Ionotropic gelling of alginate, a natural block polysaccharide with carboxylic functions, allows to form hydrogel microspheres with 98% porosity and more than 10% cation content (w/w). Supercritical drying preserves the dispersion of the get and the resulting aerogels have specific surface as high as 500 m(2) g(-1). In this communication, it is shown that the thermal degradation of alginate aerogels allows to form aggregated nanocrystals of the mineral phase corresponding to the cation used in gelling. The aggregates of nanocrystals of carbonate and oxide retain the morphology of the parent gel particles and a significant fraction of their porosity. (c) 2007 Elsevier B.V. All rights reserved.