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The precipitation of calcium carbonate is studied as a function of the change of partial pressure of CO₂. The experimental results of the kinetics of CaCO₃ precipitation are reported. The influence of the magnesium ion in a solution initially containing 360 mg l⁻¹ of Ca²⁺ and containing increasing amounts of MgCl₂ • 6H₂O (from 0 to 1 080 mg l⁻¹ Mg²⁺) is reported on : — the nucleation time, — the initial growth rate, — the allotropie variety and the shape of the crystals. The magnesium ion moves the equilibrium system to the right and the aragonite form is precipitated. As for the crystals of calcite which remain, dosages by atomic absorption and X-rays show a calcite containing more and more magnesium as the concentration of Mg²⁺ in the solution increases.