A rock-forming mineral Ca₂(Al₂Fe³⁺)Si₃O₁₂(OH) and a member of the epidote group of minerals, which includes zoisite, clinozoisite, and allanite; sp. gr. 3.4–3.5; hardness 6.5; monoclinic; normally pistachio-green, but also various shades of green, yellow, grey, or black; vitreous lustre; crystals are prismatic parallelograms, often rod-like and isometric, often showing radial, fibrous, and columnar masses; cleavage perfect {001}; it occurs in hydrothermal formations, especially associated with altered, basic, igneous rocks, and also in contact metamorphic zones with quartz, chlorite, calcite, and sulphides, and can also replace various minerals, such as amphiboles, which break down under late-stage hydrothermal alteration.