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| 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 | no distinct cleavage | 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
Y=c Z=a O.A.P. = (010) |
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Refractive Indices
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1.62-1.64 |
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| 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) | |
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Photomicrograph of forsterite in plane light. Forsterite may display pale yellow pleochroism and no distinct cleavage. This sample has been partially altered to serpentine in the marble. Select image and roll cursor over to view in crossed polarized light. |
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Photomicrograph of forsterite in a calcite marble under crossed polarized light. This sample shows second order interference colors. The forsterite interference color is 2nd order pink. Diopside grains are also visible. |