The morphology of vaterite aggregates of framboidal structure built up of smaller particles is described. It is shown that the transformation into calcite occurs partly within the framboidal vaterite aggregate, leading to much higher transformation rates compared to the transformation of solid non-framboidal vaterite spherulites. The transformation from inside the framboidal vaterite leads to the formation of single crystal calcite rhombs as observed for the transformation of non-framboidal vaterite spherulites. (c) 2005 Elsevier B.V. All rights reserved.