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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. |
Optic Orientation | X=b
Y=c Z=a O.A.P. = (010) |
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Refractive Indices
alpha = beta = gamma = delta = |
1.635-1.827
1.651-1.869 1.670-1.879 0.035-0.052 |
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Max Birefringence | 0.033 - 0.052 | |
Elongation | ||
Extinction | parallel | Elongate crystals display parallel extinction. |
Dispersion | ||
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) |