Smith College - Geology 222b - Petrology
Petrographic Data File

Orthopyroxene (Opx)

Enstatite (Mg end member): MgSiO3

Ferrosilite (Fe end member): FeSiO3

Solid solution between Enstatite and Ferrosilite.
Crystal System
2/m 2/m 2/m
Crystal Habit
Massive, irregular, stubby prismatic. Longitudial sections typically rectangular.

Hardness 5-6  
Specific Gravity 3.20-4.00  
Good cleavage on (210)
Parting on (100) and (010)
Cleavage in two directions, intersecting at nearly right angles.
Hand Sample Color Brown to green/brown to green/black. High in Mg typically green/brown, high Fe tends to be darker, green/black.
Streak White to gray.  
Grayish, yellowish or greenish white to olive green/brown. Pale pink to green pleochroism

Optic Sign
Biaxial (+ or -)
Enstatite and ferrosilite (+). Hypersthene (-)
Large range in 2V is created by variations in iron content.
Optic Orientation
X = b, Y = a, Z = c

Refractive Indices
alpha =
beta =
gamma =
delta =

Variations in Mg and Fe content determines refractive indices; higher Fe associated with higher indices.
Max Birefringence
Typically first order greys, high iron may display, lower second order.
parallel to c axis
Tends to be length slow.
Parallel in longitudinal sections and symmetrical in basal sections.
r > v
2V is larger in red light than violet light.
Distinguishing Features
Low birefringence, first order colors. Parallel extinction in longitudinal sections, pale pink to green pleochroism. Approximatly 90º cleavage planes. Thin irregular and wavy lamellae common.
Orthopyroxene high in Mg is common in gabbro and peridotite groups. Fe-rich orthopyroxene is typically found in diorite, syenite and granite. With Ca-clinopyroxene, plagioclase and olivine in mafic and ultramafic igneous rocks.
Associated Minerals Feldspars, clinopyroxene, garnet, biotite and hornblende.
Elizabeth Thomas (2003), Andrea Gohl (2007) and Emma Hall (2013).

Introduction to Mineralogy, William D. Nesse, 2000. Introduction to Optical Mineralogy, William D. Nesse, 1991. Minerals in Thin Section, Dexter Perkins and Kevin R. Henke.


Photomicrograph of orthopyroxene (low birefringence, skinny lamellae) surrounded by clinopyroxene (higer birefringence, coarse lamellae).  Sample SIC-3.  Large photo is1.7 mm wide. Image by Elizabeth Thomas.

Orthopyroxene - Sample SIC-3. Image by Elizabeth Thomas.
Orthopyroxene - Sample SIC - 4. First order greys and thin lamellae. Image by Andrea Gohl.
Orthopyroxene - Sample SIC - 3. Orthopyroxene with high relief and thin irregular lamellae. Image by Andrea Gohl.
Orthopyroxene - Sample SIC -5. Orthopyroxene seen in the center. Note plagioclase (left of orthopyroxene) with typical twinning. Image by Andrea Gohl.

Orthopyroxene - Sample BV203. The width of the field of view is 2.28 mm. Image by Emma Hall.

WWW Images University of North Carolina
UCLA Petrographic Workshop

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