Precursor Mineral Phases of Forming Mollusk Shell Nacre: A Study of Hydrated Samples

Mollusk shell nacre is an advanced functional material composed of layers of flat tablet‐shaped crystals of aragonite. The formation and growth of individual tablets can be studied on the forming surface. Such studies reveal that a disordered precursor phase of hydrated and anhydrous amorphous calcium carbonate (ACC) is first formed which then transforms into a single crystal of aragonite. Here, the formation of nacre is studied from living specimens of the bivalve Brachidontis pharaonis , using an environmental scanning electron microscope (ESEM) and cryo‐SEM under hydrated conditions. It is shown that the forming tablets are rounded ellipsoids that form inside an organic gel. The ellipsoids have an aspect ratio of ≈2 irrespective of size, as in aragonite. Etching the surface in the ESEM reveals rounded and triangular etch pits, indicating the presence of ACC and aragonite domains. Spherical particles embedded in a gel‐like organic material that is observed as they mature decrease in size presumably due to dehydration. These diverse observations lead to the conclusion that the forming ellipsoids are composed of co‐existing hydrated ACC phases, nascent aragonite, and crystalline aragonite. The type and the proportions of different phases are changed if care is not taken to maintain this sample under hydrated conditions.