The Many Colors of Iron

Unofficial Geology Law #3

If there's a color, iron can make it.

Iron, in all its wonderous forms, can cause most of the common colors found on Earth.

ColorIron PhaseExample MineralPhoto
RedFe(III)Hematite (Fe2O3)
PinkFe(II) + Mn(II)Rhodonite (Mn2+,Fe2+,Mg,Ca)- SiO3
Orange (Sienna)Fe(III)O HydroxideLimonite (FeO(OH)*H2O)
Yellow(1) Fe(III) Hydrated

(2) Fe(III) Sulfate
(1) Ferrihydrite (2Fe2O3*H2O)

(2) Coquimbite (Fe2(SO4)3*9H2O)
GreenFe(II) HydroxideFougerite ("green rust")
Green, Dark (rare)Fe(III) ChlorideMolysite (FeCl3)
Greenish- BlueFe(II) SulfateMelanterite (FeSO4 * 7H2O)
Blue- GrayFe(III) CubicMaghemite (Fe2O3)
Blue (rare)Fe(II) Phosphate + WaterVivianite (Fe3(PO4)2*8H2O)
PurpleFe(III)"Purple Ochre" (Fe2O3)
Purple (Violet)Fe(III) + Fe(II) Sulfate"Caput Mortuum" Hematite
Black(1) Fe(II)

(2) Fe(II,III)
(1) Wustite (FeO)

(2) Magnetite (Fe3O4)

But, Why?

Electron transitions cause most of the colors that our eyes can see (i.e., colors in the visible part of the electromagnetic spectrum). Metal-bearing molecules undergo a wider variety of transitions than non-metal-bearing molecules, and so can have a wider range of colors.

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