Aragonite

Sphere of aragonite

A carbonate mineral, is a form of calcite and is one of the three naturally forms of calcium carbonate (aragonite, calcite, vaterite).
It is formed by biological and physical processes including precipitation from marine and freshwater environments.
The crystal lattice is different from calcite, resulting in orthorhombic crystals.
Aragonite is thermodynamically unstable at standard temperature and pressure and will alter to calcite on a scape of 10 000 000 years.
It is used in aquaria, for the replication of reef conditions, provides necessary materials for sea life and keeps the pH of the water to natural level.
Aragonite removes pollutants like zinc, cobalt and lead from contaminated water.

Properties:
Category: carbonate mineral
Formula CaCO3
40,04% calcium; 12% carbon; 47,96% oxygen
56,03% CaO; 43,97% Co2
Variable formula: (Ca, Sr, Pb, Zn)CO3 (common impurities)
Crystal system: orthorhombic
Color: white, red, yellow, orange, green, purple, grey, blue, brown
Habit: stalactitic, prismatic, acicular, columnar, globular, coralloidal
Fracture: subconchoidal
Tenacity: brittle
Hardness: 3,5-4 Mohs
Luster: vitreous, resinous on fracture surfaces
Streak White
Transparent to translucent
Specific gravity: 2,95
Optical properties: biaxial (-)
Solubility: dilute acid
Rocks: igneous, sedimentary, metamorphic
Special characteristics: fluorescence (pale rose, yellow, white, bluish), phosphorescence (greenish, white in longwave UV or/and yellowish in shortwave UV)

Fluorescence:
longwave: yellow main color, white, yellow, orange, red, blue, bluish, pink pale; strong intensity
midwave: pink pale main color, white, bluish pale, yellow, red, orange, pink strong; medium intensity
shortwave: bluish-white main color, white, yellow, orange, red, pink pale, green
Frequency very often to all
Phosphorescence:
longwave and shortwave: green, bluish-white; intensity strong; frequency often
Main activator Mn; other: UO2 (uranyl ion), organic impurities, Sm, Dy
Sm replacing Ca: 603, 640 nm
Mn: 630 mn (120nm half-width)
Dy: 482, 486, 478, 479 nm
Fluorescence green due to Uranium races.
Mn is considered as the principal activator of red fluorescence (in the past was considered strontium, not anymore).

Tests:
Hardness 3,5-4; streak white
Fluorescence blue, pink, yellow, cream
Phosphorescence in green, white in longwave and/or yellow in shortwave UV
Clear speciments display double refraction.
Strong effervercence

Calcite is more common.
Some aragonite crystals are actually calcite pseudomorph after aragonite.
A rare but popular pseudomorph is copper after aragonite.
Aragonite is the main component of many organic substances like pearl and coral.
The iridescent surface of a pearl is in fact a layer of aragonite secreted by mollusks.
Flos Ferri is a variety of aragonite that is very fragile and may break easily.

To distinguish aragonite from calcite it is necessary optical or x-ray testing.

Varieties:
Alabaster - fine grained variety of Gypsum, may be translucent, banded type of aragonite in the form of Travertine or Tufa.
Flos Ferri - a very fragile and filiform variety of aragonite resembling coral.
Mossottite - strontium rich variety of aragonite, mixture of aragonite and Strontianite (Ca,Sr)CO3 also known as Strontian Aragonite
Nicholsonite - zinc rich of aragonite (Ca,Zn)CO3 also known as zincian aragonite
Onyx marble - travertine or Tufa in the mineral forms aragonite or calcite that exhibits color banding
Satin Spar - fibrous variety of Gypsum that may describe a fibrous form of calcite or aragonite
Tarnowitzite - lead rich aragonite variety, mixture of aragonite and cerussite (Ca, Pb)CO3 also known as Plumboan Aragonite
Cerussite - a lead carbonate, PbCO3; is isomorphous with aragonite
Travertine - formed form hot springs that contain calcium rich water that bubbles up to the earth and cools down, the water eliminates the calcium and is porous; Travertine is usually aragonite but may be calcite.
Tufa - aragonite or calcite formed from precipitating water traped in organic matter like leaves, twigs and moss.

Polymorphs:
Calcite
Vaterite
Isomorphous:
Cerussite

Common minerals associated: Quartz, Calcite, Gypsum, Albite, Azurite, Chalcopyrite, Bornite

Distinguishing similar minerals:
Calcite - distinguished by perfect rhombohedral cleavage, slightly softer, different crystal habit
Cerussite, Witherite and Strontianite - heavier
Hemimorphite - harder, heavier

Relationship to other species:
Cerussite - PbCO3
Strontianite - SrCO3
Witherite - BaCO3
Smithsonite - ZnCO3
Siderite - FeCO3
Celestine - SrSO4

Sources:
https://www.minerals.net/mineral/aragonite.aspx
https://www.mindat.org/min-307.html
http://webmineral.com/data/Calcite.shtml#.XbDIaOgzZrR
http://www.fluomin.org/uk/fiche.php?id=87&name=ARAGONITE