Smith College - Geology 222b - Petrology

Petrographic Data File

Formula (MgFe)2SiO4 Complete solid solution between Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite).
Crystal System orthorhombic  
Crystal Habit  granular masses or rounded grains  
Cleavage no distinct cleavage conchoidal fracture
Color/Pleochroism Olive or yellowish-green in hand samples.  Colorless to pale green in thin section.  Weak, pale green pleochroism in thin section. Can be a yellowish color when rich in iron. 
Optic Sign Biaxial (-); or Biaxial (+)  if 2V =82-90 it is forsterite and Biaxial (+)
if 2V=46-90 it is fayalite and Biaxial (-)
2V 82-90; forsterite
46-90; fayalite

Varies depending on composition.

Click here for graph.

Optic Orientation X=b 
O.A.P. = (010)
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 
Max Birefringence 0.033 - 0.052  
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= 7. G = 3.22 to 4.39. Specific gravity increases and hardness decreases with increasing Fe. Streak is colorless or white. 
Occurrence Occurs in a wide variety of volcanic rocks, both as phenocrysts and as groundmass. Olivine with intermediate Fe-Mg composition is common in mafic and ultramafic volcanic rocks and plutonic igneous rocks. Iron rich olivines occur in alkaline and acid plutonic rocks.
Editors Micehelle Arsenault (01), Jamie Mitchell (03), Angelie Peterson (02), Cheryl Mawaka (04), Lauren Magliozzi (12)

Photomicrograph of olivine in plane light. Olivine typically displays pale green pleochroism and no distinct cleavage. Mag. 10x.
Photomicrograph of olivine under crossed polarized light.  These crystals show 2nd order interference colors.  This sample also shows some plagioclase feldspar with albite twinning.   
Photomicrograph of olivine in basalt in crossed polars with cleavage planes parallel to crosshairs of microscope, showing "extinct" view. Select the image and then move cursor over image to view the stage rotated 45 degrees, where the mineral shows maximum transmitted light. These two images demonstrate the extinction angle of olivine.
Photomicrograph of olivine in basalt (ED99-J) in plane light. Select image and then move cursor over image to view in crossed polarized light, where crystals show high bifringence. Magnification 1.5x.
Photomicrograph of olivine phenocrysts, in a matrix of Hawaiian basalt, in plane light. Select image and then move cursor over image to view in crossed polarized light, where crystals show high birefringence. Magnification 5x.

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