Muscovite dissolution kinetics as a function of pH at elevated temperature

Abstract

Mineral reactivity can play an important role in fracture-controlled fluid networks where maintaining or increasing permeability is a goal, such as enhanced geothermal systems. In these systems, dissolution generates new void space, removes cement and physically transports less reactive mineral grains, while secondary precipitation acts to narrow or seal off fluid pathways. Sheet silicate mineral reactivity is likely to affect permeability evolution at the elevated temperatures of geothermal reservoirs because of the high reactive surface area and prevalence of these minerals in hydrothermal zones. To better describe the reactivity of one common sheet silicate, muscovite, we conducted kinetic dissolution experiments using flow-through reactors at temperatures of 100–280°C and a pH range of 2–9. Surface area-normalized muscovite dissolution rates ranged from 0.17–155·10−11molm−2s−1 over this temperature range, but showed little variation with pH above 150°C. Aluminum was released to solution nonstoichiometrically with respect to dissolved silica, most likely resulting from secondary precipitation of an aluminum oxy-hydroxide identified as boehmite (γ-AlO(OH)(s)) by X-ray diffraction in reaction products from experiments conducted at pH≤6. Surface area-normalized muscovite dissolution rates, Ratemus (molm−2s−1), can be described from 25 to 280°C with the following kinetic rate equation: Ratemus=3∙10−3∙e−44R∙T∙aH+0.8+9∙10−6∙e−45R∙T+5∙10−1∙e−61R∙T∙aOH−0.6∙1−e−∆GrRT where the rate and pre-exponential factors are in molm−2s−1; the activation energies, E, are in kJmol−1; aH+ and aOH− represent the activities of H+ and OH−, respectively; R (kJmol−1K−1) is the gas constant; T is the temperature in Kelvins; and ΔGr (kJmol−1) is a measure of how close the aqueous solution is to muscovite equilibrium. The rate equation is constrained by our new data literature rates and has been evaluated against previous formulations with varying dependence on reaction affinity. Although 150°C muscovite rates from Oelkers et al. (2008) show a systematic dependence on reaction affinity, incorporating this dependence did not accurately reproduce the higher-temperature rates. We recommend the rate equation shown above, with an affinity term that slows reaction rates only when solutions are close to equilibrium, for simulating the dissolution of muscovite under geothermal conditions.