Probing the application of kinetic theory to Mg-phyllosilicate growth with Si isotope doping

Abstract

The Principle of Detailed Balance (PDB) played a defining role in the derivation of the widely-used Transition State Theory rate law equation and serves as an important link between geochemical kinetics and thermodynamics. Although significant improvements have been made in applying the PDB to comparatively simple systems (e.g., SiO2-water), experimental verification of the PDB is lacking for more complex minerals such as the phyllosilicates. Providing kinetic constraints on the rates of phyllosilicate growth from supersaturated solutions is particularly important for constructing facies models, constraining element cycling in the lacustrine and marine environments, and interpreting paleo-biogeochemistry. Here, we use 29Si isotopic doping techniques to quantify the rates of reaction between Mg-phyllosilicate substrates (amorphous Mg-silicate (a talc-like phase) and crystalline talc) and supersaturated solutions at 21 and 60 °C. The results show that the ratio of the forward and backward rates of amorphous Mg-silicate-water reaction approaches unity as the saturation state of the solution approaches the apparent solubility of amorphous Mg-silicate. The precipitation rates coupled with equivalent dissolution rates obtained from experiments with amorphous Mg-silicate substrates appear to obey the same rate function as the precipitation rates coupled with negligible dissolution rates obtained from experiments with crystalline Mg-silicate substrates over the degrees of supersaturation we explore, suggesting that the elementary step limiting the rate of precipitation remains the same. Accordingly, our results demonstrate that the PDB is applicable to amorphous Mg-phyllosilicate-water reactions, thereby reinforcing the use of TST rate equations to describe Mg-phyllosilicate growth. The experimental data can also be taken as evidence that the apparent solubility of amorphous Mg-silicate, a concept previously explained using the kinetic theory of nucleation and growth, also has a thermodynamic meaning, in that it represents a metastable equilibrium with the poorly crystalline phase. The measured, non-negligible forward and backward rates suggest that, even in this metastable state where little if any net reaction is occurring, isotopic signatures can be reset. Moreover, the significant discrepancy between the heterogeneous net growth rates on the amorphous Mg-silicate substrate versus those measured on crystalline talc and sepiolite substrates indicates that mineral crystallinity likely plays a key role in mineral growth during diagenesis.