Echinoderms:
The defining characteristic of echinoderm skeletons is the presence of plates (ossicles) composed of porous single crystals of Mg-calcite. Disarticulation of these skeletons releases these plates as sand-size debris. Many echinoderm fragments display distinctive geometric shapes related to the pentameral symmetry of these organisms or to shapes related to structures such as stems and spines. In transmitted light microscopy, distinctive features of echinoderm fragments are their shapes, a 'dusty' appearance related to mineral infillings of pores, and the unit extinction.