A process for preparing a composition including synthetic mineral particles, in which a hydrogel which is a precursor of the synthetic mineral particles is prepared via a coprecipitation reaction between at least one compound including silicon, and at least one compound including at least one metal element, characterized in that the coprecipitation reaction takes place in the presence of at least one carboxylate salt of formula R2—COOM′ in which: —M′ denotes a metal chosen from the group made up of Na and K, and —R2 is chosen from H and alkyl groups including fewer than 5 carbon atoms. A composition including synthetic mineral particles which is obtained by such a process is also described.