Hornblende (Amphibole)
complex Ca-Mg-Fe silicate · Monoclinic · also called amphibole, black amphibole
Hornblende is the most common dark mineral in intermediate igneous rocks such as diorite and andesite. Its 60/120-degree cleavage angle is the field test that separates the amphibole family from the pyroxenes, which cleave near 90°.
Properties
- Mohs hardness
- 5–6
- Streak
- white, gray
- Lustre
- vitreous
- Colour
- Dark green to black, with a glassy shine on cleavage faces.
- Cleavage
- Two cleavages meeting at about 56° and 124° — a distinctly diamond-shaped cross-section. This is the standard way to separate amphibole from pyroxene.
- Fracture
- Uneven
- Specific gravity
- 3–3.4 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm hornblende
- Cleavage angles near 60°/120°, best seen by looking down the end of a broken crystal.
- Long, slender, needle-like or bladed crystals.
- Slightly softer than feldspar; a good knife will mark it.
What hornblende gets confused with
Each of these is settled by one test. Run that test first.
Augite cleaves at close to 90°; hornblende at about 60°/120°.
Tourmaline is harder and has no cleavage.
Where it is found
Diorite, andesite, amphibolite, and many gneisses.
Is it worth anything?
None.