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Mineraly.sk - lepidokrokit.jpg
A sample of lepidocrocite
Category Oxide minerals
(repeating unit)
Strunz classification 04.FE.15
Crystal symmetry Orthorhombic (2/m 2/m 2/m) - dipyramidal
Unit cell a = 3.88 Å, b = 12.54 Å, c = 3.07 Å; Z = 4
Formula mass 88.85
Color Ruby-red to reddish brown; light reddish to red-orange in transmitted light; gray-white in reflected light
Crystal habit Flattened scales aggregated into plumose groups and rosettes; massive, bladed to fibrous or micaceous
Crystal system orthorhombic
Cleavage {010} Perfect
Mohs scale hardness 5
Luster sub metallic
Streak Dull orange
Diaphaneity Transparent
Specific gravity 4
Optical properties Biaxial (-)
Refractive index nα = 1.940 nβ = 2.200 nγ = 2.510
Birefringence δ = 0.570
Pleochroism Strong; X = colorless to yellow; Y = orange, yellow, dark red-orange; Z = orange, yellow, darker red-orange
2V angle Measured: 83°
References [1][2][3]

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.

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.