A rate law for sepiolite growth at ambient temperatures and its implications for early lacustrine diagenesis

Abstract

The magnesium silicate mineral sepiolite is an important component of both alkaline lake and siliceous marine sediments. However, apart from historical constraints on its equilibrium solubility and effective solubility in surface waters, little is known about the rate at which it grows from pore waters during diagenesis. To place constraints on the early evolution of these pore waters, we have performed a series of sepiolite-seeded batch experiments at room temperature with varying concentrations of Mg and SiO2(aq) at pH = 9.2 and 10.1. In general, the slopes of time-series measurements of Mg and Si concentrations are consistent with heterogeneous growth of a phase with sepiolite stoichiometry during the experiments within calculated uncertainties, although some slopes are more consistent with the growth of a phase with kerolite stoichiometry. In total, thirty-six individual rates were derived from the pH = 9.2 experiments and nine individual rates were determined from the pH = 10.1 experiments. In the case of the pH = 9.2 data set, fitting a simplified Transition State Theory rate law to the Si-based sepiolite precipitation rates yields:Rate(molessepiolite/m2/s)=10-16.0±0.3Ω-10.44±0.04 and, for the pH = 10.1 experiments:Rate(molessepiolite/m2/s)=10-15.0±0.2Ω-10.23±0.02where Ω is the saturation index for crystalline, stoichiometric sepiolite. These results are consistent with a strong dependence of growth kinetics on pH and saturation state. The saturation state dependence is significantly stronger at pH = 9.2 than it is at pH = 10.1, such that, at extreme supersaturations approaching the critical supersaturation for homogeneous sepiolite nucleation, the calculated pH = 9.2 rates are faster than the pH = 10.1 rates. Inferences based on the uncertainties on the calculated rate parameters and the predictive capacity of the rate law suggest that these parameters can also predict the growth rates of kerolite without introducing significant additional uncertainty. Using these experimental constraints, we present a conceptual model for the evolution of pore waters during the early stages of diagenesis in sepiolite-bearing sediments. This conceptual model, which implies that exceptional sepiolite supersaturations observed in alkaline lake waters rapidly diminish as porewaters are separated during sediment burial, is in agreement with field measurements of pore waters obtained from Mg-silicate-bearing sediments.