AMORPHOUS-TO-CRYSTALLINE TRANSITIONS IN CALCITE BIOMINERALS

Calcitic biominerals such as the outer layer of mollusk shells, or sea urchin tests, spines, teeth, and spicules are composites of calcite and organic molecules. In diffraction and spectromicroscopy experiments they behave as single crystals of calcite. Yet they do not exhibit the familiar rhombohedral crystal habit, and they do not cleave along the (104) calcite planes.

Their fascinating, intricate, and highly specialized morphologies are possible because of an amorphous calcium carbonate (ACC) precursor phase. Here we present evidence of two ACC precursors in sea urchin spicules and teeth (1, 2), determine the sequence of phases, and their energetics (3). Twenty-nanometer resolution analysis of residual phases inside fresh sea urchin spicules highlights the difference between one biologically controlled-transition (hydrated ACC→anhydrous ACC), and one thermodynamically driven transition (anhydrous ACC→calcite).

1. Politi Y, Metzler RA, Abrecht M, Gilbert B, Wilt FH, Sagi I, Addadi L, Weiner S, & Gilbert PUPA (2008) "Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule" Procs Natl Acad Sci USA 105, 17362-17366.

2. Killian CE, Metzler RA, Gong YT, Olson IC, Aizenberg J, Politi Y, Addadi L, Weiner S, Wilt FH, Scholl A, et al. (2009) "The mechanism of calcite co-orientation in the sea urchin tooth" J Am Chem Soc 131, 18404-18409.

3. Radha AV, Forbes TZ, Killian CE, Gilbert PUPA, & Navrotsky A (2010) "Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate" Procs Natl Acad Sci USA in press.