title: Crystal nucleation and near-epitaxial growth in nacre.
abstract: Nacre is the iridescent inner lining of many mollusk shells, with a unique lamellar structure at the sub-micron scale, and remarkable resistance to fracture. Despite extensive studies, nacre formation mechanisms remain incompletely understood. Here we present 20-nm, 2°-resolution polarization-dependent imaging contrast (PIC) images of shells from 15 mollusk species, mapping nacre tablets and their orientation patterns. These data show where new crystal orientations appear and how similar orientations propagate as nacre grows. In all shells we found stacks of co-oriented aragonite (CaCO₃) tablets arranged into vertical columns or staggered diagonally. Near the nacre-prismatic (NP) boundary highly disordered spherulitic aragonite is nucleated. Overgrowing nacre tablet crystals are most frequently co-oriented with the underlying aragonite spherulites, or with another tablet. Away from the NP-boundary all tablets are nearly co-oriented in all species, with crystal lattice tilting, abrupt or gradual, always observed and always small (plus or minus 10°). Therefore aragonite crystal growth in nacre is near-epitaxial. Based on these data, we propose that there is one mineral bridge per tablet, and that "bridge tilting" may occur without fracturing the bridge, hence providing the seed from which the next tablet grows near-epitaxially.
authors.affiliation: Department of Physics, University of Wisconsin-Madison, 1150 University Avenue, Madison, WI 53706, USA.
authors.forename: Ian C
authors.initials: IC
authors.lastname: Olson
authors.forename: Adam Z
authors.initials: AZ
authors.lastname: Blonsky
authors.forename: Nobumichi
authors.initials: N
authors.lastname: Tamura
authors.forename: Martin
authors.initials: M
authors.lastname: Kunz
authors.forename: Boaz
authors.initials: B
authors.lastname: Pokroy
authors.forename: Carl P
authors.initials: CP
authors.lastname: Romao
authors.forename: Mary Anne
authors.initials: MA
authors.lastname: White
authors.forename: Pupa U P A
authors.initials: PU
authors.lastname: Gilbert
mesh_terms: D060105:Animal Shells / Q000737:chemistry / Q000648:ultrastructure*; D000818:Animals; D020342:Finite Element Analysis; D007091:Image Processing, Computer-Assisted; D008974:Mollusca; D060734:Nacre / Q000032:analysis* / Q000737:chemistry; D056951:Photoelectron Spectroscopy / Q000379:methods
chemical_list: D060734:Nacre
keywords: Aragonite; Biomineral; Bridge tilting; Diffraction; Epitaxial; Epitaxy; Low-angle grain boundary; Mesocrystal; Micro-X-ray; Mollusca; PEEM; PIC-mapping; Tablet; XANES