Formation of submicron magnesite during reaction of natural forsterite in H2O-saturated supercritical CO2

Natural forsterite was reacted in bulk liquid water saturated with supercritical CO2 (scCO(2)) and scCO(2) saturated with water at 35 80 degrees C and 90 atm. The solid reaction products were analyzed with nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and confocal Raman spectroscopy. Two carbonate phases, nesquehonite (MgCO3 center dot 3H(2)O) and magnesite (MgCO3), were identified with the proportions of the two phases depending on experimental conditions. In liquid water saturated with scCO(2), nesquehonite was the dominant carbonate phase at 35 80 degrees C with only a limited number of large, micron size magnesite particles forming at the highest temperature, 80 degrees C. In contrast, in scCO(2) saturated with H2O magnesite formation was identified at all three temperatures: 35, 50, and 80 degrees C. Magnesite was the dominant carbonation reaction product at 50 and 80 degrees C, but nesquehonite was dominant at 35 degrees C. The magnesite particles formed under scCO(2) saturated with H2O conditions exhibited an extremely uniform submicron grain-size and nearly identical rhombohedral morphologies at all temperatures. The distribution and form of the particles were not consistent with nucleation and growth on the forsterite surface. (C) 2014 Elsevier Ltd. All rights reserved.