OCHRE PRODUCING MINERALS
Hematite α-Fe2O3
Colors: Red range (purple, pink, black, orange)
Goethite α-FeOOH
Colors: Brownish-yellow range (yellowish-green (small particles) to yellow-browns (large particles) )
Limonite FeO⋅OH + FexOx
Colors: Yellowish-red range (composite mixtures)
Lepidocrocite 𝛾-FeO⋅OH
Colors: Brownish range
Maghemite γ-Fe2 O3
Colors: Brown-reddish range (dark red, black-brown-red)
Magnetite Fe3 O4
Colors: Black range (Black, dark red, black- brown)
Vivianite Fe2+3(PO4)2 . 8H2O (iron-phosphate)
Colors: white, blue, blue-green, green, black
Siderite FeCO3 (iron-carbonate)
Colors: Brownish-orange, Brownish-red, Brownish-yellow
Basic color change info:
There are five rudimentary ways to change iron-based colors:
- temperature
- oxygen
- light/radiation
- water
- weathering/time
Exposure or admixtures with other elements, minerals and organic material also change the way we experience the color of iron-based minerals.
You can force transformation of color with the following:
Oxide dehydration (removing water)
Oxide hydration (adding water)
Intervalence charge transfer (exciting ions)
To learn more in depth, a great iron-oxide laboratory resource is:
Iron Oxides From Nature to Applications edited by Damien Faivre.
For pigment foragers and natural paint makers:
Colors From the Earth by Anne Wall Thomas (thought out of print) is the classic text.