title: Biological control of crystallographic architecture: hierarchy and co-alignment parameters.
abstract: Mytilus edulis prismatic calcite and nacre layers exhibit a crystallographic structural hierarchy which differs substantially from the morphological hierarchy. This makes these biomaterials fundamentally different from classical crystalline materials. Morphological building units are defined by their surrounding organic matrix membranes, e.g. calcite fibers or nacre tablets. The crystallographic building units are defined by crystallographic co-orientation. Electron backscatter diffraction quantitatively shows how crystallographic co-orientation propagates across matrix membranes to form highly co-oriented low-mosaic composite-crystal grains, i.e. calcite fiber bundles with an internal mosaic spread of 0.5° full width at half maximum (FWHM) or nacre towergrains with an internal mosaic spread of 2° FWHM. These low-mosaic composite crystals form much larger composite-crystal supergrains, which exhibit a high mosaicity due to misorientations of their constituting calcite fiber bundles or nacre towergrains. For the aragonite layer these supergrains nucleate in one of three aragonite {110} twin orientations; as a consequence the nacre layer exhibits a twin-domain structure, i.e. the boundaries of adjacent supergrains exhibit a high probability for misorientations around the aragonite c-axis with an angle near 63.8°. Within the supergrains, the constituting towergrains exhibit a high probability for misorientations around the aragonite a-axis with a geometric mean misorientation angle of 10.6°. The calcite layer is composed of a single composite-crystal supergrain on at least the submillimeter length scale. Mutual misorientations of adjacent fiber bundles within the calcite supergrain are mainly around the calcite c-axis with a geometric mean misorientation angle of 9.4°. The c-axis is not parallel to the long axis of the fibers but rather to the (107) plane normal. The frequency distribution for the occurrence of misorientation angles within supergrains reflects the ability of the organism to maintain homoepitaxial crystallization over a certain length scale. This probability density is distributed log-normally which can be described by a geometric mean and a multiplicative standard deviation. Hence, those parameters are suggested to be a numerical measure for the biological control over crystallographic texture.
authors.affiliation: Ludwig Maximilian University Munich, GeoBioCenter and Department of Earth- and Environmental Sciences, Theresienstrasse 41, D-80333 Munich, Germany. Electronic address: bernd.maier@lmu.de.
authors.forename: B J
authors.initials: BJ
authors.lastname: Maier
authors.affiliation: Ludwig Maximilian University Munich, GeoBioCenter and Department of Earth- and Environmental Sciences, Theresienstrasse 41, D-80333 Munich, Germany.
authors.forename: E
authors.initials: E
authors.lastname: Griesshaber
authors.affiliation: Ludwig Maximilian University Munich, GeoBioCenter and Department of Earth- and Environmental Sciences, Theresienstrasse 41, D-80333 Munich, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Alexa
authors.affiliation: Central Facility for Electron Microscopy, University of Ulm, Albert Einstein Allee 11, D-89069 Ulm, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Ziegler
authors.affiliation: Oxford Instruments, Halifax Road, High Wycombe HP12 3SE, UK.
authors.forename: H S
authors.initials: HS
authors.lastname: Ubhi
authors.affiliation: Ludwig Maximilian University Munich, GeoBioCenter and Department of Earth- and Environmental Sciences, Theresienstrasse 41, D-80333 Munich, Germany.
authors.forename: W W
authors.initials: WW
authors.lastname: Schmahl
mesh_terms: D060105:Animal Shells / Q000737:chemistry*; D000818:Animals; D002119:Calcium Carbonate / Q000737:chemistry; D003461:Crystallography; D004583:Electrons; D008855:Microscopy, Electron, Scanning; D049880:Mytilus edulis / Q000737:chemistry*; D060734:Nacre / Q000737:chemistry; D012399:Rotation
chemical_list: D060734:Nacre; D002119:Calcium Carbonate
keywords: Biomineralization; Bivalve; Electron backscatter diffraction; Hierarchical architecture; Nacre