Boracite is a mineral of special interest on account of its optical anomalies. Small crystals bounded on all sides by sharply defined faces are found in considerable numbers embedded in gypsum and anhydrite in the salt deposits at Luneburg in Hanover, where it was first observed in 1787. In external form these crystals are cubic with inclined hemihedrism, the symmetry being the same as in bleride and tetrahedrite. Their habit varies according to whether the tetrahedron, the cube, or the rhombic dodecahedron predominates. This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band). ... Quartz crystal A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. // Chemical structure Gypsum from New South Wales, Australia Heating gypsum above approximately 150°C (302°F) partially dehydrates the mineral by driving off exactly 75% of the water contained in its chemical structure. ... Anhydrite is a mineral - anhydrous calcium sulfate, CaSO4. ...

Crystals of Boracite

Twins with a tetrahedron face as twin-plane are sometimes observed. The crystals vary from translucent to transparent, are possessed of a vitreous luster, and are colorless or white, though often tinged with grey, yellow or green. The hardness is as high as 7, and the specific gravity is 30. As first observed by R. J. Hafly in 1791, the crystals are markedly pyroelectric; a cube when heated becomes positively electrified on four of its corners and negatively on the four opposite corners. In some crystals, the smaller and dull tetrahedral faces are situated at the analogous poles (which become positively electrified when the crystal is heated), and the larger and bright tetrahedral faces s at the antilogous poles. For the file system called Lustre, see Lustre (file system) Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock or mineral. ... Relative density (also known as specific gravity) is a measure of the density of a material. ...

The characters so far enumerated are strictly in accordance with cubic symmetry, but when a crystal is examined in polarized light, it will be seen to be doubly refracting, as was first observed by Sir David Brewster in 1821. Thin sections show twin lamellae, and a division into definite areas which are optically biaxial. By cutting sections in suitable directions, it may be proved that a rhombic dodecahedral crystal is really built up of twelve orthorhombic pyramids, the pieces of which meet in the center and the bases coincide with the dodecahedral faces of the compound (pseudo-cubic) crystal. Crystals of other forms show other types of internal structure. When the crystals are heated these optical characteristics change, and at a temperature of 265 the crystals suddenly become optically isotropic. On cooling, however, the complexity of internal structure reappears. Various explanations have been offered to account for these optical anomalies of boracite. Some observers have attributed them to alteration, others to internal strains in the crystals, which originally grew as truly cubic at a temperature above 265. It would, however, appear that there are really two crystalline modifications of the horacite substance, a cubic modification stable above 265 and an orthorhombic (or monoclinic) one stable at a lower temperature. This is strictly analogous to the case of silver iodide, of which cubic and rhombohedral modifications exist at different temperatures; but whereas rhombohedral as well as pseudo-cubic crystals of silver iodide (iodyrite) are known in nature, only pseudo-cubic crystals of boracite have as yet been met with. Silver iodide (chemical symbol: AgI) is a chemical compound used in photography and cloud seeding. ...

Chemically, boracite is a magnesium borate and chloride with the formula Mg7C12B16O30. A small amount of iron is sometimes present, and an iron-boracite with half the magnesium replaced by ferrous iron has been called huyssenite. The mineral is insoluble in water, but soluble in hydrochloric acid. On exposure it is liable to slow alteration, owing to the absorption of water by the magnesium chloride: an altered form is known as parasite. The chloride ion is formed when the element chlorine picks up one electron to form the anion (negatively charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and are also called chlorides. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... The chemical substance hydrochloric acid is the aqueous (water-based) solution of hydrogen chloride (HCl) gas. ...

In addition to embedded crystals, a massive variety, known as stassfurtite, occurs as nodules in the salt deposits at Stassfurt in Prussia: that from the carnallite layer is compact, resembling fiIle-grained marble, and white or greenish in color, whilst that from the kainite layer is soft and earthy, and yellowish or reddish in color. Kainite (MgSO4·KCl·3H2O) is a mineral salt that consists of potassium chloride and magnesium sulfate and is used as a fertilizer. ...

This article incorporates text from the 1911 Encyclopædia Britannica, which is in the public domain. The Eleventh Edition of the Encyclopædia Britannica (1911) in many ways represents the sum of knowledge at the beginning of the 20th century. ... The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ...