Anorthite
Calcium-rich plagioclase, the mineral of the Moon's first crust and of the oldest solids in chondrites.
Key facts
- Formula
- CaAl2Si2O8
- Family
- Silicates
- Crystal system
- Triclinic
What it is
Anorthite is the calcium end of the plagioclase series. Each calcium atom in its framework is balanced by an extra aluminium atom in place of silicon. It crystallises from melts at high temperature and is relatively light, so in a large body of magma its crystals tend to float.
Where it is found
Lunar highland rocks called anorthosites are made of more than 90% anorthite-rich plagioclase. They are thought to have floated to the top of the Moon's early magma ocean. Anorthite also occurs in many calcium-aluminium-rich inclusions in carbonaceous chondrites and in eucrites and angrites.
How it is recognised
It is white or grey, often with fine twin striations. Microprobe analysis gives anorthite contents above about An90.
Did you know?
The Genesis Rock collected on Apollo 15 is made almost entirely of anorthite-rich plagioclase.
Anorthite in CAIs was used to show that aluminium-26, a short-lived radioactive isotope, was present when the first solids formed.