CRYSTALLIZATION OF CALCIUM-CARBONATE IN PRESENCE OF MAGNESIUM AND POLYELECTROLYTES

The results of X-ray diffraction and morphological investigations carried out on calcium carbonate crystals grown from solutions containing different Mg2+/Ca2+ molar ratios and polyelectrolytes are reported. Polyelectrolytes containing charged groups important in biomineralization processes are used. Polymers containing sulphate (carrageenans K and J) and carboxylate (polyacrylate, poly-L-glutamate and poly-L-aspartate) charged groups do not influence the magnesium substitution for calcium in calcite. When the Mg2+/Ca2+ molar ratio is high, the control of polymorph type seems to be dictated almost exclusively by magnesium ions independent of the presence of poly-L-glutamate, polyacrylate or carrageenans. In these conditions only aragonite crystallizes. The behaviour of poly-L-aspartate, which assumes a beta-sheet structure for long stretches of chain in the presence of calcium ions, is remarkably different. Since, in the presence of this polymer, calcite crystallizes together with aragonite also when Mg2+/Ca2+ molar ratio is high, it can be assumed that the beta-sheet structure of poly-L-aspartate plays an important role in determining the calcite crystallization in the presence of magnesium ions.