Mineral, FeCO₃; sp. gr. 3.8–4.0; hardness 3.5–4.5; trigonal; grey to grey-brown or yellowish-brown, translucent when pure; white streak; vitreous lustre; uneven fracture; crystals rhombohedral with curved faces, but also occurs massive, granular, fibrous, compact, botryoidal, and earthy in habit; cleavage perfect rhombohedral {1011}; widespread in sedimentary rocks, especially clays and shales where it is concretionary and makes clay into ironstone, also as a gangue mineral in hydrothermal veins together with other metallic ores (e.g. pyrite, chalcopyrite, and galena) and as a replacement mineral in limestone; dissolves slowly in cold, dilute hydrochloric acid, which effervesces when warmed.