Reaction rates and textural development of hydrolysis reactions in the system MgO-SiO2-H2O

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Experiments in the simplified systems MgO-SiO2-H2O (MSH) and MgO-SiO2-H2O-CO2 (MSHC) have been conducted by using hydrothermal diamond anvil cells to investigate reaction rates and the resulting textures at temperatures and pressures consistent with the temperatures and pressures of the Earth's crust. The conditions and simplified systems of the experiments serve as approximations for geologic environments wherein magnesium-rich rocks (i.e., mafic, ultramafic, and magnesium-rich carbonate rocks) are hydrothermally altered by silica-rich fluids. The hydrolysis reaction rates and textures that result from the irreversible interactions of olivine and magnesite with aqueous fluids in the presence of quartz have been characterized.

Reaction rates have been determined by a new approach developed during this study, which uses in situ observation of reactant volume loss to determine the growth rate of the products of hydrolysis reactions. In addition, some experiments were analyzed by real-time synchrotron radiation analysis to identify the phases in the reactions and to provide semi-quantitative constraints on the kinetic data. Experiments performed in this study resulted in the development of several textural varieties. Talc grown during this study exhibited both fibrous and platy habits, textural variations that appeared to be controlled by: variations in the density of the aqueous phase, surface area of starting materials, rate of temperature increase, and the presence of strong chemical gradients. The primary growth of fibrous talc in these experiments demonstrates that the production of fibrous talc does not require the pseudomorphism of a fibrous precursor as previously suggested.