Electrodeposition of CaCO3 was studied under the influence of magnesium ions present in a carbonically pure water. The investigation was performed using electrochemical, gravimetric and optical methods. The chronoamperometric measurements confirmed the inhibiting property of Mg2+ when its concentration is higher than 120 mg L−1 in a solution containing 160 mg L−1 of Ca2+. The optical technique led the nucleation-growth process to be accessed by means of an optical microscope positioned behind a transparent electrode. The increase of Mg2+ concentration changed drastically the calcite morphology. At 360 mg L−1 of Mg2+, the calcite morphology was optically amorphous but the Raman spectrum confirmed its structure. The crystal growth was recorded in situ and the image analysis software characterised the nucleation process as well as the growth rate of the crystals. It allowed the influence of the Mg2+ ions on the crystallisation process of CaCO3 to be quantified.