Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Mg2SiO4 Mg endmember of olivine (Mg,Fe)2SiO4.
Crystal System orthorhombic 2/m2/m2/m
Crystal Habit  granular masses or rounded, embedded grains  
Cleavage Cleavage: indistinct on {010},{100} conchoidal fracture, brittle
Color/Pleochroism Pale, yellow-green to olive-green. Colorless to pale green in thin section.  Weak, pale green pleochroism in thin section. Darker with increasing Fe, moving away from pure forsterite.  
Optic Sign Biaxial (+)   
2V 82-90

Varies depending on composition.

Optic Orientation X=b 
O.A.P. = (010)

Refractive Indices
alpha = 
beta = 
gamma = 


Extinction parallel   Elongate crystals display parallel extinction.
Distinguishing Features Colorless to olive green in thin section. Second-order interference colors. High relief. Lack of cleavage. H= 6.5-7. G = 3.2. Streak is colorless or white. May be confused with epidote, which has cleavage and a is pistachio-green in hand sample.
Occurrence Occurs in a wide variety of volcanic rocks, both as phenocrysts and as groundmass. Occasionally found in metamorphosed carbonate-bearing rocks (crystalline dolomitic limestones) associated with calcite, dolomite, diopsite, epidote-group minerals, grossular garnet, tremolite, and related Ca-Mg-bearing minerals.
Editors Micehelle Arsenault (01), Jamie Mitchell (03), Angelie Peterson (02), Cheryl Mawaka (04), Stephanie Moore (07), Eli Molitors Bergman (17)

Photomicrograph of forsterite in a calcite marble in plane light. Forsterite may display pale yellow pleochroism and no distinct cleavage. This sample has been partially altered to serpentine in the marble. This sample shows second order interference colors.  The forsterite interference color is 2nd order pink. Diopside grains are also visible. Select image and roll cursor over to view in crossed polarized light.

Photomicrograph of forsterite in metamorphosed carbonate under plane-polarized light. Forsterite is colorless in thin section under plane-polarized light, and thus displays no pleochroism. Under cross-polarized light, this forsterite grain displays first order blue. Click on then mouse over the image to change from plane-polarized to cross-polarized light.

Photomicrograph of optic axis figure of forsterite grain in metamorphosed carbonate. Click on then mouse over the image to view the figure with the quartz plate inserted, displaying positive optic sign. A positive optic sign distinguishes forsterite from other compositions of olivine.
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