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.

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]

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]