Enstatite
The magnesium pyroxene that dominates enstatite chondrites and aubrites, and many chondrules.
Key facts
- Formula
- MgSiO3
- Family
- Silicates
- Crystal system
- Orthorhombic (clinoenstatite is monoclinic)
What it is
Enstatite is a pyroxene, a silicate built of long chains of silicon–oxygen groups, with magnesium between the chains. The iron-bearing members of the series are called orthopyroxene in general. Its name comes from the Greek for opponent, because it resists melting.
It has two common forms. Orthoenstatite is orthorhombic. Clinoenstatite, a monoclinic form, often appears in chondrules that cooled quickly, where it grows as thin twinned stripes.
Where it is found
Almost iron-free enstatite makes up more than half of the enstatite chondrites (EH and EL) and nearly all of the aubrites, which formed in the most oxygen-poor conditions known in the Solar System. Iron-bearing orthopyroxene is the main mineral of diogenites from Vesta and is abundant in ordinary chondrites, where its iron content helps fix the class, and in the Martian meteorite ALH 84001. Enstatite was also caught in dust from comet Wild 2.
How it is recognised
It is colourless, white or pale grey to brownish, with two cleavages almost at right angles. In aubrites it forms large white crystals. Under the microscope orthopyroxene shows low grey interference colours and straight extinction; clinoenstatite shows fine twins.
Did you know?
The asteroid Šteins, visited by Rosetta, has the colours of enstatite-rich aubrites.
Enstatite chondrites have oxygen isotopes almost the same as Earth's, so some scientists think Earth grew partly from similar material.