Grunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O22(OH)2. It is an iron-rich amphibole related to cummingtonite. The origin of the name is Amosa, the acronym for the mining company “Asbestos Mines of South Africa”. It was discovered in 1853 and named after Emmanuel-Louis Gruner (1809–1883), a Swiss-French chemist who first analyzed it.
- Category: Inosilicate
- Formula: Fe7Si8O22(OH)2
- Crystal system: Monoclinic
- Crystal class: Prismatic (2/m) (same H-M symbol)
Fig: Grunerite (amphibole group)
It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors ranging from green, brown to dark grey. The Mohs hardness is 5 to 6 and the specific gravity is 3.4 to 3.5.
- Formula mass: 1,001.61 g/mol
- Color: Ashen, brown, brownish green, dark gray
- Crystal habit: Columnar
- Mohs scale hardness: 5 – 6
- Luster: Vitreous
- Streak: Colorless
- Diaphaneity: Translucent to opaque
- Specific gravity: 3.45
- Density: 3.4 – 3.5
Occurrence: Common in medium- to high-grade metamorphosed iron formations. A product of contact metamorphism, and in some blueschist facies metaquartzites.
Amosite is a rare asbestiform variety of grunerite that was mined as asbestos predominantly in the eastern part of the Transvaal Province of South Africa.
Association: Magnetite, hematite, hedenbergite, riebeckite, fayalite, garnet, quartz.