Chromite Identification Guide

What Chromite Looks Like

Chromite (FeCr2O4) is an iron-chromium oxide of the spinel group. It is iron-black to brownish-black, with a metallic to submetallic or sometimes dull/resinous luster, and is opaque. Crystals are rare and form small octahedra (isometric system); far more commonly it occurs as granular masses, disseminated grains, or massive bands in dark ultramafic rocks. Fresh surfaces may show a slightly brownish cast.

Step-by-Step Field ID Checklist

1. Note the color and luster. Black with a metallic to submetallic sheen, opaque.
2. Check the streak — the key test. Chromite gives a brown to dark brown streak (not red, not black). This separates it from magnetite (black streak) and hematite (red streak).
3. Test magnetism. Chromite is weakly magnetic at most; it is usually only feebly attracted to a strong magnet, unlike strongly magnetic magnetite.
4. Test hardness. Mohs 5.5 — scratches glass with effort; harder than many sulfides.
5. Feel the weight. Dense (SG ~4.5-4.8).
6. Look at the host rock. Found in dark green/black ultramafic rocks (dunite, peridotite, serpentinite).

Key Diagnostic Tests

• Hardness: 5.5.
• Streak: Brown to dark brown (diagnostic).
• Cleavage: None; uneven to conchoidal fracture.
• Density: ~4.5-4.8 g/cm3.
• Magnetism: Weak/feeble (Cr-rich chromite is essentially non-magnetic; Fe-rich varieties slightly more so) — contrasts sharply with strongly magnetic magnetite.
• Acid: Insoluble, inert.

Common Look-Alikes and How to Tell Them Apart

• Magnetite: Strongly magnetic with a black streak; chromite is weakly magnetic with a brown streak. This pair of tests is the decisive separation.
• Ilmenite: Black with a black-to-brownish streak but typically only weakly magnetic and platy; ilmenite occurs more in mafic igneous rocks and often shows a slight reddish-brown streak; chemistry/host rock distinguishes.
• Hematite: Red-brown streak (chromite's is brown, hematite's more cherry-red); hematite often earthy or micaceous.
• Franklinite: Brown streak too, but found in zinc deposits (Franklin, NJ) and slightly magnetic.
• Sphalerite/dark sulfides: Softer, resinous, with pale or yellow-brown streak and often cleavage.

Where It Is Typically Found

Chromite is the only ore of chromium and forms by magmatic segregation in ultramafic and mafic igneous rocks — dunite, peridotite, pyroxenite — and in layered intrusions and ophiolite complexes, as well as in serpentinites and placer deposits derived from them. Major occurrences include the Bushveld Complex (South Africa), the Great Dyke (Zimbabwe), the Ural Mountains (Russia), Kazakhstan, Turkey, Finland, India, and the Stillwater Complex (Montana, USA).

Field Tips and Common Mistakes

The decision tree for any heavy black grain is simple: streak first, magnet second. A brown streak narrows it to chromite, ilmenite, or franklinite; a strong magnetic pull then points to magnetite (black streak) instead. Chromite's feeble magnetism trips up beginners who expect every black, dense, "iron" mineral to grab a magnet, so test with a powerful neodymium magnet and note only a weak tug. Because chromite rarely forms good crystals, identification leans heavily on context: massive or disseminated black grains in green-black serpentinite or peridotite, sometimes in podiform schlieren bands, are almost always chromite. Do not confuse the brownish tint on fresh chips with rust; it is intrinsic. When sampling stream sediment over ultramafic terrain, chromite reports as heavy, rounded black sand grains alongside magnetite and platinum-group minerals, and a hand magnet will preferentially pull the magnetite, leaving chromite behind in the pan.