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Groutite from the Black Water Mine, Black Mesa Basin, Apache County, Arizona, USA
Category Oxide mineral
(repeating unit)
Strunz classification 4.FD.10
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pbnm
Unit cell a = 4.56, b = 10.7
c = 2.87 [Å]; Z = 4
Color Jet-black
Crystal habit Wedge or lens-shaped crystals; acicular, striated prisms
Twinning Reported, unknown law
Cleavage Perfect on {010}; less perfect on {100}
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 3.5 - 4.0
Luster Brilliant submetallic to adamantine
Streak Dark brown
Diaphaneity Opaque
Specific gravity 4.144
Optical properties Biaxial (+)
Refractive index nα = 2.100 - 2.200 nγ = 2.100
Pleochroism Very strong; X = very dark brown to black; Y = yellowish brown
2V angle Measured: 40° to 50°
References [1][2][3]

Groutite is a manganese oxide mineral with formula Mn3+O(OH). It is a member of the diaspore group and is trimorphous with manganite and feitknechtite. It forms lustrous black crystals in the orthorhombic system.

Cluster of groutite crystals with kaolinite filling half of a vug in a manganese nodule (size: 3.5 x 3 cm) from the Emilie Mine, Peine, Lower Saxony, Germany

It occurs in weathered banded iron formations, metamorphosed manganese ore bodies and hydrothermal ore environments.[1] It was first described in 1945 for an occurrence in the Mahnomen mine, Cuyuna Range, Crow Wing County, Minnesota and named for petrologist Frank Fitch Grout (1880–1958), of the University of Minnesota.[3]