Ferrihydrite

Ferrihydrite is an iron oxyhydroxide with nominal formula 5Fe2O3·9H2O. Although it is an accepted mineral there is much controversy about its crystal structure and crystal chemistry. It forms by rapid precipitation when reduced-iron-bearing ground water (or hydrothermal water) encounters oxygenated water.
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General Information:
 Chemical Formula:
Fe+++2O3·0.5(H2O)
 Composition:
(Molecular Weight = 168.70 gm)
Iron 66.21 % Fe
Hydrogen 0.60 % H 
Oxygen 33.19 % O 
 Empirical Formula:
Fe3+2O3·0.5(H2O)
 IMA Status:
Approved IMA 1971
Physical Properties:
 Color:
yellow brown or dark brown.
 Density:
3.8
 Streak:
yellowish brown
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