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.
| Color | Iron Phase | Example Mineral | Photo |
| Red | Fe(III) | Hematite (Fe2O3) |  |
| Pink | Fe(II) + Mn(II) | Rhodonite (Mn2+,Fe2+,Mg,Ca)- SiO3 |  |
| Orange (Sienna) | Fe(III)O Hydroxide | Limonite (FeO(OH)*H2O) |  |
| Yellow | (1) Fe(III) Hydrated (2) Fe(III) Sulfate | (1) Ferrihydrite (2Fe2O3*H2O) (2) Coquimbite (Fe2(SO4)3*9H2O) |  |
| Green | Fe(II) Hydroxide | Fougerite ("green rust") |  |
| Green, Dark (rare) | Fe(III) Chloride | Molysite (FeCl3) |  |
| Greenish- Blue | Fe(II) Sulfate | Melanterite (FeSO4 * 7H2O) |  |
| Blue- Gray | Fe(III) Cubic | Maghemite (Fe2O3) |  |
| Blue (rare) | Fe(II) Phosphate + Water | Vivianite (Fe3(PO4)2*8H2O) |  |
| Purple | Fe(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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