When calcium carbonate crystallizes from a mixed solution of calcium chloride and sodium hydrogen carbonate in ordinary water, the crystal size decreases with increasing strength of the magnetic field. This means that either the field accelerates nucleation or increases the rate of growth of nuclei, so that a larger fraction survives Ostwald ripening. At high pH, i.e. precipitated with sodium carbonate, or when heavy water is used as solvent, there is no effect of the field. Therefore, proton transfer is the rate determining step, and quantum effects (Pauli exclusion principle) are important. The magnetic field influences proton spin relaxation. (C) 2004 Elsevier B.V. All rights reserved.