Actinolite is an amphibole silicate mineral with the formula Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2 As the formula states, Mg and Fe are variables. Mg and Fe ions are easily replaced in the crystal structure, with actinolite as an intermediate member in a series between magnesium rich tremolite, and iron rich ferro-actinolite. Fibrous actinolite is classed as an asbestos mineral. This specimen from South Wales, Australia is composed of actinolite and sphalerite with actinolite as the main component. The fibrous nature of actinolite can be seen in the inset, which is an enlargement of part of the main face, which has been cut and semi-polished. The sphalerite is "rubbly" in nature with no good crystals apparent.