Chromite is the only commercial ore of chromium: a dark, heavy, metallic mineral, specific gravity 4.5-4.8, found in ultramafic rock, serpentinite belts and heavy-mineral stream sands. Identify it by its weight for its size, a streak test on unglazed porcelain, and a magnet, and confirm it by weighing a lump in air and in water before committing any fuel to smelting it.
Part of Stainless Steel Production
Finding and identifying chromite, the iron-chromium oxide mineral FeCr₂O₄ from which all stainless-steel chromium is made.
Chromite Hides In A Few Specific Rock Types
The carrier names three settings: ultramafic rock formations, serpentinite belts, and alluvial deposits where heavy minerals collect in stream sands. Ultramafic and serpentinite rock are dark, dense, often greenish and slippery-looking rocks; they weather to thin, poor soils with distinctive sparse vegetation. A district with those rocks is the place to look; a district of limestone, sandstone or granite is not.
Chromite occurs as black grains and lumps, sometimes in bands or pods within the rock. Where it has weathered out, its weight concentrates it in stream gravels. Panning a stream that drains serpentinite and looking for dense black grains is the cheapest survey available. The general prospecting methods are in Ore Identification; note that its tests are written for iron and copper ores, not chromium.
What You Need
- Unglazed porcelain (the back of a tile or plate) for the streak test.
- A magnet.
- A balance and a bucket of water, to find specific gravity.
- A hammer to break a fresh surface; weathered faces are misleading.
- A pan for stream sediments.
Method: Three Tests Before You Commit Fuel
Step 1: Look and lift. Fresh surface: black to dark brown with a metallic luster, per the carrier. Lift the lump: it should feel noticeably heavy for its size.
Step 2: Streak it. Drag a fresh edge across unglazed porcelain. The carrier says chromite leaves no streak and magnetite leaves a black one. My recollection of standard mineralogy references is that chromite leaves a dark brown streak, not none, so treat the carrier’s “no streak” with caution and rely on the magnet and weight tests below. This is flagged as unconfirmed.
Step 3: Magnet. Magnetite is strongly attracted. Chromite is weakly or not attracted, as I recall; again unsourced, so use it only to separate from magnetite, never to confirm chromite on its own.
Step 4: Specific gravity. Specific gravity is the weight in air divided by the weight lost when submerged.
Worked Example
A black lump weighs 100 g in air and 78 g when hung from a thread and fully submerged. It displaces 100 - 78 = 22 g of water. Specific gravity = 100 / 22 = 4.5, which is the bottom of the carrier’s 4.5-4.8 range. A lump scoring 3.5 is not chromite.
For the metal content: chromite is FeCr₂O₄. With atomic masses of Cr about 52.0, Fe about 55.8 and O about 16.0 (from my recall of standard values), the formula mass is 55.8 + 2 × 52.0 + 4 × 16.0 = 223.8, and chromium is 104.0 / 223.8 = 46% of pure chromite. Real ore is diluted with other minerals, so the usable chromium is less. I cannot source a typical ore grade from the corpus; assay a representative sample before planning production.
Common Mistakes
- Taking any heavy black rock for chromite. Magnetite, hematite and manganese oxides are also dark and dense. Run the weight and magnet tests together.
- Testing a weathered surface. Break the rock open first.
- Smelting before assaying. Reduction needs a furnace above 1,600 °C (Ferrochrome Production); a day of fuel on unverified rock is a day lost.
Safety
Crushing ore
Breaking and crushing rock throws splinters and makes fine dust. Wear eye protection and wet the rock while crushing; chromite dust carries chromium compounds I recall as hazardous to the lungs, though I have no corpus source for an exposure figure. See Dust and Respiratory Protection.
What’s Next
- Learn next: Ferrochrome Production - reducing the ore to an alloy
- Combine with: Nickel Sources - the second alloying metal for austenitic grades