 | Aragonite | Calcite |
|---|---|---|
| Introduction (from Wikipedia) | Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3. It is formed by biological and physical processes, including precipitation from marine and freshwater environments. | Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380–470 °C, and vaterite is even less stable. |
| Category | Carbonate mineral | Carbonate mineral |
| Formula (repeating unit) | CaCO3 | CaCO3 |
| Strunz classification | 05.AB.15 | 05.AB.05 |
| Crystal symmetry | Orthorhombic (2/m 2/m 2/m) - dipyramidal | Trigonal 32/m |
| Unit cell | a = 4.95 Å, b = 7.96 Å, c = 5.74 Å; Z = 4 | a = 4.9896(2) Å, c = 17.0610(11) Å; Z=6 |
| Color | White, red, yellow, orange, green, purple, grey, blue and brown | Colorless or white, also gray, yellow, green, |
| Crystal habit | Pseudohexagonal, prismatic crystals, acicular, columnar, globular, reniform, pisolitic, coralloidal, stalactitic, internally banded | Crystalline, granular, stalactitic, concretionary, massive, rhombohedral. |
| Crystal system | Orthorhombic | Trigonal hexagonal scalenohedral (32/m), Space Group (R3 2/c) |
| Twinning | Polysynthetic parallel to {100} cyclically on {110} | Common by four twin laws |
| Cleavage | Distinct on {010}, imperfect {110} and {011} | Perfect on [1011] three directions with angle of 74° 55' |
| Fracture | Subconchoidal | Conchoidal |
| Tenacity | Brittle | Brittle |
| Mohs scale hardness | 3.5-4 | 3 (defining mineral) |
| Luster | Vitreous, resinous on fracture surfaces | Vitreous to pearly on cleavage surfaces |
| Streak | White | White |
| Diaphaneity | Translucent to transparent | Transparent to translucent |
| Specific gravity | 2.95 | 2.71 |
| Optical properties | Biaxial (-) | Uniaxial (-) |
| Refractive index | nα = 1.529 - 1.530 nβ = 1.680 - 1.682 nγ = 1.685 - 1.686 | nω = 1.640–1.660, nε = 1.486 |
| Birefringence | δ = 0.156 | δ = 0.154–0.174 |
| Solubility | Dilute acid | Soluble in dilute acids |
| Other characteristics | Fluorescence: pale rose, yellow, white or bluish; phosphorescence: greenish or white (LW UV); yellowish (SW UV) | May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent |
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