actinolite

Actinolite is a mineral in the amphibole group with the formula Ca2(Mg,Fe)5Si8O22(OH)2. It represents the magnesium-iron member of the tremolite–actinolite series, with tremolite being the Mg-rich end member and ferro-actinolite the Fe-rich end member. The name derives from Greek aktinos, “ray,” in reference to its fibrous crystals.

Actinolite forms during regional or contact metamorphism of calcium- and magnesium-rich rocks. It is common in greenschist facies and as a constituent of actinolite schists, and it may occur with chlorite, epidote, serpentine, and calcite in metamorphosed limestones or basalts. It can appear as elongated prisms or as fibrous aggregates, and is typically green to greenish-gray; darker varieties occur with higher iron content. Cleavage is two directions at about 56 and 124 degrees, a characteristic trait of amphiboles. Hardness is about 5–6 on the Mohs scale; specific gravity ranges roughly 3.0–3.3.

Actinolite is chiefly of interest as a rock-forming mineral and as an indicator of metamorphic conditions. It may appear in decorative stones and is sometimes encountered as an asbestos-form of the mineral; fibrous varieties are part of the amphibole asbestos group, and inhalation of fibers poses health hazards; accordingly, handling of fibrous actinolite is regulated in many jurisdictions.