It may be formed by the reaction of potassium chromate and iron(III) nitrate. This reaction forms iron(III) chromate and potassium nitrate. It also can be formed by the oxidation by air of iron and chromium oxides in a basic environment:
4 Fe2O3 + 6 Cr2O3 + 9 O2 → 4 Fe2(CrO4)3
References
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Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, Florida: CRC Press. pp. 4–63. ISBN0-8493-0594-2.