(ORTHOPYROXENES) (Mg,Fe)SiO3
Crystal system: Orthorhombic
Refractive index: 1.650-1.788
Birefringence: 0.007-0.020
Cleavage: 2 good, 88o
Twinning:
Colour in PPL: Mg-rich colourless, Fe-rich reddish or greenish
Pleochroism: Mg-rich none, others weak to strong
Relief: moderate to high
Distinction: from clinopyroxenes by pleochroism, lower birefringence and straight extinction
Occurrence: ultra-mafic rocks and medium to high-grade metamorphic rocks
Minerals in the Orthopyroxene group are:
- Enstatite
- Orthoferrosilite
