CALCITE AND ARAGONITE FORMATION FROM AQUEOUS CALCIUM HYDROGENCARBONATE SOLUTIONS - EFFECT OF INDUCED ELECTROMAGNETIC-FIELD ON THE ACTIVITY OF CACO3 NUCLEI PRECURSORS

Calcite and aragonite formation from aqueous solution compete in the temperature range 353-373 K at a pressure of 0.1 MPa. Despite the fact that the calcitic phase is more stable than the aragonitic phase, often the latter phase is formed. In this paper the kinetic features related to this reaction are investigated and it is shown that a fast precipitation rate enhances the formation of calcite, which is the phase with higher entropy content. Several routes are possible for increasing the precipitation rate. The experimental evidence illustrated in this paper shows that the precipitates obtained by thermal evaporation of aqueous calcium hydrogencarbonate solutions previously exposed to the action of an induced electromagnetic low-frequency (ELF) field, have an XRD pattern where calcite is the predominant phase. After the same thermal treatment, the untreated aqueous solutions yield a precipitate formed by aragonite. The effect of the induced ELF field on the calcite : aragonite ratio in the precipitate is non-linear with the average modulus of the electromagnetic force. Data analysis allows the experimental evidence to be correlated with a faster precipitation rate due to the eff ect of the induced ELF on the surface chemistry of the pre-existing calcium carbonate nuclei in the hydrogencarbonate solutions at room temperature. Both calcite and aragonite micrograins obtained through thermal evaporation of treated solutions are about three times larger than those obtained from untreated aqueous solutions. The mechanism of facilitated crystal growth is discussed in relation to the modification in surface activity induced by the ELF field on the pre-existing calcium carbonate nuclei.