Abstract
Calcium carbonate is the most abundant biomineral and a compd. of great industrial importance. Its pptn. from soln. has been studied extensively and was often shown to proceed via distinct intermediate phases, which undergo sequential transformations before eventually yielding the stable cryst. polymorph, calcite. In the present work, we have investigated the crystn. of calcium carbonate in a ...
Abstract
Calcium carbonate is the most abundant biomineral and a compd. of great industrial importance. Its pptn. from soln. has been studied extensively and was often shown to proceed via distinct intermediate phases, which undergo sequential transformations before eventually yielding the stable cryst. polymorph, calcite. In the present work, we have investigated the crystn. of calcium carbonate in a time-resolved and non-invasive manner by means of energy-dispersive X-ray diffraction (EDXRD) using synchrotron radiation. In particular, the role of silica as a sol. additive during the crystn. process was examd. Measurements were carried out at different temps. (20, 50 and 80 °C) and various silica concns. Expts. conducted in the absence of silica reflect the continuous conversion of kinetically formed metastable polymorphs (vaterite and aragonite) to calcite and allow for quantifying the progress of transformation. Addn. of silica induced remarkable changes in the temporal evolution of polymorphic fractions existing in the system. Essentially, the formation of calcite was found to be accelerated at 20 °C, whereas marked retardation or complete inhibition of phase transitions was obsd. at higher temps. These findings are explained in terms of a competition between the promotional effect of silica on calcite growth rates and kinetic stabilization of vaterite and aragonite due to adsorption (or pptn.) of silica on their surfaces, along with temp.-dependent variations of silica condensation rates. Data collected at high silica concns. indicate the presence of an amorphous phase over extended frames of time, suggesting that initially generated ACC particles are progressively stabilized by silica. Our results may have important implications for CaCO3 pptn. scenarios in both geochem. and industrial settings, where soln. silicate is omnipresent, as well as for CO2 sequestration technologies. [on SciFinder(R)]
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