Aragonite is a polymorph of calcium carbonate, CaCO3. It was named after the province of Aragon, Spain, where it occurs as pseudo-hexagonal twins.
Calcite is the more common polymorph encountered in the environment. Aragonite is classified in the orthorhombic crystal system whereas calcite is variously classified as belonging to the in the triclinic (Glazer, 1987), hexagonal/trigonal (Berry, Mason, Dietrich, 1983) or rhombohedral (Hurlbut, 1970) crystal systems.
Other polymorphs of CaCO3 include the unstable vaterite (hexagonal) which occurs in gastropods and carbide dumps, as well as the high temperature and pressure Calcite-IV and Calcite-V which are not stable in near-surface environments.
Aragonite is harder and more dense than calcite, and has a characteristic acicular habit. It has two cleavage angles whereas calcite has the familiar three perfect cleavage angles.
Aragonite is mostly encountered in the marine environment, often the product of biological chemical activity such as the development of an organism's shell. When the organism dies, shell aragonite will in time revert to calcite in the near surface environment in the presence of fresh water. Outside of biological influences, the deposition of aragonite is generally considered the product of high pressure and / or high temperature on calcite, or chemical influence.
Given its instability in the freshwater environment, deposition of aragonite in the spelean environment is unexpected. In the spelean environment, calcite deposits are frequently coloured by humic acids whereas aragonite tends to be either white or coloured by metal impurities such as copper.