Ferrosilite
The iron end of the orthopyroxene series, whose share in pyroxene helps classify meteorites.
Key facts
- Formula
- FeSiO3
- Family
- Silicates
- Crystal system
- Orthorhombic
What it is
Ferrosilite is the iron end member of the low-calcium pyroxene series that runs from enstatite, MgSiO3, to ferrosilite, FeSiO3. Pure ferrosilite is rare in nature; what matters most in meteorites is the ferrosilite number, Fs, the share of iron in a pyroxene.
Where it is found
Pyroxene in H, L and LL chondrites has Fs values of roughly 15 to 19, 19 to 22 and 22 to 26 respectively, and these help assign a class. Pyroxene in diogenites and eucrites from Vesta is more iron-rich still. Shocked iron-rich pyroxene can turn into hemleyite or hiroseite.
How it is recognised
In practice the Fs value is measured with the electron microprobe on several grains. In hand specimens orthopyroxene is brownish and shows two cleavages at nearly right angles.
Did you know?
Fs and Fa, the ferrosilite and fayalite numbers, are the two figures most often quoted in a meteorite classification.
Equilibrated chondrites have pyroxene grains with almost identical Fs values; unequilibrated ones show a wide spread.