Biomineralization on crinoid echinoderms. Characterization of crinoid skeletal elements using TEM and STEM microanalysis.

Columnals of Neocrinus blakei, a modern species of stalked crinoid, were studied using a variety of analytical techniques. Analyses of the magnesium calcite of the crinoid stereom using powder X-ray diffraction and electron microprobe analysis yield a composition of Ca 88Mg 12C03. Scanning transmission electron microscopy (STEM) microanalytical data indicate that Mg incorporation into the calcite structure of the crinoid stereom is random and homogeneous to at least the 20 nm level. There appear to be no variations in composition at this level either within or between structural entities of the crinoid columnal stereom. TEM reveals a heterogeneity of contrast which may be due to incorporation of organic material or some other substance which is non-crystalline in character. Single-crystal X-ray diffraction data indicate that the individual skeletal plates are single crystals which yield diffuse and imperfect X-ray reflections due to a mosaic structure. Subsequent selected area electron diffraction (SAD) photographs via TEM, using various sizes of SAD apertures, indicate that the crystallites making up the mosaic structure are (order of magnitude) about 1.0 micrometer in size. The presence of mosaic structure in the single crystal skeletal elements may at least in part explain the lack of cleavage in fracture surfaces of echinoderm skeletal material. Based on these data, as well as data from skeletal elements of other deep water, stalked crinoids, we feel that these results may be applicable to crinoids in general, at least those existing in relatively constant temperature environments. The single-crystal nature of crinoid high magnesium calcite, and its remarkable homogeneity of composition suggest that a large "vital effect" (i. e., biologic control of skeletal deposition) mediates the mineralization process.