Akaganeite

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Akaganeite
A piece of the mineral akaganeite. Exhibit of the "Earth and Man" Museum in Sofia, Bulgaria. Discovered in Kaskasnyunchorr, Khibiny Massif, Kola Peninsula, Russia
General
Category	Oxide mineral
Formula	Fe3+O(OH,Cl)
IMA symbol	Akg[1]
Strunz classification	4.DK.05
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	I2/m
Unit cell	a = 10.561, b = 3.031 c = 10.483 [Å]; β = 90.63°; Z = 1
Identification
Color	Yellowish brown, rusty brown
Luster	Earthy
Streak	Brownish yellow
Diaphaneity	Transparent, translucent
Specific gravity	3.52

Akaganeite, also written as the deprecated Akaganéite,^[2] is a chloride-containing iron(III) oxide-hydroxide mineral, formed by the weathering of pyrrhotite (Fe_1−xS).

Akaganeite is often described as the β phase of anhydrous ferric oxyhydroxide FeOOH, but some chloride (or fluoride) ions are normally included in the structure,^[3] so a more accurate formula is FeO
_0.833(OH)
_1.167Cl
_0.167.^[4] Nickel may substitute for iron, yielding the more general formula (Fe³⁺
,Ni²⁺
)
₈(OH,O)
₁₆Cl
_1.25^[5]

Akaganeite has a metallic luster and a brownish yellow streak. Its crystal structure is monoclinic and similar to that of hollandite BaMn
₈O
₁₆, characterised by the presence of tunnels parallel to the c-axis of the tetragonal lattice. These tunnels are partially occupied by chloride anions that give to the crystal its structural stability.^[4]