FeSO₄ + 2K₂C₂O₄ → K₂[Fe(C₂O₄)₂] + K₂SO₄
Iron(II) sulfide react with potassium oxalate to produce potassium dioxalatoferrate(II) and potassium sulfate.
The mineral Humboldtine has the formula Fe²⁺(C₂O₄). 2H₂O
The color of the mineral is yellow to amber yellow.
It is a rare authigenic organic mineral occuring on fracture surfaces in coal deposits and rarely in granitic pegmatites and hydrothermal mineral deposits.