Chromite
FeCr₂O₄ · Cubic
Chromite is the only ore of chromium and looks much like magnetite. The brown streak and its weak magnetic response separate the two, and its geological company — serpentine, olivine, ultramafic rock — is a strong hint.
Properties
- Mohs hardness
- 5.5
- Streak
- brown
- Lustre
- submetallic, metallic
- Colour
- Brownish black, often with a slightly duller finish than magnetite.
- Cleavage
- No cleavage.
- Fracture
- Uneven
- Specific gravity
- 4.5–4.8 (water = 1)
- Magnetism
- Weakly magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm chromite
- Dark brown streak, versus magnetite's black.
- At most weakly magnetic.
- Almost always found in or near serpentinite and other ultramafic rock.
What chromite gets confused with
Each of these is settled by one test. Run that test first.
Magnet test and streak: magnetite is strongly magnetic with a black streak.
Where it is found
Ultramafic igneous rocks and the serpentinites derived from them; also in placer sands nearby.
Is it worth anything?
Negligible specimen value.