Spar is an old mining or mineralogical term for a crystal with clearly discernible crystal faces. Spar breaks or cleaves readily into rhombic, cubic or platy fragments with smooth, shiny surfaces. The word was widely used by miners and alchemists for any non-metallic mineral resembling gypsum, and the Old English form spaerstān referred to its crystalline projections. In mineralogy, spar is therefore related to spear.
Miners today often use spar alone for any bright crystalline substance. More commonly, spar refers specifically to light-coloured non-metallic minerals that cleave easily, such as feldspar, calcite or baryte. Baryte (BaSO₄) is the main source of barium and is also called heavy spar, from the Greek barys, meaning heavy.
The suffix survives in many mineral names, including feldspar, fluor-spar, calcite spar, Iceland spar and satin spar. These names reflect the historical practice of classifying minerals by appearance and cleavage rather than by modern chemical composition and crystal structure.
Calcite commonly forms dogtooth spar clusters in vugs and caves. Iceland spar is a transparent variety of calcite famous for double refraction and was once used to make polarising prisms and optical instruments. Baryte is used for its high specific gravity and chemical stability in drilling muds, fillers and radiation shielding.
Spar commonly forms underwater, either in the phreatic zone below the water table, a key region for cave formation, or as pool spar in still pools. If a cave is flooded and stalactites are submerged, structures called bottlebrushes may form.
Mineral ions dissolved in water are deposited over thousands of years, mostly as calcite, gypsum and sometimes halite, quartz and fluorite, accumulating layer by layer. These deposits record the chemical evolution of cave water, palaeoclimate and groundwater-flow history.
Spar sometimes forms in air where solutions seep from cave walls or through porous sediment. When it grows in air it usually consists of gypsum or selenite. It may also occur as small needle-like crystals in sediment, and other spar can be found at the tips of gypsum chandeliers.
The morphology of air-grown spar depends on evaporation rate, solution supply and cave ventilation. Rapid evaporation may produce fine needle-like or frosty crystals, whereas slow growth may produce larger selenite or calcite crystals.
In modern mineralogy, spar is not a formal classification term but a historical descriptive word. It remains in use in speleology, mining, gemmology and commerce, but formal descriptions should use the mineral species name and chemical composition. The value of the spar concept lies in the record it preserves of an early stage of mineral identification based on appearance, cleavage and lustre.