Kinetics of glauconite dissolution in anoxic conditions as a function of pH and temperature

Abstract

Because it is an abundant, divalent cation-bearing mineral in sedimentary rocks and hydrocarbon reservoirs worldwide, glauconite has likely played a role in Earth’s carbon cycle over geologic time and may be important for ongoing efforts to geologically store anthropogenic CO2. Yet, due to its complex chemistry and redox sensitivity, glauconite dissolution kinetics have so far been difficult to constrain. To fill this significant knowledge gap, we have undertaken a study to quantify the far-from-equilibrium rates of glauconite dissolution using a novel experimental apparatus specifically designed to explore mineral dissolution kinetics under strictly anoxic conditions. Steady-state glauconite dissolution rates were measured at varying pH from 1.7 to 11.2 and temperature from 24 to 80 °C. Temporal evolution of the differences between cation concentrations in the inlet and outlet solutions exhibits stoichiometric or close-to-stoichiometric glauconite dissolution for Fe, Mg, and Si. Fitting the rates calculated from Si release during the experiments to a standard Transition State Theory-derived, far-from-equilibrium rate law yields:k=2.18×10-12·exp-32.2R·1T-1Tr·aH+0.37+2.95×10-14·exp-37.5R·1T-1Tr,where k is the rate constant (mol m−2 s−1) at the temperature (T, Kelvin) and H+ activity (aH+) of interest, Tr is the reference temperature (298.15 K), and R is the ideal gas constant (8.314 × 10−3 kJ mol-1 K−1). Our experimental results show that the mechanism of glauconite dissolution is highly dependent on temperature and on pH in acidic solutions. Geochemical calculations based on the fitted rate equation predict that complete carbonation of glauconite in a system with a 10:1 water–rock ratio and 50 bar of CO2 fugacity can be expected after 17.5, 11.9, 7.1, and 3.8 kyr at 35 °C, 45 °C, 60 °C and 80 °C, respectively. While the lower-temperature simulations generally agree with previously published modelling efforts, the higher temperature reactions are significantly faster than previously predicted. These results highlight the importance of reservoir temperature for glauconite diagenesis and suggest that, when appropriate attention is paid to reservoir temperature during site selection, glauconite carbonation may present significant opportunities for CO2 mineralization.