Ferrihydrite

Ferrihydrite
Mine drainage from Ohio. The orange coating on the logs is ferrihydrite.
General
Category	Oxide minerals
Formula (repeating unit)	(Fe3+)2O3·0.5H2O
IMA symbol	Fhy[1]
Strunz classification	4.FE.35
Dana classification	04.03.02.02
Crystal system	Hexagonal
Crystal class	Dihexagonal pyramidal (6mm) H-M symbol: (6mm)
Space group	P63mc
Unit cell	a = 5.958, c = 8.965 [Å]; Z = 1
Identification
Formula mass	168.70 g/mol
Color	Dark brown, yellow-brown
Crystal habit	Aggregates, microscopic crystals
Streak	Yellow-brown
Diaphaneity	Opaque
Density	3.8 g/cm3
References	[2][3][4][5]

Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide mineral at the Earth's surface,^[6][7] and a likely constituent in extraterrestrial materials.^[8] It forms in several types of environments, from freshwater to marine systems, aquifers to hydrothermal hot springs and scales, soils, and areas affected by mining. It can be precipitated directly from oxygenated iron-rich aqueous solutions, or by bacteria either as a result of a metabolic activity or passive sorption of dissolved iron followed by nucleation reactions.^[9] Ferrihydrite also occurs in the core of the ferritin protein from many living organisms, for the purpose of intra-cellular iron storage.^[10][11]