Bischofite

BISCHOFITE (has been named after the German scientist Gustav Bischof * EN: bischofite; DE: Bischofit; FR: bichofite; ES: bischofita; RU: бишофит) is the mineral of the class of halides, MgCl2•6H2O. It sometimes contains the impurity of Br (up to 1%). It crystallizes itself into the monoclinic crystal system. The crystal structure is sub-molecular; there are at its base the isolated Mg(H2O)6Cl2 groups, which are similar to molecules. It forms the short-prismatic and acicular crystals, more often the aggregates: granular, foliated, fibrous. There are characteristic the secondary gliding twins. It is colourless or white, sometimes coloured with the finely dispersed hematite into the red colour. The lustre is vitreous or matte. It is brittle. The cleavage is absent. The hardness is 1-2. The density is approximately 1600 kilograms per cubic metre. It is hygroscopic, deliquesces during the exposure to the air (especially cold). It is well soluble within water and alcohol. It has the pungent, bitter flavour. It is macroscopically determined with difficulty, and, as the consequence of this, was considered rare during a long time. According to origin, it is mainly sedimentary, chemogenic (although it has been also found within the volcanic sublimates of the Vesuvius volcano).

Bischofite is the final product of the halogenesis, which is emerging at the final stage of the forming of the salt depositions of the sulfate type, which are rich in magnesium. Bischofite is the characteristic, but usually secondary, mineral of the salt depositions, especially of the potassium ones. It is also known among the products of crystallization of the brine within the modern continental salty lakes (the Lake Saki within the Crimea peninsula; the Lake Elton within the Volgograd province of the Russian Federation, and others), where it deposits itself prevalently during the periods of droughts, especially during the decreasing of temperature. The largest clusters of bischofite, including its industrial depositions, are forming themselves during the crystallization within the evaporating marine halmeic water bodies. Depending on the degree of metamorphism of the brine under the influence of the addition of the fresh waters, they distinguish the two lines of the development of the halogenic process within the marine halmeic basins: the sulphate one (in case of the absence of metamorphism, and of the preservation of MgSO4 within the brine), and the sulfate-free one (in case of the significant manifestation of metamorphism of the brine, and of the loss of MgSO4 by this brine).

The sulfate-free deposits represent the greatest practical interest in relation to the forming of the industrial deposits of bischofite. These sulfate-free deposits are usually confined to the marginal depressions of cratons, and have formed themselves during the drying of the epi-continental marine basins. For the forming of the independent large depositions of bischofite, the brine must pass sequentially through several intermediate water bodies, which ensures the spatial isolation of the final products of its crystallization. Bischofite also forms itself as the secondary mineral on the surface of carnallite during its decomposition because of the action of the water. Within the certain deposits of the potassium salts (Saxony, within the East Germany; Ozinki, within the Saratov province, within the USSR, and others) the layers of the bischofite rocks reach the thickness of several metres. The largest deposits of bischofite (with the thickness of many metres, and with the wide areal distribution) have been discovered near the Volgograd city; there has been started its experimental-industrial operation by the method of the borehole leaching.

Bischofite is the valuable and cheap raw material for the obtainment of magnesium and its compounds, and for the production of the magnesia cement; it is used for the impregnation of wood with the goal of providing it with strength.