Ultrastructural and microanalytical results from echinoderm calcite: Implications for biomineralization and diagenesis of skeletal material

Magnesian calcite skeletal elements of the modern crinoid echinoderm Neocrinus blakei were studied using high resolution TEM, high voltage TEM and STEM microanalysis. Unlike inorganic magnesian calcites which are compositionally heterogeneous, magnesium in these skeletal calcites is homogeneous to at least the 0.1 μm level. While a mosaic structure exists in echinoderm calcite, high voltage TEM reveals the absence of defects or dislocation features which should exist as a consequence of the structure. By comparison, inorganic magnesian calcites show a plethora of defects and dislocation features. High resolution lattice fringe images of the echinoderm calcite exhibit a kinking of fringes between mosaic domains, the boundaries of which are largely coherent. Large scale dislocation structures are not observed. Such a 'stressed' lattice structure, if pervasive, explains conflicting observations concerning the 'single crystal' or 'polycrystalline aggregate' nature of echinoderm calcite. The microstructural and microchemical data demonstrate strong organismal control of skeletal deposition in Echinodermata. Both ultrastructural and compositional heterogeneity/homogeneity should be assessed when determining the susceptibility of skeletal material to diagenetic change.