The primary chromium ore is the mixed metal oxide chromite, FeCr₂O₄, found as brittle metallic black crystals or granules.
Chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air.
The chromium is oxidized to the hexavalent form, while the iron forms iron(III) oxide, Fe₂O₃:
4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂
Subsequent leaching of this material at higher temperatures dissolves the chromates, leaving a residue of insoluble iron oxide.
Normally the chromate solution is further processed to make chromium metal, but a chromate salt may be obtained directly from the liquor.

Chromate containing minerals are rare.
Crocoite, PbCrO₄, which can occur as spectacular long red crystals, is the most commonly found chromate mineral.
Rare potassium chromate minerals and related compounds are found in the Atacama desert.
Among them is lópezite – the only known dichromate mineral.
The primary chromium ore is the mixed metal oxide chromite, FeCr₂O₄, found as brittle metallic black crystals or granules. Chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms iron(III) oxide, Fe₂O₃:
4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂
Subsequent leaching of this material at higher temperatures dissolves the chromates, leaving a residue of insoluble iron oxide. Normally the chromate solution is further processed to make chromium metal, but a chromate salt may be obtained directly from the liquor.
Chromate containing minerals are rare. Crocoite, PbCrO₄, which can occur as spectacular long red crystals, is the most commonly found chromate mineral. Rare potassium chromate minerals and related compounds are found in the Atacama desert. Among them is lópezite – the only known dichromate mineral.
Chromate and dichromate