An overgrowth from an echinoderm fragment is a single crystal of calcite in lattice continuity with the calcite crystal of the host. The development history of echinoderm overgrowths during cementation has been elucidated by the identification of iron-bearing and iron-free calcites within the overgrowths by employing staining techniques.
The iron-bearing parts of the overgrowths described do not stain uniformly for iron. Instead they exhibit a succession of light and dark stained sub-zones, which represent the shapes of the overgrowths during their development.
The pattern of zoning around basal sections of crinoid ossicles is comparatively simple. The innermost zones follow the outlines of the ossicles. Later zones adopt first quasi-hexagonal outlines with curved sides, and then true hexagonal zones of prismatic habit. Such basal sections of ossicles give well-centred conoscope images and are therefore, oriented perpendicular to the c-axis directions of their calcites.
Vertical sections through overgrowths show a more complex zoning: above and below the articulating surfaces of ossicles the zones are wide, and have a marked zig-zag outline, with the axes of the spires parallel to the vertical axes of the ossicles and parallel to the c-axes of the crystals. Along the sides of the ossicles the zones are narrow and straight. The multiple terminations in the c-axis direction reduce in number away from the surface of the ossicle and, if the original pore-spaces are sufficiently large, ultimately yield a single scalenohedron-type termination. The multiple terminations have their origins on the articulating surfaces of the ossicles. In life these surfaces were perforated by the entrances to the numerous minor canals which ramify the ossicles. The multiplicity of terminations resulted from more rapid crystal growth from the surface of the ossicle between canals (c-axis direction) than from the surfaces of the canals themselves (direction perpendicular to the c-axis).