celestite Mineral SrSO₄ which may form a solid solution series with barite; sp. gr. 3.9–4.0; hardness 3.0–3.5; orthorhombic; faint blue to colourless, sometimes stained red; white streak; vitreous lustre; crystals normally very tabular, resembling barite, but can also occur fibrous and granular; cleavage perfect basal {001}, present {210}, {010}; often fluorescent; occurs in sedimentary rocks, particularly dolomites, as a cavity-lining in association with barite, gypsum, anhydrite, and halite, with gypsum and anhydrite in evaporite deposits, also as a gangue mineral in hydrothermal veins with galena and sphalerite, and it will form concretionary masses in clays and marls. It is the main ore mineral for strontium and strontium compounds.

strontium (symbol Sr) Silvery-white, metallic element of the alkaline-earth metals in Group II of the periodic table. Resembling calcium physically and chemically, it occurs naturally in strontianite and celestite and is extracted by electrolysis. It was isolated in 1808 by English chemist Sir Humphry Davy. Strontium salts are used to colour flares and fireworks red. The isotope Sr⁹⁰ (half-life 28 years) is a radioactive element present in fallout, from which it is absorbed into milk and bones; it is used in nuclear reactors. Strontium salts are used in fireworks and signal flares. Properties: at.no. 38; r.a.m. 87.62; r.d. 2.554; m.p. 769°C (1416°F); b.p. 1384°C (2523°F).