Lepidocrocite (γ-FeO(OH)), also called esmeraldite or hydrohematite, is an iron oxide-hydroxide mineral. Lepidocrocite has an orthorhombic crystal structure, a hardness of 5, specific gravity of 4, a submetallic luster and a yellow-brown streak. It is red to reddish brown and forms when iron-containing substances rust underwater. Lepidocrocite is commonly found in the weathering of primary iron minerals and in iron ore deposits. It can be seen as rust scale inside old steel water pipes and water tanks.

Mineraly.sk - lepidokrokit.jpg
A sample of lepidocrocite
CategoryOxide minerals
(repeating unit)
Strunz classification4.FE.15
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupCmcm
Unit cella = 3.88, b = 12.54
c = 3.07 [Å]; Z = 4
Formula mass88.85 g/mol
ColorRuby-red to reddish brown; light reddish to red-orange in transmitted light; gray-white in reflected light
Crystal habitFlattened scales aggregated into plumose groups and rosettes; massive, bladed to fibrous or micaceous
Cleavage{010} Perfect
Mohs scale hardness5
Lustersub metallic
StreakDull orange
Specific gravity4
Optical propertiesBiaxial (-)
Refractive indexnα = 1.940 nβ = 2.200 nγ = 2.510
Birefringenceδ = 0.570
PleochroismStrong; X = colorless to yellow; Y = orange, yellow, dark red-orange; Z = orange, yellow, darker red-orange
2V angleMeasured: 83°

The structure of lepidocrocite is similar to the boehmite structure found in bauxite and consists of layered iron(III) oxide octahedra bonded by hydrogen bonding via hydroxide layers. This relatively weakly bonded layering accounts for the scaley habit of the mineral.

It was first described in 1813 from the Zlaté Hory polymetallic ore deposit in Moravia, Czech Republic. The name is from the Greek lipis for scale and krokis for fibre.

Layered crystal structure of lepidocrocite. Iron atoms shown as brown spheres; lattice oxygen as red spheres; and hydroxyl oxygen as magenta spheres. Hydrogen atoms are not shown.