Spar (mineralogy)
Spar is a term that is used to refer to a category of crystals that have readily discernible faces. Crystallized, earthy, and some metallic, a spar will easily break into rhomboidal, cubical, or laminated fragments with polished surfaces, but without regard to the ingredients of which they are composed.
The various "spar" minerals were a historical term among miners and alchemists for any nonmetallic mineral akin to gypsum, known in Old English as spærstān, spear stone, referring to its crystalline projections. Thus, the word spar in mineralogy has the same root as "spear," by way of comparison to gypsum, as a common natural crystal forming in spearlike projections.
Amongst miners the term "spar" today is frequently used alone to express any bright crystalline substance. [1] Most frequently, spar describes easily cleaved, lightly colored nonmetallic minerals such as feldspar, calcite or barite. Barite (BaSO4), the main source of barium, is also called "heavy spar" (Greek "barys" means "heavy"). The largest type of spar is of the phreatic variety, since it essentially much more time to grow than average spar. It grows best where the water is just barely saturated with minerals. Some caves are like giant geodes, the walls and ceilings completely lined with spar. Jewel Cave and others in the Black Hills of South Dakota contain large quantities of spar.[1]
Formation
Generally, a spar will form underwater, either in a phreatic zone, or below the water table, the essential place where most caves form. [2] Minerals in the water, mostly calcite or gypsum, but sometimes even halite, quartz, and fluorite, are deposited through the course of thousands of years, building up on each other.
Growth in the air
Sometimes, spar will form in the air due to solutions seeping out of the cave's walls or through porous sediments. When grown in the air, it is often made of gypsum or selenite. Sometimes it will form as small needles found in sediments. Others spar can be found on the tips of gypsum chandeliers.
Pool spar
Pool spar are well-defined crystals that grow underwater in pools, and are a subtype of spar crystal, usually composed of calcite. [2] Often the pool in which the crystals grew has evaporated, or fills only seasonally. Pool spar is often found on the bottom of shelfstone.
Most pool spar is nicknamed "dogtooth" because it comes to a jagged point. Other pool spar, especially ones formed along the flat surface of the pool, may even form in a triangular shape.
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