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BORACITE

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Originally appearing in Volume V04, Page 242 of the 1911 Encyclopedia Britannica.
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BORACITE , a See also:

mineral of See also:special See also:interest on See also:account of its See also:optical anomalies. Small crystals bounded on all sides by sharply defined faces are found in considerable See also:numbers embedded in See also:gypsum and See also:anhydrite in the See also:salt deposits at See also:Luneburg in See also:Hanover, where it was first observed in 1787. In See also:external See also:form these crystals are cubic with inclined hemihedrism, the symmetry being the same as in See also:blende and See also:tetrahedrite. Their See also:habit varies according to whether the See also:tetrahedron (fig. 1), the See also:cube (fig. 2). or the rhombic See also:dodecahedron (fig. 3) predominates. Penetration twins with a tetrahedron See also:face as twin-See also:plane are sometimes observed. The crystals vary from translucent to transparent, are possessed of a vitreous lustre, and are colourless or See also:white, though often tinged with See also:grey, yellow or See also:green. The hardness is as high as 7 on See also:Mohs' See also:scale; specific gravity 3.0. As first observed by R. J.

Haiiy 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 a crystal such as represented in fig. 3, the smaller and dull See also:

tetrahedral faces s are situated at the analogous poles (which become positively electrified when the crystal is heated), and the larger and See also:bright tetrahedral faces s' at the antilogous poles. The characters so far enumerated are strictly in accordance with cubic symmetry, but when a crystal is examined in polarized See also:light, it will be seen to be doubly refracting, as was first observed by See also:Sir See also:David See also:Brewster in 1821. Thin sections show twin-lamellae, and a See also: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 apices of which meet in the centre and the bases coincide with the dodecahedral faces of the See also:compound (pseudo-cubic) crystal. Crystals of other forms show other types of See also:internal structure. When the crystals are heated these optical characters See also: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 See also:grew as truly cubic at a temperature above 265°. It would, however, appear that there are really two crystalline modifications of the boracite substance, a cubic modification See also:stable above 265° and an orthorhombic (or See also:monoclinic) one stable at a See also:lower temperature. This is strictly analogous to the See also:case of See also:silver iodide, of which cubic and See also: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.

Chemically, boracite is a See also:

magnesium borate and chloride with the See also:formula Mg7C12B16O30. A small amount of See also:iron is sometimes See also:present, and an iron-boracite with See also:half the magnesium replaced by ferrous iron has been called huyssenite. The mineral is in-soluble in See also:water, but soluble in hydrochloric See also: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 See also:parasite. In addition to embedded crystals, a massive variety, known as stassfurtite, occurs as nodules in the salt deposits at See also:Stassfurt in See also:Prussia: that from the carnallite layer is compact, resembling See also:fine-grained See also:marble, and white or greenish in See also:colour, whilst that from the kainite layer is soft and earthy, and yellowish or reddish in colour. (L. J.

End of Article: BORACITE

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