Smith College - Geology 222b - Petrology

Petrographic Data File

Augite
Property
Value
Comments
Formula Ca (Mg, Fe) [SiO3]2 A clinopyroxene, augite is an intermediate member of diopside and hedenbergite
Crystal System Monoclinic
Crystal Habit Short prismatic crystals, elongate along c axis, with 4 or 8- sided cross sections; Grains often anhedral; May be granular, massive, columnar or lamellar  
Cleavage {110} at 87 degrees and 93 degrees; Imperfect cleavage 90 degree clevage visible in thin section
Color Usually colorless, gray, pale green, pale brown or brownish green in thin section  
Pleochroism x=pale green or bluish green
y=pale greenish, brown, green or bluish green
z=pale brownish green, green or yellow-green 		Usually weak, but varies.
Optic Sign Biaxial (+)
2V 55-65
Optic Orientation Y=b
a = 9.73
b = 8.91
c = 5.25
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 
1.688- 1.712
1.701- 1.717
1.713- 1.737
0.021- 0.025
Max Birefringence 0.025
Elongation Z^c=39-48
  
Extinction Inclined (approximately 45 degrees) except extinction is parallel when viewed down (010); Sometimes displays hourglass extinction  
Dispersion r > v  
Twinning Commonly displays simple and lamellar twinning on {100} and {001}; They may combine to form a herringbone pattern.  
Luster Vitreous  
Hardness 5-6  
Relief High Positive  
Point Group 2/m  

Distinguishing Features

 High relief; 90 degree cleavage visible in thin section
Occurrence Common in subsilicic igneous rocks (gabbros, basalts, auganites). Found locally in gneisses and granulates.
Editors

Sarah Katchpole ('02), Devon Powers ('04), Sienna Tinsley (Amherst '08)


 
Photomicrograph of augite in plain polarized light. The augite is the light colored mineral with high relief.
Photo of same augite phenocryst in cross polarized light.
Augite phenocryst in K-84 showing 90-degree cleavage.
The same phenocryst in crossed polarized light.  Note the simple twin.


Augite crystal from sample K-84 showing cleavage at 90-degrees. Shown in plain polarized light.



This crystal shows twinning in an augite crystal from sample K-84 shown in cross polarized light.
Augite from the Stillwater Igneous Complex showing exsolution lamellae of orthopyroxene.
Exsolution lamellae in augite from slide SIC6. Notice the tiny dark exsolution bands within some of the larger exsolution bands.

This is amphibole, NOT AUGITE, which is a pyroxene. Notice the 60 degree and 120 degree cleavages that help you tell the difference between the two. This can be seen in slide K84.
WWW Images U.C.L.A. Petrographic Workshop

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