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,

2 comments:

  1. UnknownDecember 22, 2013 at 4:12 PM

    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.

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    Replies
    1. David ZakharovDecember 22, 2013 at 6:37 PM

      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.

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