An important member of the monoclinic calcium-rich amphiboles Ca2(Mg,Fe2+)5[Si4O11]2(OH,F)2, which forms a series with ferroactinolite; sp. gr. 3.02–3.44; hardness 5.0–6.0; monoclinic; white to greyish-white; vitreous lustre; crystals simple, long, prismatic, and acicular, often in thin, radial, rod-like, fibrous, and felted masses; cleavage perfect prismatic {110}; widespread amphibole mineral in igneous rocks, and also in metamorphosed, crystalline limestones, dolomites, and in schists and hornfels. It is named after the Tremola Valley near St. Gotthard, Switzerland. See also jadeite.