aragonite

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One of three naturally occurring mineral forms of calcium carbonate (CaCO₃). The other forms (or polymorphs) are the abundant mineral calcite and the relatively rare mineral vaterite. Still other forms of calcium carbonate are known, but only as products of laboratory experiments. The name aragonite comes from Aragon, a province in Spain where especially fine specimens occur. See also Calcite.

Aragonite has an orthorhombic crystal structure in which layers of calcium (Ca) atoms alternate with layers of offset carbonate (CO₃) groups. A common crystallographic feature of aragonite is twinning, in which regions of crystal are misoriented as though they were mirror images of each other. This can give rise to a pseudohexagonal symmetry which is readily identified in large crystals (see illustration). Aragonite crystals are usually colorless or white if seen individually; however, aggregates of small crystals may exhibit different colors. Most aragonites are nearly pure calcium carbonate; however, small amounts of strontium (Sr) and less commonly barium (Ba) and lead (Pb) may be present as impurities.

Aragonite. Pseudohexagonally twinned specimen from Girgenti, Sicily. (American Museum of Natural History specimens)

At the low temperatures and pressures found near the Earth's surface, aragonite is metastable and should invert spontaneously to calcite, which is stable at these conditions. This, in part, explains why calcite is far more abundant than aragonite. However, at low temperatures the transformation of aragonite to calcite effectively occurs only in the presence of water, and aragonite may persist for long periods of geologic time if isolated from water. Increased temperature also promotes the transformation to calcite. Despite being metastable, aragonite rather than calcite is sometimes the favored precipitate from certain solutions, such as seawater, in which magnesium (Mg) ions inhibit precipitation of calcite.

Aragonite occurs most abundantly as the hard skeletal material of certain fresh-water and marine invertebrate organisms, including pelecypods, gastropods, and some corals. The accumulated debris from these skeletal remains can be thick and extensive, usually at the shallow sea floor, and with time may transform into limestone. Most limestones, however, contain calcite and little or no aragonite. The transformation of the aragonite to calcite is an important step in forming limestone and proceeds by the dissolution of aragonite followed by the precipitation of calcite in the presence of water. This process may take more than 100,000 years. See also Limestone.

Other occurrences of aragonite include cave deposits (often in unusual shapes) and weathering products of calcium-rich rocks.

Environment

Deposits around warm and hot springs, some ore veins, and sedimentary formations. Also secreted by mollusks.

Single crystals, most often long slender needles, also in tabular plates. Trillings (three intergrown individuals) common, looking like short hexagonal prisms or even hexagonal plates. Re-entrant angle visible in fresh, sharp crystals in the center of each apparent prism of these pseudohexagons. Striations on the apparent base suggest the three individuals. Also in crusts and stalactites.

Colorless, white, pale violet, light blue, or light yellow. Luster vitreous; hardness 3Ɖ-4; specific gravity 2.9-3.0; fracture subconchoidal; cleavage poor, parallel to prism and side pinacoids. Brittle; translucent to transparent; commonly fluorescent and phosphorescent.

Calcium carbonate, like calcite (56.0% CaO, 44.0% CO ₂ ) with strontium (mossottite), lead (tarnowitzite), and sometimes zinc (nicholsonite).

Dissolves with bubbles in cold hydrochloric acid, like calcite. Strong solutions give a precipitate of white needles or granules of calcium sulfate when sulfuric acid is added to the hydrochloric acid solution.

Recognizable as a carbonate by its effervescence in hydrochloric acid; as the aragonite group by lack of a conspicuous cleavage; as a calcium carbonate by its sulfate precipitate (only in concentrated solutions) and by the weak red-orange coloration of the flame, and aragonite by the pink-violet color it assumes when a powder is boiled in a test tube with a cobalt nitrate solution (calcite stays white). Also, it crumbles more on heating than calcite does and loses its fluorescence, whereas calcite tends to remain intact and to gain fluorescence.

Aragonite is far rarer than calcite, apparently being deposited from warmer solutions than calcite (but not too warm). It is found around hot springs in crusts and thick beds (which are cut as onyx marble). Fine needlelike elongated and pointed crystals were found in the Cumberland (England) iron mines. Good pseudohexagonal trillings common in the Sicilian sulfur mines. Also found as isolated six-sided rods and plates in central Spain, the former in crystal clusters with sulfur, gypsum, and celestite, the latter with gypsum in red and green shale beds often colored by red-stained clay inclusions. Trillings and sandy calcite pseudomorphs of this type have been found at Chaves (near Roswell, New Mexico), in s. France, Spain, Morocco, and at the Matsushiro gypsum mines, Ota City, Shimane Prefecture, Honshu, Japan, in medium to giant spherical crystal balls. Violet pseudohexagonal rods cross gas cavities in basalt at Suzu Gun, Ishikawa Prefecture, in the Noto Peninsula, Japan.

A white coral-like branching growth, called flos ferri (iron flowers), has been found in Austrian iron mines (and lesser examples in New Mexico and Mexico).

The iridescent inner surfaces of shells (and pearls) are composed of aragonite secreted by mollusks. The light violet cast that is common in some aragonites (in cores of those from green shale in Spain and from Japanese basalt) seems to be due to iron plus a natural irradiation. It fades from crystals exposed on outcrops but can be restored (and the balance made greenish) by irradiation in the laboratory.

• Mineral compounds

Aragonite
Aragonite from Salsignes Mine, Aude department, France Size: 30x30x20 cm
General
Category	Carbonate mineral
Chemical formula	CaCO3
Strunz classification	05.AB.15
Crystal symmetry	Orthorhombic (2/m 2/m 2/m) - dipyramidal
Unit cell	a = 4.95 Å, b = 7.96 Å, c = 5.74 Å; Z = 4
Identification
Color	White, red, yellow, orange, green , blue and brown
Crystal habit	Pseudohexagonal, prismatic crystals, acicular, columnar, globular, reniform, pisolitic, coralloidal, stalactitic, internally banded
Crystal system	Orthorhombic
Twinning	Polysynthetic parallel to {100} cyclically on {110}
Cleavage	Distinct on {010}, imperfect {110} and {011}
Fracture	Subconchoidal
Tenacity	Brittle
Mohs scale hardness	3.5-4
Luster	Vitreous, resinous on fracture surfaces
Streak	White
Diaphaneity	Translucent to transparent
Specific gravity	2.95
Optical properties	Biaxial (-)
Refractive index	nα = 1.529 - 1.530 nβ = 1.680 - 1.682 nγ = 1.685 - 1.686
Birefringence	δ = 0.156
2V angle	18°
Solubility	Dilute acid
Other characteristics	Fluorescence: pale rose, yellow, white or bluish; phosphorescence: greenish or white (LW UV); yellowish (SW UV)
References	[1][2][3]

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO₃ (the other form being the mineral calcite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with acicular crystals. Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.

Contents 1 Occurrence 2 Physical properties 3 Uses 4 Gallery 5 See also 6 References 7 External links

Occurrence

The type location for aragonite is Molina de Aragón (Guadalajara, Spain), 25 km from Aragon for which it was named in 1797.^[1] An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia. In the USA, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves. Massive deposits of oolitic aragonite sand are found on the seabed in the Bahamas.

Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite). Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, respectively. The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone.

Aragonite is metastable and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown.^[4]

Physical properties

Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 10⁷ to 10⁸ years. The mineral vaterite, also known as μ-CaCO₃, is another phase of calcium carbonate that is metastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.

Uses

In aquaria, aragonite is considered essential for the replication of reef conditions in aquariums.^{[citation needed]} It not only is the material that the sea life is evolved to use and live around, but also keeps the tank's pH close to its natural level.

Gallery

• 

  Aragonite crystals from Cuenca, Castile-La Mancha, Spain

• 

  Cluster of twinned aragonite from Morocco

• 

  Remnant biogenic aragonite (thin, rainbow-colored shell) on the ammonite Baculites (Pierre Shale, Late Cretaceous, South Dakota).

• 

  Scanning electron microscope image of aragonite layers in the nacre of a blue mussel (Mytilus edulis).

See also

• Aragonite sea
• Ikaite, CaCO₃·6H₂O
• Monohydrocalcite, CaCO₃·H₂O
• Nacre, otherwise known as "Mother-of-Pearl"
• Oolitic aragonite sand

References

1. ^ ^{a b} Mindat.org
2. ^ Handbook of Mineralogy
3. ^ Webmineral data
4. ^ Runnegar, B. (1985). "Shell microstructures of Cambrian molluscs replicated by phosphate". Alcheringa: an Australasian Journal of Palaeontology 9 (4): 245–257. doi:10.1080/03115518508618971.  edit

External links

• The Ochtinska aragonite cave
• Kosovo Caves Aragonite Formations

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