Cummingtonite
![](/Images/godic/202501/07/Amphibole_-_Cummingtonite_w-_chlorite_in_schist_Magnesium_iron_silicate_3800_foot_level_Homestake_Mine_Lawrence_COunty_South_Dakota_20711951.jpg")
![Fibrous, brownish crystals of cummingtonite - Locality: Dannemora Mine, Uppsala Län, Uppland, Sweden](/Images/godic/202501/07/Cummingtonite_-_Dannemora_Mine,_Uppsala_Län,_Uppland,_Sweden1951.jpg")
Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe)2(Mg,Fe)5Si8O22(OH)2, magnesium iron silicate hydroxide.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being metastable.