Actinolite is composed of Ca2 (Mg,Fe)5[Si4 O11](OH)2 and belongs to the same polymorph series as tremolite. It has a monoclinic crystal structure, and the crystals are long columns. The aggregates are radial, granular or blocky. The fibrous type is known as actinolite asbestos. It has various colours of green, while the lustre is vitreous, the Mohs hardness is 5.5–6, and the density is 3.0–3.3/cm3. It is often found in contact-metamorphosed rocks.