Calcite
Calcite is calcium carbonate, CaCO₃, with trigonal crystals and a hardness of 3 that is the reference for that number on the Mohs scale. It is the main mineral of limestone and marble, and clear crystals called Iceland spar show strong double refraction.
Calcite is calcium carbonate, CaCO₃, with trigonal crystals and a hardness of 3 that is the reference for that number on the Mohs scale. It is the main mineral of limestone and marble, and clear crystals called Iceland spar show strong double refraction.
🪨 Geology Guide Specimen: Open Calcite (Iceland Spar) in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 8 mineral classes of Dana / Strunz Classes, and test your eye in LithoClade.
Rhombohedral Cleavage: How Do You Recognize It?
Calcite splits along three perfect directions that meet at angles close to 75 and 105 degrees, yielding slanted blocks, rhombohedra. It also fizzes briskly in cold dilute hydrochloric acid, releasing carbon dioxide, which is the simplest field test for carbonate.
Double Refraction: What Is Iceland Spar?
Clear calcite splits a beam of light in two, so a line of text seen through it appears doubled. The effect was described in 1669 and became important in the study of polarized light, and some historians suggest Norse navigators used such crystals to find the Sun.
Rock Builder: Why Is Calcite Everywhere?
Organisms build shells and skeletons of calcite, which accumulate as fossiliferous limestone and chalk, and heat turns them into marble. Calcite cements many rocks, grows as cave formations, and its burial and weathering are part of the carbonate-silicate cycle.
Calcite at a Glance
| Property | Value |
|---|---|
| Type / variety | CaCO₃ • Trigonal Rhombohedral Carbonate |
| Composition | CaCO₃ (Trigonal Calcium Anion Complex) |
| Geology Guide group | Minerals (8 Major Chemical Groups) |
| Mineral Group | Carbonates |
| Mohs hardness | Mohs 3 (reference standard for 3) |
| Texture / habit | Slanted rhombohedra, pointed 'dogtooth' scalenohedra, or flat 'nailhead' habits |
| Density | 2.71 g/cm³ |
| Lustre and cleavage | Perfect 3-directional rhombohedral cleavage (75°/105°); strong double refraction |
| Geological setting | Limestones, marbles, hydrothermal veins, and cave speleothems |
| Diagnostic feature (Geology Guide) | Transparent slanted rhombohedral crystal (Mohs 3 standard) that splits along three perfect non-90° planes, splits printed text placed underneath it into a double image, and fizzes briskly in cold dilute HCl. |
| Formula | CaCO₃ |
| Cleavage | Three directions, rhombohedral |
See Also
- Aragonite — the polymorph with the same formula
- Marble — a rock made of recrystallized calcite
- Carbonate-Silicate Cycle — the cycle that moves carbon through carbonates