Actinolite is an amphibole silicate mineral with the chemical formula Ca2(Mg,Fe)5Si8O22(OH)2. The name actinolite is derived from the Greek word aktis (ἀκτίς), meaning "beam" or "ray", because of the mineral's fibrous nature. (This word is also the origin of the name of the chemical element actinium.)
Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca2Mg5Si8O22(OH)2, and iron-rich ferro-actinolite, Ca2Fe5Si8O22(OH)2. Mg and Fe ions can be freely exchanged in the crystal structure. Actinolite is commonly found in metamorphic rocks, such as contact aureoles surrounding cooled intrusive igneous rocks. It also occurs as a product of metamorphism of magnesium-rich limestones.