Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/408

390 MINERALOGY 137. OPAL, 9Si0 2 , H 2 to 3Si0 2 , H,0 . Amorphous ; fracture conchoidal ; very brittle. H. 5 - 5 to 6 - 5; G. =2 to 2 2. Transparent to opaque; vitreous, inclining to resinous. Colourless, but often white, yellow, red, brown, green, or grey, with a beautiful play of colours. B. B. decrepitates and becomes opaque, but is infusible; in the closed tube yields water; almost wholly soluble in solution of potash. C.c. : silica, with 5 to 13 per cent, water. Most opals are mixtures of various minerals. The following varieties may be noticed: (1) Hyalite, Glassy Opal, or Mailer s Glass: transparent, colourless, very glassy; small botry- oidal, or incrusting; Kaiserstuhl in the Breisgau, Schemnitz, Silesia, Moravia, Mexico, and other places. (2) Fire Opal or Girasol: trans parent ; brilliant vitreous lustre ; bright hyacinth-red or yellow ; Zimapan in Mexico, and the Faroes. (3) Noble Opal, semi-trans parent or translucent ; resinous, inclining to vitreous ; bluish or yellowish white, with brilliant prismatic colours; most show double refraction and are binaxal; in irregular masses or veins near Eperies in Hungary; Australia. (4) Common Opal: semitransparent, vitreous; white, yellow, green, red, or brown; Hungary, also Faroes, Iceland, the Giant s Causeway, and the Western Isles of Scotland. (5) Semi-opal: duller and less pellucid; Wood Opal or Lithoxylon: with the form and texture of wood distinctly seen; Hungary, Bohemia, and other countries. (6) Menilite: compact, reniform; opaque and brown or bluish grey; Menilmontant, near Paris. (7) OpalJasper: blood-red, brown, or yellow. (8) Cacholong: opaque, dull, glimmer ing, or pearly, and yellowish or rarely reddish white; in veins or reniform and incrusting ; Faroes, Iceland, the Giant s Causeway. One variety is named Hydrophane, from imbibing water, and becoming translucent. (9) Siliceous Sinter: deposited from the Geyser and other hot springs; and Pearl Sinter: incrusting volcanic tufa at Santa Fiora in Tuscany (Fiorite), and in Auvergne. 138. ZIRCON, Zr0 2 , Si0 2 . Pyramidal; P 84 20 . Crystals, ooP, P; often with 3P3 ; also oopoo , P ; or oopoo (s), oo P (l), P (P), 3P3 (a:), Poo (t), 4P4 (y), 5P5 (z), (fig. 318, also 86, 87, 585). Chiefly prismatic or pyramidal, and in rounded grains. Trans parent to opaque ; vitreous, often ada mantine. Earely white, generally grey, yellow, green, or fre quently red and brown. B. B. loses its colour, but is in fusible. Not affected by any acid except concentrated s. acid, after long digestion. C.c. : 66 - 3 zirconia and 33 - 7 silica, with to 2 iron peroxide Fig. 318. Fig. 319 (sp. 138). as colouring matter. Miask, Arendal, Sweden, Belgium (at Nil-St- Vincent), Carinthia, Tyrol, Ceylon, and North America; in Scot land, Scalpay in Harris (fig. 319), Lewis (Hyacinth), Sutherland, Ross. The colourless varieties are sold for diamonds. The more brilliantly coloured are named hyacinths, and are valuable gems. SULPHIDES, SELENIDES, TELLUR1DES, &c. 139. PYRITE, FeS 2 . Cubic ; semitesseral dominant (figs. 320 to 323, also 67 to 77, and 26 to 34). Pentagonal-dodecahedron in excess; or striae, produced by oscillation of it with faces of the cube, visible. Often distorted, as in the cubo-octahedral twin (fig. 323). Sometimes massive and in pseudomorphs. Cl. cubic or octahedral, difficult ; brittle. H. =6 Fig. 820. Fig. 321. Fig. 322. to 6 5; G. =4 9 to 5 2. Brass-yellow, often somewhat gold- yellow; streak brownish black, when broken emits smell of sulphur. In closed tube sulphur sublimes. B.B. on charcoal burns with blue flame, and odour of sulphurous acid. In inner flame fuses to magnetic bead. Sol. in n. acid, with deposition of sulphur. C.c. : iron 467, sulphur 53 3; often contains gold in visible grams, when broken. Common to rocks of all ages. Tomnadashin , Used to be cut in Birnam, Scotland; Cornwall, England; Elba and Traversella; Peru; Rossie, Middletown, and Schoharie in U.S. Auriferous pyrites, Berezoff (Siberia), Adelfors (Sweden), Mexico. facets and set as an ornament, under the name of marca- sites; also for striking fire in the old firelocks, whence the name of firestone; now used for manufacture of sulphuric acid. 140. MARCASITE, FeS Right prismatic; 106" 5 . Crystals tabular, thin prismatic, or pyramidal. Fig. 323 (sp. 139). Twins very frequent, also cockscomb-like groups, or spherical nnd staln^titip PI mT* fi.o^-.- . i,4.Ai^ n A .f>r and stalactitic. Cl. to 4 9. Greyish bronze- yellow to greenish grey, often with brown crust ; streak greenish grey or brownish black. B.B., &c., like pyrite. Very prone to decomposition, being changed into green vitriol, which may be detected by the tongue. Spear Pyrites are twins like fig. 325 ; Littmitz, Przibram. uneven ; brittle. G. = 4 65 Fig. 324. Fig. 326. Fig. 327. Hepatic Pyrites or Lebcrlcies, liver- brown, generally decomposing; Harz, Saxony, Sweden. Cockscomb Pyrites; Derbyshire and the Harz. Kyrosite contains arsenic. 141. MISPICKEL, FeS 2 + FeAs. Right prismatic ; ooP (J/) 111 12 (fig. 326). Twins common; also massive or columnar. Cl. o>P; fracture uneven; brittle. H. = 5 5 to 6; G. = 6 to 6 2. Silver-white to steel-grey ; streak black. In closed tube yields first a red then a brown sublimate, lastly metallic arsenic. B. B. on char coal fuses to a black magnetic globule. Sol. in n. acid, with separation of arsenious acid and sulphur. C.c.: 34 3 iron, 46 1 arsenic, 19 6 sulphur; some times silver or gold, or 5 to 9 of cobalt. Cornwall, Freiberg, Zinnwald, Sweden, Franconia, America. 142. LEUCOPYRITE, FeAs. Right prismatic ; ooP (d) 122 26 ; Poo (o) 51 20 . Crystals like fig. 327; generally massive or columnar. Cl. basal; fracture uneven; brittle. H. =5 to 5 5 ; G. =7 to 7 4. Silver-white with darker tarnish; streak greyish black. B. B. emits strong smell of arsenic, and fuses to a black magnetic globule. C.c. : iron 27 2, arsenic 72 8 ; sometimes iron 32 - 2 and arsenic 66 8 ; always some sulphur, and often nickel and cobalt. Fossum in Norway, Andreasberg, Styria, and Silesia. Spathiopyrite, from Bieber in Hesse, seems a variety. 143. COBALTITE, CoS 2 + CoAs. Cubic and hemihedral ; sometimes massive (figs. 67, 74). Cl. cubic, perfect ; brittle. H. =5 5 ; G. =6 to 6 3. Brilliant lustre. Pinkish silver-white ; tarnishes yellow or grey ; streak greyish black. B.B. with borax blue glass; evolves smell of arsenic. C.c, : cobalt 35 9, arsenic 44 9, sulphur 19 2. St Just in Cornwall, Tunaberg in Sweden, Skutterud in Norway, Querbach in Silesia. 144. GLATJCODOTE, (Co, Fe)S 2 + (Co, Fe)As a . Right prismatic ; ooPH236 . Cl. basal, perfect. H. =5;G.= 6. Lustre metallic. Greyish white ; streak black. C.c. : cobalt 247, iron 11-9, arsenic 43 2, sulphur 20 2. Huasco in Chili. 145. SMALTINE, (Co, Fe, Ni)As 2 . Cubic ; generally like fig. 27 ; also reticulated and granular com pact. Cl. octahedral; fracture uneven; brittle. H. =5 - 5; G. = 6 4 to 7 "3. Tin-white to steel-grey, with dark or iridescent tarnish ; streak greyish black. Evolves odour of arsenic, when broken or heated. C.c.: 71 4 arsenic, 28 6 cobalt; sometimes 3 to 19 iron, and 1 to 12 nickel, or 4 bismuth. Dolcoath and Redruth in Cornwall, Schneeberg, Annaberg, Tunaberg, Allemont, Chatham in Connecticut. 146. CHLOANTITE ( White Nickel), NiAs . Cubic ; generally fine granular or compact ; fracture uneven ; brittle. H. = 5 5 ; G. =6 4 to 6 6. Tin-white, rapidly tarnishing black. In the closed tube yields a sublimate of arsenic, and becomes copper-red. Gives odour of arsenic when broken. B.B. fuses with much smoke, becomes coated with crystals of arsenious acid, and leaves a brittle grain of metal. C.c.: 28 2 nickel, 71 8 arsenic, but often with cobalt. Schneeberg, Riechelsdorf, Alle

mont, Chatham in Connecticut.