The formation of CaCO3 mineral scale is a persistent and expensive problem in oil and gas production. The magnesium ion, present in formation and injection waters in downhole conditions, is a key determinant in CaCO3 scale formation. In the work reported herein, the kinetics of calcium carbonate scale formation is studied both in the bulk solution and on a metal surface. The effect of the Mg2+ ion on the scale formed on the metal surface has been studied systematically in this paper including aspects of the kinetics, morphology, and Mg/Ca ratio of the deposit on the metal surface. The Mg2+ ion adsorbs onto the deposited crystals and the ratio of Mg2+ in the deposit formed on the metal surface is proportional to the ratio of Mg/Ca in the scaling water. The distribution coefficient in surface deposition and in bulk solution is a constant and independent of the concentration of Mg2+ ions in the bulk solution. Simultaneously, the Mg2+ ion accelerates the crystal transformation from vaterite to calcite and adsorbs on the surface of vaterite and calcite causing an increase in surface roughness in addition to distortion of crystals. (c) 2006 Elsevier Ltd. All rights reserved.