The invention concerns a pigment, filler or mineral containing a natural calcium carbonate, treated with one of more providers of H.sub.3O.sup.+ ions and gaseous CO.sub.2, allowing a reduction in the weight of paper for a constant surface area without loss of physical properties when it is used as a pigment or coating filler for the said paper. In particular, the invention concerns a pigment, filler or mineral containing a natural calcium carbonate or dolomite or mixtures of talc and calcium carbonate, of kaolin and carbonate or carbonate alone or in combination with natural and/or synthetic fibres or similar, treated with one or more medium-strong or strong providers of H.sub.3O.sup.+ ions in the presence of gaseous CO.sub.2.Applications are particularly in the paper industry, obtaining in particular good sheet properties i.e. a reduction its weight for a given surface area.