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
CategoryOxide mineral
Formula
(repeating unit)
Mn3+O(OH)
IMA symbolGro[1]
Strunz classification4.FD.10
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPbnm
Unit cella = 4.56, b = 10.7
c = 2.87 [Å]; Z = 4
Identification
ColorJet-black
Crystal habitWedge or lens-shaped crystals; acicular, striated prisms
TwinningReported, unknown law
CleavagePerfect on {010}; less perfect on {100}
FractureUneven
TenacityBrittle
Mohs scale hardness3.5 - 4.0
LusterBrilliant submetallic to adamantine
StreakDark brown
DiaphaneityOpaque
Specific gravity4.144
Optical propertiesBiaxial (+)
Refractive indexnα = 2.100 - 2.200 nγ = 2.100
PleochroismVery strong; X = very dark brown to black; Y = yellowish brown
2V angleMeasured: 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]

References

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  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Handbook of Mineralogy - Groutite
  3. ^ Groutite on Mindat.org
  4. ^ a b Groutite on Webmineral