  Assemblies of nanocrystals with complex morphologies have solicited great
scientific interest. Controlling the formation of these self-assembled
structures has proved difficult. Here we present a novel transformation
reaction for transforming a single crystal of minerals into assemblies of
nanocrystals of substances with lower solubility without changing the overall
morphology. This study focusses on the transformation of rhombohedral calcite
crystals to assemblies of hydroxyapatite nanocrystals to control the
transformation process. This yielded particle assemblies of HAP nanoparticles
grossly preserving the original rhombohedral calcite morphology.
