Figure 3a projected down the crystallographic c-axis allows comparison with the calcite structure and the different polygonal arrangement of calcium. Aragonite crystallizes in the orthorhombic space group Pmcn and is one of the members of an isostructural group that includes these other cations ( Gaines et al., 1997, pp. The aragonite structure also has alternate layers of carbonate groups and cations with the triangular CO 3 pointing in opposite directions along the c-axis, but the cations are in ninefold coordination catering to larger size elements such as strontium, barium, and lead. Polymorphism of minerals implies the same chemical composition but distinct crystal structure. The crystals in each layer develop at offset positions from those of the layer immediately below, and there is no crystallographic continuity between any two neighboring tablets.Īn equally important carbonate mineral is aragonite, the common polymorph of calcite. In bivalves, the tablets are arranged in a “brick-wall” type of configuration. 1997) that the crystallographic continuity with the stacks is achieved by tablets growing through pores within the organic matrix. In gastropods such as the red abalone, evidence now exists ( Schäffer et al. 3(b) within each stack, all the tablets have the same crystallographic orientation.

In gastropods and cephalopods, the tablets are arranged in stacks, as shown in Fig. The c axes of the aragonite tablets are aligned and perpendicular to the inner surface of the shell. This example is taken from the red abalone, Haliotis rufescens the shell of this gastropod has an inner aragonitic nacreous sublayer abutting onto an outer, calcitic prismatic sublayer (see Sect. The tablets have an irregular form, as shown in Fig. The nacreous microstructure consists of aragonite tablets, ∼0.4 μm thick and 5–10 μm wide ( Gregoire 1972, Laraia and Heuer 1990, Sarikaya and Aksay 1992).

Caribsea aragonite aragamax oolite