Lepidocrocite

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.

Lepidocrocite
A sample of lepidocrocite
General
Category	Oxide minerals
Formula	γ-FeO(OH)
IMA symbol	Lpc[1]
Strunz classification	4.FE.15
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Cmcm
Unit cell	a = 3.88, b = 12.54 c = 3.07 [Å]; Z = 4
Identification
Formula mass	88.85 g/mol
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
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	[2][3][4]

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.