Aragonite-hydroxyapatite conversion in gastropod (abalone) nacre

From studies of geological and coral skeleton aragonite, it was previously proposed that aragonite converts hydrothermally to hydroxyapatite (HA) in p hosphate solution by a solid-state topotactic ion-exchange reaction in whic h the HA retains the orientation of the original aragonite. The present inv estigation of this reaction in gastropod shell nacre suggests instead that conversion for this biomineral proceeds via dissolution-recrystallization, with the preferred orientation of the product determined by the flux direct ion of the solution through the material. Conversions of various CaCO3 biom inerals in aqueous (NH4)(2)HPO4 at 140-260 degrees C show that for gastropo d nacre alone, sample form and internal ultrastructure are largely preserve d and a unique preferred orientation arises: with reaction time, HA nanocry stallites are increasingly oriented with their c-axes perpendicular to the c-axis of the original aragonite. In contrast, other CaCO3 biominerals eith er disintegrate to HA powder or contain HA crystallites with random orienta tion or the orientation of the original aragonite. We suggest that the orga nic matrix of gastropod nacre channels the reaction solution through the ma terial vertically between stacks of aragonite tablets so that sample form a nd internal ultrastructure are preserved and the HA. predominantly adopts a perpendicular c-axis orientation. Other properties of HA-converted gastrop od nacre are also discussed.