The invention relates to the selection of a natural calcium carbonate crushed to a high degree of fineness, possibly treated with one or more fatty acids or one or more of their salts or mixtures thereof, and its use as a rheology regulator for polymeric compositions.The specific surface area is 14 to 30 m.sup.2/g measured according to the BET method to ISO 4652, and the oil absorption is greater than 16 measured according to ISO 787-V (Rub-out method).The conditions of mixing of the constituents and manufacture are improved, as well as the properties of the final product.