Groutite
(repeating unit)Mn3+O(OH) IMA symbol Gro[1] 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 Identification 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 [2][3][4]
H-M symbol: (2/m 2/m 2/m)
c = 2.87 [Å]; Z = 4
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.
It occurs in weathered banded iron formations, metamorphosed manganese ore bodies and hydrothermal ore environments.[2] 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.[4]
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