ARAGONITE  , one of the

mineral forms of calcium carbonate (CaCO3), 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, how-ever, rare; twinning on the prism planes (M in the figures) being a characteristic feature of the mineral (fig . I) . Thistwinning is usually often repeated on the same plane (fig . 2), and gives rise to striations on the terminal faces (k) 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 colourless, 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.=3a-4) and specific gravity (2.93); further, it is optically biaxial, whilst calcite is uniaxial . It is brittle and has a subconchoidal fracture; on a fractured surface the lustre is decidedly resinous in character . 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, which was originally spelt arragonite . Fine groups of crystals of the same habit are found in the sulphur deposits of Girgenti in Sicily; also at Herrengrund near Neusohl in Hungary .

At many other localities the mineral takes the form of radiating groups of acicular crystals, such as those from the

haematite mines of west Cumberland: beautiful feathery forms have been found in a limestone cave in the Transvaal . Fibrous forms are also common . A peculiar coralloidal variety known as flosferri (" flower of iron ") consists of radially arranged fibres: magnificent 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 a member of the isomorphous group of minerals comprising witherite (BaCO3), strontianite (SrCO3), cerussite (PbCO3) and bromlite ((Ba, Ca)CO3); and crystals of aragonite sometimes contain small amounts of strontium or lead . A-variety known as ta.rnowitzite, from Tarnowitz in Silesia, contains about 5 % of lead carbonate . 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 . From a solution of calcium carbonate in water containing carbon dioxide crystals of calcite are deposited at the ordinary temperature, but from a warm solution aragonite crystallizes out . The thermal springs of Carlsbad deposit spherical concretions of aragonite, forming masses known as pisolite or Sprudelstein . (L . J .