Smith College - Geology 222b - Petrology
Petrographic Data File
| Orthopyroxene (Opx) |
||
|---|---|---|
| Property |
Value |
Comments |
| Formula |
Enstatite (Mg end member): MgSiO3 Ferrosilite (Fe end member): FeSiO3 |
Solid solution between Enstatite and Ferrosilite. |
| Crystal System |
Orthorhombic |
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 | |
| Cleavage |
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. | |
| Color/Pleochroism |
Grayish, yellowish or greenish
white to olive green/brown. Pale pink to green pleochroism |
|
| Optic Sign |
Biaxial (+ or -) |
Enstatite and ferrosilite (+). Hypersthene (-) |
| 2V |
50-132º |
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 = |
1.649-1.768 1.653-1.770 1.657-1.788 0.007-0.020 |
Variations in Mg and Fe content determines refractive indices; higher Fe associated with higher indices. |
| Max
Birefringence |
0.020 |
Typically first order greys, high iron may display, lower second order. |
| Elongation |
parallel to c axis |
Tends to be length slow. |
| Extinction |
Parallel in longitudinal sections and symmetrical in basal sections. | |
| Dispersion |
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. | |
| Occurrence |
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. | |
| Editors |
Elizabeth Thomas (2003), Andrea Gohl (2007) and Emma Hall (2013). | |
| References |
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 |