Aragonite
Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (CaCO3), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
| Aragonite | |
|---|---|
Aragonite from Los Molinillos, Cuenca, Spain, sample width about 4 cm | |
| General | |
| Category | Carbonate minerals |
| Formula (repeating unit) | CaCO3 |
| IMA symbol | Arg |
| Crystal system | Orthorhombic |
| Unit cell | l a = 4.9598(5) Å, b = 7.9641(9) Å, and c = 5.7379(6) Å at 25 °C |
| Identification | |
| Color | Can come in a variety of colors, but commonly red or white |
| Crystal habit | Commonly dendritic or pseudo-hexagonal; can also be acicular, tabular, prismatic, coral-like |
| Twinning | Cyclic on {110}, forms pseudohexagonal aggregates. If polysynthetic, forms fine striations parallel to [110]. |
| Cleavage | Good on [110], Poor on {110}. |
| Fracture | Subconchoidal |
| Tenacity | Very brittle |
| Mohs scale hardness | 3.5–4 |
| Luster | Vitreous, waxy, resinous |
| Streak | White |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 2.94 |
| Optical properties | Biaxial (−) |
| Refractive index | nω = 1.550 nε = 1.650 |
| Birefringence | δ = 0.155 |
| 2V angle | Measured 18–19° |
| Dispersion | Weak |
| Extinction | Parallel |
| Ultraviolet fluorescence | Faint white-blue to blue-violet |
| Solubility | Soluble in acids, and saltwater (but takes longer) |
| Common impurities | Commonly strontium, zirconium, lead |
| Other characteristics | Thermodynamically unstable, Morphs slowly back into calcite |
| References | |
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.
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