Silicates: Olivine

Composition: (Mg,Fe)₂SiO₄

Crystal system: Orthorhombic; usually granular masses or rounded grains

5.18.1
olivine xenolith in basalt

Properties:

• Vitreous lustre
• Variable color; usually olive green to dark brown
• White to grey streak
• Knife does not scratch (Mohs Hardness 6.5)
• Above average to high density depending on iron content
• Cleavage rarely apparent
• Smooth (conchoidal) fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Olivine occurs in many mafic and ultramafic igneous rocks and some high grade metamorphic rocks .

5.18.2
crystalline olivine