Cummingtonite, a hydroxide of, magnesium iron silicate is a metamorphic (transformed from other rock types) amphibole (Dark colored prism forming innosilicate minerals). Its chemical composition is magnesium iron silicate hydroxide. The Monoclinic cummingtonite has a composition that is analogous and polymorphic to orthorhombic anthophyllite, a substantially more recognizable magnesium-rich amphibole structure, which is metastable.
To some extent its composition is similar to alkali amphiboles of the type of Arfvedsonite and Glaucophane-Riebeckite. There is insignificant solubility between the two minerals because of their different crystal patterns and incapability of replacement among alkali components and ferromagnesian components in the amphibole structure.
Mg2Mg5Si8O22 (OH) 2
Derivation of Name
The mineral Cummingtonite gets its name from Cummington, Massachusetts, where it was discovered in 1824
Cummingtonite is usually appears in magnesium-rich rocks that have metaphorsed and found in amphibolites. Generally, it exists with Hornblende or Actinolite, Talc, Magnesium Clinochlore Chlorite, minerals of the Serpentine-Antigorite variety or Pyroxene (metamorphic). Cummingtonite with high magnesium content can also exist with Anthophyllite.
This mineral is not used as gemstones.
It prominently occurs in Cummington (so the name), in the states of the USA such as Massachusetts, Hampshire County; South Dakota, Lawrence County, the Homestake gold mine, Arizona and La Paz County; in Sweden; South Africa and Scotland.
Cummingtonite forms a constituent of the solid solution sequence of cummingtonite-grunerite that varies from Mg7Si8O22 (OH) 2 (magnesiocummingtonite) to the iron loaded Grunerite end member Fe7Si8O22 (OH) 2. Cummingtonite is used to describe minerals of this formula having 30 and 70 per cent of Fe7Si8O22 (OH) 2. So, cummingtonite is in the middle of the series.
Manganese also replaces (Fe, Mg) inside cummingtonite amphibole, substituting B site atoms.
These minerals are arranged in banded iron formation OF A high-grade metamorphic order, and delivers a compositional string between tirodite and dannemorite.
Cummingtonite has low concentrations of sodium, potassium and calcium. Cummingtonite leans more toward calcium substitution than connected anthophyllite.
Likewise, cummingtonite possesses lower aluminum and ferric iron than Anthophyllite.
Dark green, gray, brown beige; pale green to colorless
Mohs scale hardness
5 - 6