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Encyclopedia Britannica - Main :: INV-JED |
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IONA , or IconmmLL, an island
Scotland , 62 m. S. of Staffa and 14 m. W. of the Ross of Mull, from which it is separated by the shallow Sound of Iona. Pop. (1901) 213. It is about 31- M. long and 11 m. broad; its area being some 2200 acres, of which about one-third is under cultivation, oats, potatoes and barley being grown. In the rest of the island
coast
south
('loe, "violet "; XLBos, "stone"). It is generally called by petrographers cordierite, a name given by R. J. Hauy in honour of the French mineralogist, P. L. Cordier, who discovered its remarkable dichroism, and suggested for it the name dichroite, still sometimes used. The difference of colour which it shows in different directions is so marked as to be well seen without the dichroscope. The typical colours are deep blue, pale
lolite is a hydrous magnesium and aluminium silicate, with ferrous iron partially replacing magnesium. It crystallizes in the orthorhombic system. In hardness and specific gravity it much resembles quartz. The transparent blue or violet variety used as a gem occurs as pebbles in the gravels of Ceylon, and bears in many cases a resemblance to sapphire. The paler kinds are often called water-sapphire (saphir d'eau of French jewellers) and the darker kinds lynx-sapphire; the shade of colour varying with the direction in which the stone is cut. From sapphire the iolite' is readily distinguished by its stronger pleochroism, its lower density (about 2.6)- and its inferior hardness (about 7). Iolite occurs in granite and in true eruptive rocks, but is most characteristically developed as a product of contact metamorphism in gneiss and altered slates. A variety occurring at the contact of clay-slate and granite on the border of the provinces of Shimotsuke and Kodzuke in Japan has been called cerasite. It readily suffers chemical change , and gives rise to a number of alteration-products, of which pinite is a characteristic example.Although iolite, or cordierite, is rather widely distributed as a constituent of certain rocks, fine crystals of the mineral
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