Mechanisms and rates of plagioclase carbonation reactions

Details Mechanisms and rates of plagioclase carbonation reactions Mechanisms and rates of plagioclase carbonation reactions

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012gecoa..77...27m&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 77, p. 27-51.

Computer Science

Scientific paper

Plagioclase is one of the most abundant sources of calcium in the earth's crust, and it may play an important role for CO2 storage. This study address' the carbonation of anorthite-rich plagioclase (An67-An73) in a system with fluid transport, and under stagnant conditions.A combined approach of flow-through column and batch experiments has been used. Experimental conditions ranging from 100 to 250 °C and 20 to 120 bar and different preparations of the starting material were applied. The overall carbonation reaction consists of plagioclase dissolution coupled to a number of precipitation reactions. The flow-through column experiments at 250 °C showed stoichiometric dissolution of the plagioclase. Al-hydroxide ("proto Al-hydroxide") nucleated on the plagioclase as the first phase to precipitate. A secondary porosity developed between the shrinking plagioclase and the enclosing "proto Al-hydroxide". Calcite, as the second phase to precipitate, filled the primary pore space. A reaction front was developed separating the zone at the inlet where all the plagioclase had dissolved and the less reacted outlet of the column. Redissolution of the calcite and formation of euhedral boehmite crystals occurred when a sufficient amount of plagioclase had dissolved. Clay minerals were not precipitated in the column experiments. Between 11% and 30% of the plagioclase was dissolved within 72-168 h of reaction. A much higher extent of plagioclase dissolution was observed in the high pressure experiments compared to the low pressure. However, a smaller share of the released Ca was trapped as calcite in the high pressure experiments. Both observations are consistent with a more rapid progression of the dissolution front at high pressure.The batch experiments showed conversion of the plagioclase to a mixture of Al-hydroxide, possibly gibbsite, clays and calcite. A range in conversion from below the detection limit to 91% was observed within reaction periods of 24-72 h. Crystallinity of the feldspar was the most important factor contributing to increased reaction rates. A general positive effect of increasing temperature on the conversion is observed for all materials, whereas pressure and the addition of CaCl2 did not have any effect.The carbonation of plagioclase at stagnant conditions is slow compared to olivine at temperatures around 200 °C. However, industrial operations involving high fluid flows of CO2-water mixtures induce gradients in pH or solute concentrations, which may lead to increased reaction rates and changes in porosity/permeability.

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