-and pH-depend ent. The rate of uptake was greatest in the first 5 hr and in solution s of lower pH (higher P-CO2). Silica uptake was only nominally depende nt on solution [Mg] and temperature over the range studied. Silica upt ake was greater in ASW than in otherwise comparable experiments conduc ted in low-ionic-strength solutions. Controlled calcite precipitation rates were unaffected by the adsorption of silica, During recrystalliz ation of aragonite and Mg-calcite in low-ionic-strength solutions, sil ica uptake (1-6 mu mol g(-1)) was similar to values from near-equilibr ium experiments for synthetic calcite in ASW. Experimental Si adsorpti on values were comparable to Si adsorption values determined for modem metastable sediments (3-4 mu mol g(-1)). Despite considerable variati on in the experimental carbonate equilibrium-disequilibrium, mineralog y and solution chemistry, Si uptake at near-surface and moderate buria l conditions (25-50 degrees C) was limited to similar to 15 mu mol g(- 1) Si even in solutions maintained near opal-CT saturation over a peri od of days to weeks.