Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/399

MINERALOGY 381 11. WEHRLITE, Bi(Te 3 S). Hexagonal. Cl. basal. H. =1 to 2; G. =8 44. High lustre. Steel- grey. C.c.: bismuth 61 15, tellurium 2974, sulphur 2 33, silver 2 "07. Deutsch-Tilsen in Hungary. 12. JOSEITE, Bi 3 Te 2 (SSe) 2 . Hexagonal. Cl. basal. G. 7 93. Colour grey-black to steel- grey. Brittle. C.c.: tellurium 15 93, sulphur 3 15, selenium !< bismuth 79 15. San Jose (Brazil). A Cumberland variety yielded tellurium 673, sulphur 6 43, bismuth 84 33. corresponding to Bi 4 (TeS 4 ). 13. DIAMOND, C. Var. 1. Crystallized. Cubic; very frequently hemihedral. Crystals most generally with curved faces. Twins common on the octahedral face ; hemitropes also common (see tigs. 170, 204, 205, 207). Crystals vary remarkably in appearance (see figs. 259 to 262). Cl. octahedral. H. =10; G. =3 5 to3 6. Transparent, or translucent when of dark colour, Refracts light strongly. The back planes of diamonds reflect all the light which strikes them at an angle exceed ing 24 13 , and thence comes their peculiar brilliancy. High adamantine lustre. Colourless, but often tinged white, grey, and brown, more rarely yellow, pink, blue, green, and black, those last named being the rarest. Disperses light highly, and hence emits brilliant flashes of all the colours of the spectrum. Becomes posi tively electric by friction. B.B. infusible, but burns into carbonic Fig. 259. Fig. 260. acid in oxygen gas. "When air is excluded is unchanged at the temperature of melting cast iron, but at that of melting malleable iron is changed into a black coke, or, it is said, into graphite. Insoluble in all acids and alkalies. C.c.: carbon, with traces of silica and earths. Geologic formation apparently a laminated flexible quartz rock called itacolumite, which occurs in Brazil, the Urals, Georgia, and North Carolina, in the vicinity of places where diamonds have been found. Minute crystals have been found in xanthophyllite, and in talc slate and serpentine, in the Schischim- skian hills, near Zlatoust (Russia). They have also been obtained in Brazil imbedded in a conglomerate composed of much-worn pebbles of quartz, chalcedony, and gold, cemented by linionite or Fig. 261. Fig. 262. ferruginous clay. In South Africa they are imbedded in a steatitic clay. Diamonds were formerly obtained in India, at Panna, Raol- conda, and Golconda. So few are now obtained here that the mines are let for 1 a year. From these mines were obtained not only the Kohinoor, which was possibly the same as the great diamond mentioned by Tavernier as having been seen by him in the pos session of the Great Mogul, which weighed 280 carats, but the Regent, of 136 carats (which, not only from its size, but from the perfection of its form, is very much the finest diamond known), the Nizam, an uncut diamond of 340 carats, and the Carlow, rose- cut, 193 carats. More lately diamonds were found in great quantity in the neighbourhood of Rio Janeiro in Brazil ; they occur in two different deposits : the one called "gurgulho" consists of broken quartz covered by a bed of sand ; the other, "cascalho," consists of rolled quartz pebbles united by ferruginous clay ; both rest on talcose clays, which are the debris from talcose rocks. The first deposit affords the finest diamonds, and both contain gold, plati num, magnetite, and rutile. A dodecahedral diamond of 257 carats was lately found at Bogagem in this district; this was reduced by cutting to an oblong brilliant of 125 carat8, and is the second most valuable diamond, the Kohinoor, now reduced to an imperfectly circular brilliant of 102 carats, occupying the third place. The two coloured diamonds most worthy of note are a green diamond in the Dresden collection weighing 31 carats, which is a little deeper in tint than a beryl, and a blue diamond in the Hope collection, of 44 carats, as highly coloured as a sapphire, which it is by some considered to be. Diamonds have lately been found in very large quantities, and some of great size, north of the Cape of Good Hope; these for the most part are of yellow colour and of very inferior value. While a Brazilian cut brilliant of one carat is worth from 20 to 25, the value of the finest brilliants from the Cape is only from 3 to 4, and that of the yellow diamonds is from 2 to 2, 10s. Apart from its employment as an ornamental stone, the diamond has an intrinsic value from its being utilized for cutting glass and for grinding and polishing other gems. Of late years its usefulness has had a new application, it being employed for the drilling of rocks in tunnelling operations and in the boring of artesian wells. A singular observa tion has resulted from these last methods of utilizing it, namely, that the hardness of the African diamonds, as tested by the amount of their endurance, is markedly inferior to that of the Brazilian and Indian. So much is this recognized that, while the bort, or minute crystals, of the latter command a price of 15s. per carat, the African can be got for about 5s. The cleavage of certain of the African diamonds is so eminent that even the heat of the hand causes some of them to fall in pieces. Such diamonds, generally octahedra, may be recognized by a peculiar watery lustre ; they are called plate diamonds. The above facts give some ground for the supposition that there may be a slight difference in their composition, possibly that both may contain small, but different, quantities of hydrogen. The circumstances under which diamonds have been formed are altogether unknown. The fact of their being changed into a kind of coke at a very high temperature is an argument against their having been produced through the operation of heat, and it has long been known that an excess of carbon dissolved by molten cast iron crystallizes on cooling in the form of graphite ; yet the only attempts to form diamonds deserving of being mentioned as having been attended with any measure of success are those in which sugar charcoal was dissolved in molten silver at the temperature only or melting steel. There were thus obtained a few minute black and also colourless octahedral and cubo-octahedral crystals with curved faces, mingled with a much larger amount of graphitoidal carbon. Var. 2. Massive. In black pebbles or masses called carbonado, sometimes 1000 carats in weight. H. = 10; G. =3 012 to 3 42. C.c. : carbon except 27 to 2 07 per cent, of ash. Found in the mines of Baranco, &c., in Bahia. Var. 3. Anthracitic. Like anthracite, but scratches the diamond. In mammillar masses, partly in concentric layers, and globular. Brittle. G. =1 66. C.c.: carbon 97, hydrogen - 5, oxygen 1 5. When cut and polished, refracts and disperses light, like the diamond. Supposed from Brazil. 14. GRAPHITE, C . Hexagonal in flat crystals ; p:p 85 29 . Usually foliated, scaly, or compact. Cl. basal. H. = - 5 to 1 ; G. = 1 9 to 2 - 2. Lustre metallic. Colour and streak black to dark steel-grey ; flexible in thin laminre; very sectile; feels greasy ; leaves a mark on paper of its own colour; conducts elec tricity. B.B. burns with diffi culty ; heated with nitre, de flagrates. C.c. : carbon, with small quantities of volatile matter, and ash from 5 to 40 per cent. Strathfarrer (Inverness-shire), Mull, Craigman (Ayrshire), Borrowdale in Cumberland, Ural Mountains, Ceylon, Greenland. Used for making pencils. 15. TIN, Sn. Tetragonal in greyish white metallic grains. Reported as occur ring with Siberian gold ; with bismuthite from Guanajuato in Mexico. 16. IRON, Fe. Cubic; in grains and plates or disseminated. H. =4 5; G. = Native to 7 8. Steel-grey or iron-black. Fracture hackly, very metals, magnetic. B.B. infusible. Sol. in h. acid. Two varieties are to be distinguished, (a) Telluric Iron, in grains and plates. Almost pure iron, or contains graphite, carbon, lead, or copper, but no nickel. At Chotzen in Bohemia in limestone ; in an argillaceous sandstone in the keuper at Miihlhausen ; in Thuringia along with fossils ; in an ironstone conglomerate in Brazil, and in lava in Auvergne ; in the mine of Hackenbursr ; at Bexley, in Liberia, Africa, along with quartz, a zeolite, and magnetite ; enclosed in magnetite in Unst (Shetland) and in Sutherlandshire ; in basalt in Antrim, Ireland; in the gold sands of Brazil, the Urals, and Olah- pian (Transylvania). (b) Meteoric iron, steel-grey to silver-white.

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