Calcite
CaCO₃ · Trigonal · also called Iceland spar, calcspar, dogtooth spar
Calcite is the mineral of limestone, marble, cave formations, and most seashells. Between the acid test, the leaning rhombohedral cleavage, and hardness 3, it is one of the easiest minerals to confirm — and worth learning early, because it turns up constantly.
Properties
- Mohs hardness
- 3
- Streak
- white
- Lustre
- vitreous, pearly
- Colour
- Usually white or colourless, but trace impurities produce almost any colour.
- Cleavage
- Perfect rhombohedral cleavage in three directions, none at right angles. Struck pieces fall apart into leaning, lozenge-shaped blocks.
- Fracture
- Conchoidal, rarely seen
- Specific gravity
- 2.71 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- Fizzes on contact
- UV
- Some specimens fluoresce
Tests that confirm calcite
- Fizzes vigorously in cold dilute acid — household vinegar works, if more slowly than laboratory acid.
- Clear pieces show double refraction: a line viewed through the crystal appears twice.
- Soft enough for a copper coin to scratch it.
What calcite gets confused with
Each of these is settled by one test. Run that test first.
Acid and hardness. Calcite fizzes and is scratched by a coin; quartz does neither.
Dolomite only fizzes when you powder it first. Calcite fizzes on contact.
Gypsum is softer still — a fingernail scratches it — and does not react to acid.
Where it is found
Limestone and marble country, cave deposits, and as the white vein filling in a very wide range of rocks.
Is it worth anything?
Common and cheap. Optical-quality Iceland spar and brightly fluorescent material from certain localities are the exceptions.