The invention relates to a pigment composition in the form of an aqueous dispersion or slurry comprising: (a) porous aggregates formed by aggregation of colloidal primary particles of silica, aluminosilicate or a mixture thereof in an aqueous sol, said porous aggregates having a mean diameter from about 0.03 to about 25 [mu]m, and, (b) extender particles where the mean size of at least one dimension is larger than the mean diameter of the porous aggregates, wherein the weight ratio of porous aggregates to extender particles is from about 0.01:1 to about 3:1. The invention further relates to aprocess for its production, a compositionfor coating paper or paper board and a process for its production, and a process for coating paper or paper board and paper or paper board obtainable by the process.