Plagioclase
The feldspar series from sodium albite to calcium anorthite, the light-coloured mineral of basalts and lunar highlands.
Key facts
- Formula
- NaAlSi3O8–CaAl2Si2O8
- Family
- Silicates
- Crystal system
- Triclinic
What it is
Plagioclase is a feldspar: a framework silicate in which aluminium replaces some of the silicon and sodium or calcium fill the gaps. Its composition is given by its anorthite content, An: An0 is pure albite, An100 pure anorthite. On Earth it is the single most abundant mineral in the crust.
Where it is found
In ordinary chondrites, sodium-rich plagioclase grows from glass in chondrules as the rock is heated, so its grain size helps fix the petrologic type. Calcium-rich plagioclase is a major mineral of eucrites, howardites and lunar highland rocks; anorthosites made almost entirely of it form the Moon's ancient crust. In shocked meteorites it is often converted to maskelynite glass.
How it is recognised
It is white to grey, with flat cleavage faces that often show fine parallel striations. In thin section it is clear with low grey interference colours and characteristic stripes of repeated twinning, called albite twins.
Did you know?
The bright areas of the Moon you can see with the naked eye are bright because of plagioclase.
Plagioclase grains larger than about 0.05 mm mark an ordinary chondrite as petrologic type 6.