Smith College - Geology 222b - Petrology

Petrographic Data File

Augite
Property
Value
Comments
Formula (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 Augite is a clinopyroxene, part of a solid solution series. It is an intermediate member of diopside and hedenbergite.
Crystal System Monoclinic 2/m
Luster Vitreous, translucent.  
Specific Gravity 3.2 - 3.6  
Hardness 5-6  
Color Black to dark green in hand sample. Usually colorless, tan-gray, pale green, pale brown or brownish green in thin section. May be zoned. Presence of tinanium darkens appearance.
Streak Greenish - white  
Fracture Uneven  
Crystal Habit Short prismatic crystals. In thin section: elongate on c- axis, with 4 or 8- sided cross sections; Grains often anhedral; May be granular, massive, columnar or lamellar  
Cleavage Fair to good cleavage on {110} at 87 degrees and 93 degrees. ~90 degree clevage visible 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 or nonexistent, but varies. Darker color may mask pleochroism.
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 0.030
Elongation Z^c=39-48
  
Extinction Inclined (approximately 45 degrees). Parallel when viewed down (010); Sometimes displays hourglass extinction  
Dispersion r > v  
Relief High Positive  
Twinning, Lamellae Commonly displays simple and lamellar twinning on {100} and {001}; They may combine to form a herringbone pattern. Exsolution lamellae may be present.  

Thin Section 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), Sasha Breus ('10), Paula Burgi ('14)

 
Augite phenocryst in K-84 showing 90-degree cleavage. Rollover image in plane and cross polarized light. Note the simple twin.
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.

A rollover image of an augite crystal at 5x magnification. Note the 90 degree cleavage, though imperfect. In cross polarized light, notice high birefringence. Photomicrograph from K-84.
Rollover image of an augite crystal in SIC-10. High relief and imperfect 90 degree cleavage. Birefringence=.014, low for augite. Magnification is 5x.
Rollover image of the biaxial nature of augite. Image from augite crystal in SIC-10. Note the approximately 60 degree 2V.
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