A process for preparing a 3R1-type crystalline anionic clay comprising the steps of: a) preparing an aqueous precursor mixture comprising aluminum trihydrate or a thermally treated form thereof and a magnesium source, the magnesium source is milled before use or when present in the precursor mixture, b) aging the precursor mixture at temperatures in the range 30°-100° C. to obtain the crystalline clay product, and c) optionally shaping the product of step b). Milling of the magnesium source, either alone or in combination with the (thermally treated) aluminum trihydrate, results in a faster reaction and higher conversion to anionic clay. The resulting anionic clay can be obtained by simply drying the slurry retrieved from the reactor. There is no need for washing or filtering, and a wide range of ratios of Mg/Al in the reaction product is possible.