The structure of aragonite

Abstract

Aragonite has the same composition as calcite, both being crystalline forms of calcium carbonate, CaCO 3 . A comparison of the structures of these two crystals is interesting, because they represent one of the most striking cases of dimorphism amongst minerals. Calcite forms rhombohedral crystals, whose structure has been analysed by several workers. Aragonite crystals have orthorhombic symmetry, and are generally assigned to the holo-symmetric class. According to Miers (Mineralogy, Part II, Section XIV) etching experiments indicate that the symmetry may really be polar. The analysis which is described below places aragonite in the holosymmetric class of the orthorhombic system, and though it is always possible that the actual structure may have a lower symmetry owing to some distortion which is not revealed by X-ray examination, it would appear that this distortion is very small. Crystals of aragonite have an obviously pseudohexagonal character. This is revealed by the axial ratios, by the common terms of the crystals, and by their twinning. The ' c ' axis is tie pseudohexagonal axis. The faces (010), (110), marked 'B' and ' m ' in fig. 1a, build up a form which is very nearly hexagonal, the angle (1 - 10) : (110) being 63° 48' instead of the 60° required by hexagonal symmetry. Simple crystals are rare, as thinning about the plane (110) is nearly always present. Repeated twinning about this plane results in interpenetrating groups which are almost indistinguishable from simple hexagonal crystals (fig. 1b).