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Encyclopedia Britannica - Main :: KRO-LAP |
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LAPIS LAZULI , or azure stone,' a mineral
Petersburg
Lapis lazuli occurs usually in compact masses, with a finely granular structure; and occasionally, but only as a great
1 The Med. Gr. Xaj'oipcov, Med. Lat. lazurius or lazulus, as the names of this mineral
azure ," blue, particularly used of that colour in heraldry (q.v.) and represented conventionally in black and white by horizontal
it presents the form of the rhombic dodecahedron. Its specific gravity is 2.38 to 2.45, and its hardness about 5'5, so that being comparatively soft it tends, when polished, to lose its lustre rather readily. The colour is generally a fine azure or rich Berlin blue, but some varieties exhibit green, violet and even red tints, or may be altogether colourless. The colour is sometimes improved by heating the stone. Under artificial illumination the dark-blue stones may appear almost black. The mineral is opaque, with only slight translucency at thin edges. Analyses of lapis lazuli show considerable variation in composition, and this led long ago to doubt as to its homogeneity. This doubt was confirmed by the microscopic studies of L. H. Fischer, F. Zirkel and H. P. J. Vogelsang, who found that sections showed bluish particles in a white matrix; but it was reserved for Professor W. C. Brogger and H. Backstrom, of Christiania, to separate the several constituents and subject them to analysis, thus demonstrating the true constitution of lapis lazuli, and proving that it is a rock rather than a definite mineral species. The essential part of most lapis lazuli is a blue mineral allied to sodalite and crystallized in the cubic system, which Bragger distinguishes as lazurite, but this is intimately associated with a closely related mineral which has long been known as hauyne, or hauynite. The lazurite, sometimes regarded as true lapis lazuli, is a sulphur
formula
turquoise
great
Lapis lazuli usually occurs in crystalline limestone, and seems to be a product of contact metamorphism. It is recorded from Persia, Tartary, Tibet and China, but many of the localities are vague and some doubtful. The best known and probably the most important locality is in Badakshan. There it occurs in limestone, in the valley of the river Kokcha, a tributary to the Oxus, south of Firgamu. The mines were visited by Marco Polo in 1271, by J. B. Fraser in 1825, and by Captain John Wood
Vesuvius
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