Actinolite is an amphibole silicate mineral with the chemical formula Ca2(Mg,Fe)5Si8O22(OH)2. It forms a series with Tremolite, where the iron content in actinolite can vary from low to high, with tremolite being the iron-free end member. Actinolite's color can range from light green to dark green or even black when iron-rich.
Actinolite is commonly found in metamorphic rocks such as schists and gneisses, where it forms under conditions of low to medium grade metamorphism. It can also occur in igneous rocks like gabbro and basalt, and in some cases, it is a product of the hydrothermal alteration of primary minerals in these rocks. Actinolite often appears as needle-like crystals or in fibrous masses, which can sometimes be confused with asbestos due to its similar appearance.
The name "actinolite" comes from the Greek words for "ray" and "stone," reflecting its sometimes radiant appearance. It was first described in 1794 by R. J. Hauy. While not widely used as a gemstone due to its cleavage, fibrous varieties of actinolite have been used as ornamental stone, particularly in carvings and sculptures. The mineral's fibrous form, known as nephrite, is one of the two minerals historically used for jade.
Actinolite can be found worldwide. Some notable locations include:
As with other amphibole minerals, the fibrous form of actinolite can pose health risks if inhaled, potentially leading to respiratory issues similar to those caused by asbestos.