Groutite is a polymorph of manganite, MnO(OH). This sample is about 6 cm across. — Photo: Shannon Heinle · CC0
Groutite
A manganese oxyhydroxide of the diaspore group forming lustrous black orthorhombic crystals — trimorphous with manganite and feitknechtite.
Intro adapted from Wikipedia (CC BY-SA).
Groutite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Mn³⁺O(OH).
It crystallizes in the orthorhombic crystal system (space group Pbnm). It is a member of the Diaspore structural group.
Key properties: Mohs hardness 3.5–4; specific gravity 4.144; Dark brown streak; Brilliant submetallic to adamantine luster; refractive index 2.133; optically biaxial.
IMA symbol Gro, grandfathered status — known before the IMA’s founding, named in 1945.
A secondary mineral lining vugs in weathered banded iron formations (Minnesota, USA); manganiferous talc schists (Talcville, New York, USA); a metamorphosed stratiform zinc orebody (Franklin, New Jersey, USA).
It is found associated with Manganite, hematite, goethite, lepidocrocite, quartz, calcite.
The mineral was named to honor Professor Frank Fitch Grout (1880–1958), petrologist, of the University of Minnesota, Minneapolis, Minnesota, USA.