ACTINOLITE (from the Greek words "aktis", which means "beam" or "ray", and "lithos", which means "stone" * EN: actinolite; DE: Aktinolith, Strahlstein; FR: actinolithe; ES: actinolita; RU: актинолит) is the rock forming mineral of the amphibole group, the intermediate member of the isomorphic series from the tremolite to the ferro-actinolite, Ca2(Mg, Fe2+)5[Si4O11]2(OH, F)2. The admixtures are MnO (up to 7.4% MnO in the manganactinolite), Al, Fe3+. It crystallizes in the monoclinic crystal system. It forms radiant, columnar and asbestiform fibrous aggregates (actinolite-asbestos), less often the dense tangled fibrous masses, namely, the nephrite. The colour is green of the various shades. The hardness is 5.5-6. The density is 3170-3300 kilograms per cubic metre. The cleavage is perfect along one direction.

Actinolite is the typical metasomatic mineral. It is very common in the schists of the lower stage of the regional metamorphism, in the zones of the transformation of the ultramafic rocks, in the skarns. It is the final product of the transformation of the pyroxenes in the process of the uralitization. It is the accessory mineral of the certain acid rocks, pegmatites and carbonatites. Actinolite is the promising material, which is used in the rubber industry as the filler (during the production of the tires).