Aragonite vs Calcite
Aragonite and Calcite share a composition and differ on 3 measured properties — the question a reader asks about any pair of polymorphs.
Aragonite vs Calcite
Aragonite
CaCO3
The other calcium carbonate — same composition as calcite, different structure, and the form most shells are built from.
Calcite
CaCO3
Calcium carbonate — the mineral of limestone, marble, chalk and most shells, and the reference point for hardness 3.
| Property | Aragonite | Calcite |
|---|---|---|
| Cleavage Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | perfect rhombohedral in three directions |
| Crystal system Categorical values can be listed side by side but not ordered or subtracted. | orthorhombic | trigonal |
| Density | 2.93–2.95 g/cm³ higher | 2.71–2.72 g/cm³ |
| Lustre Categorical values can be listed side by side but not ordered or subtracted. | vitreous | vitreous |
| Mohs hardness Mohs hardness is an ordinal scale: values can be ranked, but differences and ratios between them are not meaningful. | 3.5–4 higher | 3 |
| Streak Categorical values can be listed side by side but not ordered or subtracted. | white | white |