Rt. Klein et Lm. Walter, INTERACTIONS BETWEEN DISSOLVED SILICA AND CARBONATE MINERALS - AN EXPERIMENTAL-STUDY AT 25-50-DEGREES-C, Chemical geology, 125(1-2), 1995, pp. 29-43
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.