A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5700 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfate minerals all contain one or more sulfate anions (SO4-2).

Celestite (also known as "celestine") is a strontium sulfate mineral, SrSO4. Its name has the same etymology as "celestial", meaning "sky", in reference its bluish to pale bluish crystals. This mineral has a nonmetallic luster, a clearish to whitish to pale blue to bluish-gray color, a white streak, a hardness of 3 to 3.5, two cleavages, and is noticeably heavy for its size. It forms diagenetically and also occurs in low-temperature hydrothermal vein systems.

The celestite crystal shown here is filling a cavity (vug) in Silurian dolostone. The specimen is from northwestern Ohio, which is on the southeastern flanks of the Michigan Basin. The strontium-rich fluids from which the celestite crystallized apparently traveled updip from the Michigan Basin. For more info. on Ohio celestite, see Carlson (1991, Minerals of Ohio, Ohio Geological Survey Bulletin 69, 155 pp.) and Carlson (2015 - Minerals of Ohio, Second Edition, Ohio Geological Survey Bulletin 69 (Second Edition), 290 pp.).

Locality: quarry on the southern side of Genoa, western Ottawa County, northwestern Ohio, USA

Photo gallery of celestite: