Process for preparing a mineral composition based on a mixed solid phase of calcium and magnesium carbonates, comprising a preparation, in an aqueous phase, of a suspension of a solid phase containing at least one calcium compound selected from calcium hydroxide, calcium carbonate and mixtures thereof and at least one magnesium compound selected from magnesium oxide, magnesium hydroxide, at least partially hydrated dolomite, and mixtures thereof, the weight of said solid phase being between 5% and 15% of the total weight of the suspension, a carbonation of said solid phase in suspension by injecting a gas containing CO2 into said suspension that is heated at a temperature of 55° C. to 90° C., at a flow rate of CO2 of 2.5 to 15 dm3/min/kg of said solid phase of the suspension, with a reduction of the pH of the suspension to a value of less than 9 and with a stabilization of the electrical conductivity of the suspension, the carbonation being stopped as soon as this stabilization of electrical conductivity is
