Groutite

Groutite
Groutite crystals within a manganese nodule
General
Category	Oxide mineral
Formula (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]

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 . 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 (1880–1958), of the University of Minnesota.^[4]

References[]