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Aragonite Mineral Facts:

Chemical Formula: CaCO₃ (same as Calcite)

Colors: White, gray, yellowish, sometimes green or violet.

Hardness: 3.5 to 4
Hardness varies somewhat depending on formation and impurities.

Density: 2.95
For Both the density and hardness reading for aragonite are higher than normally found for calcite.

Cleavage: One distinct cleavage on 010, one poor cleavage on 110, parallel to macropinacoid.  Fracture is Subconchoidal ; brittle.

Crystallography: Orthorhombic

Luster:. Vitreous, transparent to translucent.

Optics: (Refractive Index)  a = 1.53; b = 1.68; y = 1.685

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Composition, Structure and Associated Minerals:
Aragonite is another crystal morph of Calcium Carbonate. The other form is the well known mineral calcite. Arragonite is the less stable and less common form of Calium Carbonate, tests showing that it forms only at higher temperatures.

Aragonite commonly forms as prismatic crystals, often terminated by acute domes. Very commonly twinned on the face of the prism, and by repetition of this twinning, six sided pseudo- hexagonal crystals are formed. These pseudo-hexagonal twins are distinguished from hexagonal crystals by re-entrant angles. Individual crystals often many times twinned, and with alternately reversed striation on faces of prism and cleavage planes. Also occurs in acicular crystals, often radiating, columnar, and in globular, reniform, stalactitic, striated, and encrusting forms. Aragonite is sometimes deposited from hot springs and can be found sometimes in association with beds of gypsum and some iron oxide deposits, where it may occur in forms resembling coral. It was named for a famous occurrence at Aragon, Spain.

Identification and Diagnostics
Aragonite is not often confused with calcite, although both are forms of calcium carbonate. Both react similarly to hydrochloric acid. However, Aragonite is normally found in small thin pointed crystals. A form quite different from calcite - in addition, the cleavage, hardness and density of Aragonite are quite different from calcite. However psudomorphs of calcite after aragonite are fairly common, and these can confuse the issue.

Testing for the presence of Carbonates
The standard test for calcite or aragonite is to put a few drops of Hydrochloric acid on it. It will quickly bubble and give off carbon dioxide at a fast rate, far more easily than most other carbonates. However, both aragonite and calcite will do this, so it cannot be used to determine which form is present.

SAFETY NOTE ON CHEMICAL TESTS: These tests are given as reference material only. Chemicals can be dangerous and those not well versed in chemistry and the potential hazards of these acids and any other chemical reactions that might occur, should not perform these tests.

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