Chromates, Natural

a small class of minerals; the salts of chromium acids and certain large cations (Pb²⁺, K^–, and less frequently Ca²⁺). The class of natural chromates comprises about ten minerals, the principal one of which is crocoite. Other minerals include iranite, PbCrO₄·H₂O; tarapacaite, K₂CrO₄, phoenicochroite, Pb₃[CrO₄]₃O, and chromatite, CaCrO₄. Lopezite, or potassium dichromate, K₂Cr₂O₇, is also classified as a natural chromate. In addition, there are natural chromates in which, in addition to [CrO₄]^3–, other anions, such as [PO₄]^3–, [AsO₄]^3–, and [SiO₄]^4–, may be present.

Most of the natural chromates crystallize in lower-order systems, forming small prismatic, acicular, or tabular crystals or concretions, as well as fibrous, fine-grained, and sinter aggregates and crystalline crusts. Typical colors include bright yellow, orange, and red, caused by the presence of Cr⁶⁺. The hardness of the minerals vary from 2.5 to 3.5 on Mohs’ scale, and the density ranges from 2,700 to 3,600 kg/m³; however, the density of lead-containing chromates varies between 5,800 to 6,600 kg/m³. Potassium chromates are readily soluble in water.

Natural chromates form primarily in the zone of hypergenesis in environments with a high oxygen potential. Lead chromates are typical minerals of the oxidation zone of ore deposits occurring amid serpentinites. Potassium chromates are found primarily in deposits of sodium nitrate in Chile. Chromatite crystals have been found in cracks in limestones in the arid regions of the Middle East.