Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/398

Rh 380 MINERALOGY bodies, especially when the oxides of the heavy metals come in the room of the earths and alkalies, exerts a most essential influence on the external aspect of the species, particularly in regard to colour, specific gravity, and transparency. The varieties of hornblende, augite, garnet, epidote, and many other minerals are remarkable proofs of this influence. This intermixture of isomorphic elements confers many valuable properties on minerals, and to it this depart ment of nature owes much of its variety and beauty. Without the occasional presence of the colouring substances, especially the oxides of iron and manganese, the non-metallic combinations would have exhibited a very monotonous aspect. It is also remarkable that in some silicates the substitution of a certain portion of the metallic oxides for the earthy bases seems to be almost a regular occurrence; whilst in others, as the felspars and zeolites, this rarely happens. This fact is also of great economic interest, as drawing attention to important elements often combined with others of less value. Thus iron oxide and chrome oxide, sulphuret of copper and sulphuret of silver, nickel and cobalt, may be looked for in connexion. The general chemical formulae for such compounds is formed by writing R ( = radical or basis) for the whole isomorphic elements; and in special instances their signs are placed one below the other, con nected by a bracket, or, as is more convenient, are enclosed in brackets one after the other separated by a comma. Thus the gene ral sign for the garnet is R 3 Si.,+ ii Si, which, -when fully expressed, becomes (Ca 3, Fe 3 , Mg 3 , Mn 3 ) Si 2 + (Al, Fe, 6r)Si; and this mineral forms many varieties as the one or other element preponderates. Of the forms special to similar groups of atoms the more notable are the cubic system, special to metals proper, and binary compounds as protoxides and haloid salts ; the tetragonal to binoxides ; the rhombohedral to carbonates ; the hexagonal to sesquioxides and phosphates and their isomorphs; the prismatic to sulphates and their isomorphs. The isomorphism of minerals goes as a whole to show that form depends on the number of molecules present, and is comparatively little influenced by the nature of the molecules themselves. DESCRIPTION OF MINERAL SPECIES. The arrangement adopted in the following description of mineral species is chemical. Simple substances are considered first, in the order of their quantivalence, then binary compounds, and lastly those of more complex structure. Our limits permit of the briefest notice of the less important, in order that more space may be avail able for the delineation of the characteristic and transition forms of such as go to constitute the more important rock masses. The following abbreviations are used : H., hardness ; G., specific gravity (distilled water at 60 Fahr. and barometer 30 inches =1); cl., cleavage; sol., soluble; s. [h. or n.] acid, sulphuric [hydrochloric or nitric] acid ; B.B., before blowpipe ; ox., oxidizing ; red., reducing ; c.c., chemical composition ; com., combination. In the chemical formulas, barred letters express two equivalents, and the dots over the symbols indicate the combination with them of as many equivalents of oxygen as there are dots. In the symbolic notation the several faces of crystals are separated by semicolons, and the constituent members of combinations by commas. The lettering on the faces of the figures is for the most part that adopted by Miller. In the enumeration of crystal forms, that which is typical of the mineral is placed first. SIMPLE SUBSTANCES. 1. SULPHUR, S. (a) Right prismatic. P (p) polar edges 106 38, 84 58 , middle edge 143 17 ; ooP 101 58 ; OP (c) ; T (s) ; f oo (n). Crystals pyramidal, single or in druses ; also stalactitic, disseminated, and pulverulent. Cl. basal and ooP. H. = 1 5 to 2 5 ; G. =1 9 to 2 1. Fracture conchoidal or splintery ; brittle, sectile. Lustre resinous, streak and colour sulphur-yellow, passing into red, brown, or green. Sublimes in the closed tube. Fuses a little above the temperature of boiling water. Takes fire at 518 F., and burns with a pale blue flame with odour of sulphurous acid. C.c.: pure sulphur, occasionally mixed with traces of selenium, and when amorphous with clay or bitumen. Found chiefly in Tertiary strata. Localities: Girgenti in Sicily, with celestine ; Conil in Spain ; Bex in Switzerland : Cracow in Poland ; deposited from hot springs in Solfatara near Naples ; from hot springs in Iceland ; from sulphur springs in New York ; and in cavities of decomposing galena, cinnabar, and pyrites at several localities. (b) Oblique prismatic. The crystals of volcanic sulphur are of this form ; they occur in the neighbourhood both of extinct and of recent volcanoes. They are slender, needle-shaped, and interlacing, and have generally more or less of a red-brown tinge. Oxhaveer and Cape Reykjanes in Iceland, Sicily, and the volcanoes of the Pacific, the Chilian Andes, and California yield this variety. 2. SELENSULPHUR, S.Se. Like sulphur, but reddish brown to orange-yellow. B. B. burns with fumes of selenious acid mixed with the sulphurous. Founc in the crater of Volcano in the Lipari Islands, and Kilauea ii Hawaii. 258. 3. SELENIUM, Se. H. =2; G. =4 3. Brownish black to lead-grey; thin splinters 4. TELLURIUM, Te. Rhombohedral ; R 86 50. In minute hexagonal prisms, with
 * ranslucent and red. From Culebras in Mexico.
 * &amp;gt;asal edges replaced ; usually massive and granular. Cl. lateral

perfect, basal imperfect. H. = 2 to 2 5 ; G. = 6 &quot;1 to 6 3. Tin-white; sectile. C.c.: tellurium with a little gold and iron. Occurs at Facsebaya near Zalathna (Transylvania), and in several mines in Boulder county, Colorado ; masses 25 Ib in weight have been obtained there. 5. ARSENIC, As. Rhombohedral ; R 85 36 (fig. 258). Usually in botryoidal Tri-valei investing masses composed of numberless layers. The structure is element: fine granular, rarely columnar. H. = 3 - 5 ; G. = 5 7 to 5 93. Cl. basal. Colour black and dull, but when fresh broken very splendent and silver-white ; fracture uneven. When rubbed or heated gives out a garlic-like odour. B. B. volatile, with formation of white fumes. C.c. : arsenic, with some antimony, and traces of iron, silver, and gold. Andreasberg in the Harz, Annaberg, Sehneeberg, Freiberg, Joachimsthal, Allemont (Dau- phine), Kongsberg (Norway), the Altai, Chili, Pebble mine (Dum friesshire), Ty minim (Perthshire). 6. ANTIMONY, Sb. Rhombohedral ; R 87 35 ; but rarely crystallized, generally in foliated or granular masses. Cl. basal. H. =3 ; G. =6 6 to 6 8. Tin- white, with slight yellow tarnish. Brittle and sectile. B. B. easily fusible; volatilizes, and on charcoal leaves a white deposit, burning with a pale flame. Found at Andreasberg, Przibram (Bohemia), Sala (Sweden), Allemont, Southham in East Canada, and Borneo. 7. ALLEMONTITE, SbAs 3. Hexagonal, spherical, reniform, and investing. H. = 3 5 ; G. = 6 1 to 6 2. Lustre, when fresh, metallic. Tin- white to lead-grey, but with a blue or brown tarnish. B.B. strong odour of garlic, with residuum of oxide of antimony. C.c.: antimony 37 85, arsenic 62*15. Almost always in curved foliated laminae. Occurs at Allemont, Przibram, Schladming in Styria, Andreasberg. 8. BISMUTH, Bi. Rhombohedral ; R 87 40. Crystals, R,OR, generally distorted ; also reticulated, spear-head twins, or arborescent ; also disseminated and granular. Cl. basal, perfect. H. =2 5 ; G. -=9 6 to 9 8. Brittle and sectile. Reddish white, often tarnished grey, brown, or blue. B.B. easily fusible, even in candle flame. Volatilizes on charcoal, leaving a citron-yellow crust. Sol. in n. acid ; solution pre cipitated when thrown into water. Occurs in gneiss and clay slate in veins and disseminated, along with ores of cobalt, silver, lead, and zinc. Alva in Stirlingshire, Cumberland, Devonshire and Cornwall, Sehneeberg, Marienberg, Joachimsthal, Bieber, Modum (Norway), Falun (Sweden), Bolivia. 9. TELLURIC BISMUTH, Bi.,Te 3. Bismuth 52, tellurium 48. Virginia, Dahlonega in Georgia, Mon tana. A variety with 7 per cent, of selenium and H.=2 also occurs. 10. TETRAimtiTE, BioTe 2 S. Rhombohedral ; 3R 68 10. Almost always twins of 3R and OR, with the faces of OR at 93. Cl. basal, perfect. Sectile, and thin lamina} flexible. H. =1 to 2 ; G. =7 2 to 7 5. Steel-grey. B.B. fuses, yielding a grain of metal which ultimately volatilizes. Sol. in n. acid. C.c.: 59 6 bismuth, 35 9 tellurium, and 4 5 sulphur. Schemnitz.