Reaction of plagioclase feldspars with CO2 under hydrothermal conditions

Geological storage Of CO2 in plagioclase-bearing sediments, granitic rocks and fractured basalts is expected to have a variety of geochemical and geomechanical effects, including mineral trapping and porosity-permeability changes. The amount Of CO2 trapped by mineralisation and the extent of any changes in host rock transport or mechanical properties depend on rock composition. To date, studies of mineralisation have mainly been based on dissolution and precipitation experiments performed at ambient surface conditions, or at slightly elevated temperature and pressure. However, the reactions that occur under in-situ conditions are poorly constrained. In an attempt to determine the secondary phases formed in the plagioclase-CO2-water system under such conditions, we performed a set of batch experiments, at temperatures in the range 200-300 degrees C, using carbon dioxide pressures from 0.4 to 15 MPa. We investigated reaction of anorthite and albite. which are expected to precipitate calcite and kaolinite, and dawsonite and quartz, respectively. However, in most of our experiments, reactions precipitating clays (kaolinite and smectite or illite), boehmite, and a Mg, Ni,Fe-hydrotalcite-like phase dominated, the nickel being derived from the reaction vessel. Little or no carbonate and no dawsonite were detected. Additional feldspar-free control experiments employing Mg-rich brine showed that carbonates would be preserved in our feldspar experiments if formed, but that a carbonate substrate is needed to promote precipitation. Since the secondary phases formed in our plagioclase experiments are stable under a wide range of conditions we expect that they will also form under CO2 storage systems with suitable fluid-rock composition. From our results we conclude that clay precipitation can proceed or accompany carbonate precipitation, depending on the availability of carbonate substrate. Though an artefact in our experiments, in the context of subsurface alkaline waste injection, hydrotalcite may play a positive role here by immobilising heavy metals, while carbonates will mineralise the CO2. (C) 2008 Elsevier B.V. All rights reserved.