A process for preparing a mineral composition based on a mixed solid phase of calcium and magnesium carbonates, characterised in that it comprises - i) preparing, in an aqueous phase, 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, dolomite at least partially hydrated, and mixtures thereof, the weight of said solid phase being between 5 and 15% of the total weight of the suspension, excluding suspension of only dolomite, hydrated or not, - ii) carbonating, in a single step, said solid phase in suspension by injection of a gas containing CO 2 into said suspension heated at a temperature of 55 to 90°C at a CO 2 flow rate of 2.5 to 15 dm 3 /min/kg of said solid phase of the suspension, with a decrease in the pH of the suspension to a value of less than 9 and with a stabilization of the electrical conductivity of the suspension, the carbonating being stopped as soon as this stabilization of electrical conductivity is observed and, - iii) obtaining a mixed solid phase of synthetic calcium and magnesium carbonates formed of a crystallized calcic portion and a plate-like crystallized magnesium portion, the crystals of the calcic portion and those of the magnesium portion being aggregated in the form of composite aggregates, these aggregates being themselves at least partially agglomerated in the form of agglomerates, said calcic portion comprising at least one carbonate selected from the group consisting of calcite and mixtures of calcite and aragonite, said magnesium portion comprising the hydromagnesite in plate-like form, said mixed solid phase of carbonates of said mineral composition having a bulk density of less than or equal to 250 kg/m 3, and greater than or equal to 80kg/m 3; measured in accordance with standard EN 459.2, and a Ca/Mg molar ratio greater than 1.2 and equal to or less than 4.0. - i) preparing, in an aqueous phase, 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, dolomite at least partially hydrated, and mixtures thereof, the weight of said solid phase being between 5 and 15% of the total weight of the suspension, excluding suspension of only dolomite, hydrated or not, - ii) carbonating, in a single step, said solid phase in suspension by injection of a gas containing CO 2 into said suspension heated at a temperature of 55 to 90°C at a CO 2 flow rate of 2.5 to 15 dm 3 /min/kg of said solid phase of the suspension, with a decrease in the pH of the suspension to a value of less than 9 and with a stabilization of the electrical conductivity of the suspension, the carbonating being stopped as soon as this stabilization of electrical conductivity is observed and, - iii) obtaining a mixed solid phase of synthetic calcium and magnesium carbonates formed of a crystallized calcic portion and a plate-like crystallized magnesium portion, the crystals of the calcic portion and those of the magnesium portion being aggregated in the form of composite aggregates, these aggregates being themselves at least partially agglomerated in the form of agglomerates, said calcic portion comprising at least one carbonate selected from the group consisting of calcite and mixtures of calcite and aragonite, said magnesium portion comprising the hydromagnesite in plate-like form, said mixed solid phase of carbonates of said mineral composition having a bulk density of less than or equal to 250 kg/m 3, and greater than or equal to 80kg/m 3; measured in accordance with standard EN 459.2, and a Ca/Mg molar ratio greater than 1.2 and equal to or less than 4.0.