Nucleation and growth of calcite on native versus pyrolyzed oyster shell folia

The thin sheets of calcite, termed folia, that make up much of the shell of an oyster are covered by a layer of discrete globules that has been propos ed to consist of agglomerations of protein and mineral. Foliar fragments, t reated at 475 degrees C for 36 h to remove organic matter, were imaged by a tomic force microscopy (AFM) as crystals grew on the foliar surfaces in art ificial seawater at calcite supersaturations up to 52-fold. Crystals were a lso viewed later by scanning electron microscopy. After pyrolysis, the foli ar globules persisted only as fragile remnants that were quickly washed awa y during AFM imaging, revealing an underlying morphology on the foliar lath s of a tightly packed continuum of nanometer-scale protrusions. At intermed iate supersaturations, crystal formation was seen immediately almost everyw here on these surfaces, each crystal having the same distinctive shape and orientation, even at the outset with crystals as small as a few nanometers. In contrast, nucleation did not occur readily on non-pyrolyzed foliar surf aces, and the crystals that did grow, although slowly at intermediate super saturations, had irregular shapes. Possible crystallographic features of fo liar laths are considered on the basis of the morphology of ectopic crystal s and the atomic patterns of various surfaces. A model for foliar lath form ation is presented that includes cycles of pulsed secretion of shell protei n, removal of the protein from the mineralizing solution upon binding to mi neral, and mineral growth at relatively high supersaturation over a time fr ame of about 1 h for each turn of the cycle.