Calcite
Calcium carbonate that grew from water inside carbonaceous asteroids, and also forms as meteorites weather on Earth.
Key facts
- Formula
- CaCO3
- Family
- Carbonates, sulphates, salts and weathering products
- Crystal system
- Trigonal
What it is
Calcite is calcium carbonate, the mineral of limestone and chalk. In meteorites it is a sign of liquid water: it crystallises from water containing dissolved carbon dioxide.
Where it is found
It occurs as grains and veins in CM chondrites such as Murchison, and in CI chondrites and the Ryugu sample. Its carbon and oxygen isotopes record the temperature of the water, typically from about 0 to 70 °C. In desert finds, calcite also forms as a weathering product in cracks.
How it is recognised
It is white and fizzes in dilute acid, though acid should not be used on meteorites. In thin section it shows very high interference colours.
Did you know?
Carbonates in CM chondrites can be dated using the decay of manganese-53, giving ages within a few million years of the CAIs.
Testing a meteorite with acid destroys material, so it is not done outside laboratories.