Editing Chromate and dichromate (section)

==Natural occurrence and production==
[[File:Crocoite from Tasmania.jpg|thumb|Crocoite specimen from the Red Lead Mine, [[Tasmania]], [[Australia]]]]
The primary chromium ore is the mixed metal oxide [[chromite]], {{chem2|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, {{chem2|Fe2O3}}:
:{{chem2|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.<ref name="IndMin">{{cite book | title =Industrial Minerals & Rocks: Commodities, Markets, and Uses | edition = 7th | publisher = SME | year = 2006 | isbn = 978-0-87335-233-8 | chapter = Chromite | first = John F. | last = Papp |author2=Lipin Bruce R. | chapter-url = https://books.google.com/books?id=zNicdkuulE4C&pg=PA309}}</ref>

Chromate containing minerals are rare. [[Crocoite]], {{chem2|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.<ref>{{cite web |url=http://www.mindat.org |title=Mines, Minerals and More |website=www.mindat.org}}{{page needed|date=February 2020}}</ref>

As chromate is [[isostructural]] to [[sulfate]], sulfate and chromate minerals can form [[solid solution]]s such as [[hashemite (mineral)|hashemite]], and chromate minerals are often listed alongside [[sulfate mineral]]s in mineral classification schemes such as [[Nickel-Strunz classification]].