Ottrélite from Ottré, Belgien — Photo: Kritzolina · CC BY-SA 4.0
Ottrélite
Ottrélite is an IMA-recognized mineral species in the silicate class, with the chemical formula Mn²⁺Al₂O(SiO₄)(OH)₂.
It crystallizes in the monoclinic crystal system. It is a member of the Chloritoid structural group.
Key properties: Mohs hardness 6–7; specific gravity 3.52; Greenish, grayish streak; Vitreous to nearly adamantine on cleavages luster; optically biaxial.
IMA symbol Otr, grandfathered status — known before the IMA’s founding, named in 1842.
Formed under nonshearing stress conditions in veins and cavities within low- to medium-grade metamorphic rocks; also a medium-temperature hydrothermal alteration mineral.
It is found associated with Dickite, kaolinite, rutile, andalusite, chlorite, pyrophyllite, davreuxite, quartz.
The name refers to its occurrence at Ottr¶e, Belgium.