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(Mg,Fe)2SiO4
(ferro-magnesian silicate)
Comprises two end-members,
Forsterite (Mg2SiO
4) and
Fayalite (Fe2
SiO4)
Crystal system: Orthorhombic
Refractive index: 1.635-1.879
Birefringence: 0.035-0.052
Cleavage: imperfect in Mg-rich members, moderate in Fe-rich members
Twinning: none
Colour in PPL: Mg-rich members colourless, Fe-rich members pale yellow
Pleochroism: Fe-rich members pale yellow to orange-yellow
Relief: moderate (forsterite) to very high (fayalite)
Distinction: from clinopyroxenes by higher birefringence, poorer cleavage and common alteration; from epidote by
oblique extinction of later
Occurrence: olivines are found in ultramafic igneous rocks and some mafic igneous rocks; Mg-rich members in thermal
metamorphism of dolomites, Fe-rich members in iron-rich regionally-metamorphosed sediments
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Olivine - Plain polarized light - Puebla - Mexico
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