| Title | Sheet Silicate Dissolution at Elevated Temperature: A Comparison of Dissolution Kinetics of Chlorite, Illite, and Biotite |
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| Authors | Megan SMITH, Susan CARROLL |
| Year | 2016 |
| Conference | Stanford Geothermal Workshop |
| Keywords | kinetics, dissolution, chlorite, illite, biotite |
| Abstract | Sheet silicates are important and abundant minerals in hydrothermal systems, making up significant portions of solid-phase volume in some geothermal reservoirs. The reactivity of these sheet silicates is of special interest when these minerals occur within naturally occurring geothermal fractures or in adjoining wallrock, where large volumes of out-of-equilibrium fluids cycle through the geothermal system. We have identified a lack of kinetic data for many sheet silicates at temperatures above 100 °C, and have been conducting kinetic dissolution experiments to close this data gap for three specific minerals: chlorite (Mg-Al-silicate), illite (K-Al-silicate), and biotite (K-Mg-Al-silicate). Experiments were conducted over 100-280 °C and a pH range of 3-9, and the resulting rates were combined with pre-existing lower-temperature data (where available) to produce kinetic rate equations applicable at (sub-280 °C) geothermal temperatures. Specifically, we find that previously published rate equations from 25 °C data both over- (in the case of chlorite) and underpredict (in the case of illite) rates at higher-temperature geothermal conditions. Our experiments show that illite is the most reactive of the three sheet silicates, with rates increasing by over 1000 times from 100-280 °C at a given pH over a 3-4 day timescale, compared to only a 10-50fold increase in chlorite and biotite rates at the same conditions. We find that illite dissolution under acid conditions is accompanied by rapid co-precipitation of aluminum oxy-hydroxide solids, with no secondary precipitation noted under neutral or alkaline conditions. In comparison, chlorite and biotite experiments showed no evidence of secondary precipitates, and rates showed little variability with pH ≥ 5 as solutions approached equilibrium with magnesium-hydroxide phases. We present kinetic rate equations that should be helpful for better prediction of the long-potential term impacts of geochemical alteration on reservoir permeability. |