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ARAGONITE

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Originally appearing in Volume V02, Page 314 of the 1911 Encyclopedia Britannica.
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ARAGONITE , one of the See also:

mineral forms of See also:calcium carbonate (CaCO3), the other See also:form being the more See also:common mineral See also:calcite. It crystallizes in the orthorhombic See also:system, and the crystals are either prismatic or acicular in See also:habit. See also:Simple crystals are, how-ever, rare; twinning on the See also:prism planes (M in the figures) being a characteristic feature of the mineral (fig. I). Thistwinning is usually often repeated on the same See also: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, See also:white or yellowish, transparent or translucent, has a vitreous lustre, and, in fact, is not unlike calcite in See also:general See also:appearance. It may, however, always be readily distinguished from calcite by the See also: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 See also:surface the lustre is decidedly resinous in See also:character. The mineral was first found, as reddish twinned crystals with the form of six-sided prisms, at See also:Molina in See also:Aragon, See also:Spain, where it occurs with See also:gypsum and small crystals of ferruginous See also:quartz in a red See also:clay. It is from this locality that the mineral takes its name, which was originally spelt arragonite. See also:Fine See also:groups of crystals of the same habit are found in the See also:sulphur deposits of See also:Girgenti in See also:Sicily; also at Herrengrund near Neusohl in See also:Hungary.

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

haematite mines of See also:west See also:Cumberland: beautiful feathery forms have been found in a See also:limestone See also:cave in the See also:Transvaal. Fibrous forms are also common. A See also:peculiar coralloidal variety known as flosferri (" See also:flower of See also:iron ") consists of radially arranged See also:fibres: magnificent See also:snow-white specimens of this variety have See also:long been known from the iron mines of See also:Eisenerz in See also: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 See also:group of minerals comprising See also:witherite (BaCO3), See also:strontianite (SrCO3), See also:cerussite (PbCO3) and See also:bromlite ((Ba, Ca)CO3); and crystals of aragonite sometimes contain small amounts of See also:strontium or See also:lead. A-variety known as ta.rnowitzite, from Tarnowitz in See also: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 See also:kind (paramorphs) are not infrequently met with in nature, whilst in fossil shells the See also:original nacreous layer of aragonite has invariably been altered to calcite. From a See also:solution of calcium carbonate in See also:water containing See also:carbon dioxide crystals of calcite are deposited at the See also:ordinary temperature, but from a warm solution aragonite crystallizes out. The thermal springs of See also:Carlsbad See also:deposit spherical concretions of aragonite, forming masses known as pisolite or Sprudelstein. (L. J.

End of Article: ARAGONITE

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