BORACITE , a
See also:mineral of
See also:interest on account of its
See also:optical anomalies . Small crystals bounded on all sides by sharply defined faces are found in considerable numbers embedded in
See also:gypsum and
See also:anhydrite in the
See also:salt deposits at
See also:Luneburg in Hanover, where it was first observed in 1787 . In
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 tetrahedron (fig . 1), the
See also:cube (fig . 2). or the rhombic 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' 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 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: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 apices of which meet in the centre and the bases coincide with the dodecahedral faces of the 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 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 amagnesium borate and chloride with the
See also:formula Mg7C12B16O30 . A small amount of 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 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 . In addition to embedded crystals, a massive variety, known as stassfurtite, occurs as nodules in the salt deposits at
See also:Stassfurt in Prussia: that from the carnallite layer is compact, resembling
See also:fine-grained 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 .
BORAGE (pronounced like " courage "; possibly from ...
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