ARAGONITE is a carbonate mineral and its formula is calcium carbonate. It has the same formula as Calcite and Vaterite, but has a different crystal structure. They are tabular, prismatic or needle-like, often with steep pyramidal or chisel-shaped ends, and can form columnar or spreading aggregates. Multiple twin crystals that appear hexagonal in shape are common. Although aragonite sometimes resembles calcite, it is easily distinguished by the absence of rhombic cleavage. Samples can be white, colorless, gray, yellowish, green, blue, reddish, purple or brown. Aragonite is found in oxidized areas of ore deposits and in evaporites, hot spring deposits and caves. It is also found in some metamorphic and igneous rocks and is formed by biological and physical processes, including precipitation from marine and freshwater environments.
NAME: For its first-noted occurrence in the Aragon region, Spain
POLYMORPHISM & SERIES: Trimorphous with calcite and vaterite
PRODUCT DESCRIPTION:
MINERAL : ARAGONITE (Carbonate Mineral)
CHEMICAL COMPOSITION : Ca Co3
HARDNESS (Mohs Scale) : 3.5 to 4.0
MEASUREMENTS Approx : 78.8 x 27.1 MM
WIEGHT : 74 gram
ORIGIN : HELMAND PROVINCE , AFGHANISTAN
ARAGONITE is the high pressure polymorph of calcium carbonate. As such, it occurs in high pressure metamorphic rocks such as those formed at subduction zones.
The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal.
Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.
CHEMICAL PROPERTIES
Formula: CaCO3
Common Impurities: Sr,Pb,Zn
ARAGONITE PHYSICAL PROPERTIES
CRYSTAL HABIT: Orthorhombic
COLOR: Colorless to white or grey, often stained various hues by impurities, such as blue, green, red or violet; colourless in transmitted light.
STREAK: Uncolored/white.
LUSTER: Vitreous, Resinous
CLEAVAGE: Distinct/Good On {010} distinct; On {110} and {011} very indistinct.
DIAPHANEITY: Transparent, Translucent
MOHS HARDNESS: 3 4
TENACITY: Brittle
DENSITY: 2.947
FRACTURE: Sub-Conchoidal
OCCURRENCE
It turns into calcite over geological time. Primary sediment in warm marine waters such as oolites and carbonate mud, an essential clastic sedimentary component as the hard parts of the shells and skeletons of many marine micro-organisms; also from evaporite deposits; in sinter in hot springs and in stalactite in caves; characteristic of high pressure, low temperature (blueschist facies) metamorphism; as amygdullary in basalt and andesite; It is a secondary component in altered ultramafic rocks.
ARAGONITE is a high pressure polymorph of calcium carbonate. Therefore, it occurs in high pressure metamorphic rocks such as those formed in subduction zones.
ARAGONITE is metastable at low pressures near the Earth’s surface and is therefore often replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown. It can also be synthesized by adding a solution of calcium chloride in water-ethanol mixtures at ambient temperatures or to a sodium carbonate solution at temperatures above 60 °C (140 °F).
DISTRIBUTION
Many localities, but fine crystals are uncommon.
HELMAND PROVINCE,AFGHANISTAN
From Molina, Guadalajara Province, Spain.
Fine crystals from Racalmuto, Cianciana, and Agrigento, Sicily, Italy.
At Dogn´acska and Spania Dolina (Herrengrund), Slovakia.
From Tarnowitz, Silesia, Poland.
At the Erzberg, near Eisenerz, Styria, and from Leogang, Salzburg, Austria.
On the Spitzberg, Horenz, near B´lina, Czech Republic.
From Frizington and Cleator Moor, Cumbria, England.
Fine examples at the Touissit mine, near Oujda, and from Tazouta, near Sefrou, Morocco.
Large crystals from Tsumeb, Namibia.
In the USA, in caves at Bisbee, Cochise Co., Arizona; large crystals from near Lake Arthur, Chavez Co., also near Santa Rosa, Guadalupe Co., New Mexico; in the Passaic mine, Sterling Hill, Ogdensburg, Sussex Co., New Jersey
USES
In aquaria, aragonite is considered essential for the replication of reef conditions. Aragonite provides the materials necessary for much sea life and also keeps the pH of the water close to its natural level, to prevent the dissolution of biogenic calcium carbonate.
Aragonite has been successfully tested for the removal of pollutants like zinc, cobalt and lead from contaminated wastewaters.
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