Cummingtonite

Cummingtonite is a mineral belonging to the amphibole group. It is a complex silicate of iron and magnesium, with the chemical formula (Fe, Mg)7Si8O22(OH)2. Cummingtonite is a member of the orthorhombic amphibole series, specifically a solid solution series that ranges from grunerite (iron-rich) to cummingtonite itself, and then to its magnesium-rich end-member, anthophyllite. However, cummingtonite is typically used for compositions where magnesium is significantly present.

This mineral often occurs in metamorphic rocks, particularly those of medium to high grade. It can be found in schists, gneisses, and amphibolites, often forming as a result of the metamorphism of iron-rich sedimentary rocks or mafic igneous rocks. Cummingtonite typically appears as prismatic or acicular crystals, or in fibrous or lamellar aggregates. Its color can vary from pale green to gray or brown, and it has a vitreous luster.

The mineral was first described in 1824 from a locality in Cummington, Massachusetts, after which it is named. Cummingtonite is not a particularly common mineral but is found in various metamorphic terranes worldwide. It is relatively soft, with a Mohs hardness of 5 to 6, and has a specific gravity around 3.1 to 3.4. While it does not possess significant economic value, it is an important indicator mineral for understanding the geological conditions of metamorphic environments.