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Crystallized plates of reddish-brown, silver metallic-lustre hisingerite crystals
Category Silicate mineral
(repeating unit)
Strunz classification 9.ED.10
Crystal system Monoclinic
Unknown space group
Unit cell a = 5.4, b = 9.03
c = 14.99 [Å]; β = 98.32°; Z = 4
Color Black, brownish-black
Crystal habit Massive, compact; acicular, may be minutely spherical.
Cleavage None
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 2.5 - 3.0
Luster Vitreous, resinous, greasy
Streak Yellowish brown, green
Diaphaneity Transparent to translucent
Specific gravity 2.43 - 2.67
Optical properties Biaxial (-)
Refractive index nα = 1.715 nγ = 1.730
Birefringence δ = 0.015
References [1][2][3]

Hisingerite is an iron(III) phyllosilicate mineral with formula Fe3+2Si2O5(OH)4·2H2O. A black or dark brown, lustrous secondary mineral, it is formed by the weathering or hydrothermal alteration of other iron silicate and sulfide minerals.[1]

It was first described in 1828 for an occurrence in Riddarhyttan, Vastmanland, Sweden. It was named after Wilhelm Hisinger (1766–1852), a Swedish chemist.[3]

There are also aluminian hisingerite variety in which one of the iron atoms is replaced by aluminium and chrome-alumina-hisingerite variety in which chromium substitutes for iron.[2]