ARAGONITE, one of the mineral forms of calcium car bonate the other form being the more common mineral calcite. It crystallizes in the orthorhombic system, and the crystals are either prismatic or acicular in habit. Simple crystals are, however, rare; twinning on the prism planes being a charac teristic feature of the mineral. This twinning is usually repeated often on the same plane and gives rise to striations on the terminal faces of the crystals; often, also, three crystals are twinned together on two of the prism planes of one of them, producing an apparently hexagonal prism. The mineral is colour less, white or yellowish, transparent or translucent, has a vitreous lustre, and, in fact, is not unlike calcite in general appearance. It may, however, always be readily distinguished from calcite by the absence of any marked cleavage, and by its greater hardness (H.=31-4) and specific gravity (2.93) ; further, it is optically biaxial, whilst calcite is uniaxial.
The mineral was first found as reddish twinned crystals with the form of six-sided prisms at Molina in Aragon, Spain, where it occurs with gypsum and small crystals of ferruginous quartz in a red clay. It is from this locality that the mineral takes its name. Fine groups of crystals of the same habit are found in the sulphur deposits of Girgenti in Sicily. Fibrous forms are also common. A peculiar coralloidal variety known as flosferri ("flower of iron") consists of radially arranged fibres : mag nificent snow-white specimens of this variety have long been known from the iron mines of Eisenerz in Styria. The calcareous secretions of many groups of invertebrate animals consist of aragonite (calcite is also common) ; pearls may be specially cited as an example.
Aragonite is the more unstable of the two modifications of calcium carbonate. A crystal of aragonite when heated becomes converted into a granular aggregate of calcite individuals : altered crystals of this kind (paramorphs) are not infrequently met with in nature, whilst in fossil shells the original nacreous layer of aragonite has invariably been altered to calcite. The thermal springs of Carlsbad deposit spherical concretions of aragonite, forming masses known as pisolite or Sprudelstein.