groutite

{{infobox mineral

| name = Groutite

| image = Groutite-rare-09-33b.jpg

| alt =

| caption = Groutite crystals within a manganese nodule

| category = Oxide mineral

| formula = Mn³⁺O(OH)

| IMAsymbol = Gro{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight =

| strunz = 4.FD.10

| dana =

| system = Orthorhombic

| class = Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)

| symmetry = Pbnm

| unit cell = a = 4.56, b = 10.7
c = 2.87 [Å]; Z = 4

| color = Jet-black

| colour =

| 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 = 3.5 - 4.0

| luster = Brilliant submetallic to adamantine

| streak = Dark brown

| diaphaneity = Opaque

| gravity = 4.144

| density =

| polish =

| opticalprop = Biaxial (+)

| refractive = n_α = 2.100 - 2.200 n_γ = 2.100

| birefringence =

| pleochroism = Very strong; X = very dark brown to black; Y = yellowish brown

| 2V = Measured: 40° to 50°

| dispersion =

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| references = [http://rruff.geo.arizona.edu/doclib/hom/groutite.pdf Handbook of Mineralogy - Groutite][http://www.mindat.org/min-1756.html Groutite on Mindat.org][http://www.webmineral.com/data/Groutite.shtml Groutite on Webmineral]

}}

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.

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.