Augite |
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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 |
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Refractive Indices alpha = beta = gamma = delta = |
1.688- 1.712 1.701- 1.717 1.713- 1.737 0.021- 0.025 |
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Max Birefringence | 0.030 | |
Elongation | Z^c=39-48 |
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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) |