Augite (Pyroxene)
(Ca,Mg,Fe)₂Si₂O₆ · Monoclinic · also called pyroxene
Augite is the dominant dark mineral in basalt and gabbro. It is short and blocky where hornblende is long and bladed, and its cleavage meets at right angles rather than at 60 degrees.
Properties
- Mohs hardness
- 5.5–6
- Streak
- white, gray
- Lustre
- vitreous, dull
- Colour
- Dark green-black to brown-black; typically duller than hornblende.
- Cleavage
- Two cleavages meeting at close to 90°, giving a squarish cross-section.
- Fracture
- Uneven
- Specific gravity
- 3.2–3.6 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm augite
- Near-90° cleavage angles and short, stubby, blocky crystals.
- Common in basalt as small dark grains.
- Duller lustre than hornblende.
What augite gets confused with
Each of these is settled by one test. Run that test first.
Cleavage angle: 90° for augite, 60°/120° for hornblende.
Where it is found
Basalt, gabbro, and other magnesium- and iron-rich igneous rocks.
Is it worth anything?
None.