Actinote is a mineral belonging to the amphibole group of inosilicate minerals. It is an intermediate member in a solid solution series between tremolite (calcium-magnesium amphibole) and ferro-actinolite (calcium-iron amphibole). The chemical formula for actinote is Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂.
Actinote is characterized by its monoclinic crystal system, prismatic habit, and frequently occurs as radiating or fibrous aggregates. Its color ranges from bright green to greyish-green or greenish-black, depending on the iron content. The higher the iron content, the darker the color.
Actinote has a Mohs hardness of 5 to 6 and a specific gravity between 3.0 and 3.3. It exhibits perfect cleavage in two directions along the prismatic planes. Its luster is vitreous to silky.
Actinote is typically found in metamorphic rocks, particularly in regionally metamorphosed rocks such as schists and greenstones. It can also occur in altered igneous rocks. It is often associated with other metamorphic minerals such as chlorite, epidote, and quartz.
Actinolite is sometimes used as a gemstone when it exhibits chatoyancy, which is a cat's-eye effect. Nephrite jade is a compact variety of actinolite-tremolite solid solution.