Olivine: Forsterite-Fayalite series

General formula of olivine: (Mg,Fe)2[SiO4]

Forsterite - Fayalite
nα=1.636 - 1.827
nβ=1.651 - 1.869
nγ=1.669 - 1.879

δ=0.033 - 0.052
2V=(+86)° - (-47)°

Thin section pictures:

Partly replaced by amphibole olivine (center). Note the characteristic cleavage.
Troctolite, the Kola Peninsula. PPL.

Forsterite 84, 2V=90.

An aggregate of secondary minerals (smectice+serpentine+chlorite?) forms a complete pseudomorph after olivine in basalt. The remains of olivine are enclosed in an OPX reaction rim (SiO2+Olivine=Enstatite). Skinner Butte, OR, USA,


  1. So, to tell the difference between the two biaxial minerals an optic axis figure is best to estimate the 2V angle. Any other distinct optical properties to differentiate.

    1. I can't come up with any optical property to differentiate fayalite and forsterite right away. However, paragenesis should give you a good idea what you're dealing with. For instance, you would suggest fayalite if you see olivine-looking mineral in rhyolite or Fe-rich scarn. Fayalite is relatively rare mineral and I believe, its alteration products are different from the products of forsterite alteration.