Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/407

Rh MINERALOGY 389 134. TANTALIC OCHRE. Powdery; brown; vitreous. Pennikoja in Finland. 2. OXIDES OF CARBON-SILICON FAMILY (BINOXIDES). 135. QUARTZ, Si0 2. Hexagonal ; the purest varieties tetartohedral. The primary pyramid P has the middle edge = 103 34, and the polar edges = Fig. 302. Fig. 303. Fig. 304. Fig. 305. often perfect. Very frequently it appears Fig. 306. Fig. 307. Fig. 308. Fig. 301. 133 44, and is as a rhombohedron R (or JP), with polar edges = 94 Crystals often of ooP, P or oo P, P, 4P, the forms ooP and 4P being combined .. in an oscillatory manner, producing striae on the face of the prism (figs. 303, 304, 305); also ooP, P, }(2P2), the last face appearing as a rhomb replacing the alter nate angles between the two other forms (figs. 307, 308). They are prismatic, or py ramidal, or rhombohedral, when P is divided into R and -R; the latter very often wanting. Many faces plagihedral, as in figs. 302, 306, 309. Twins common, with parallel axes, and either merely in juxtaposition (see fig. 178) or interpene trating. Crystals often distorted, as in figs. 310 to 313. The crystals occur either single, attached, or imbedded, or in groups and druses. Most fre quently granular, massive, fibrous, or columnar; also in pseudomorphs, petrifactions, and other forms. Cl. rhombohedral along R, very imper fect; prismatic along oP, still more imperfect ; fracture conchoidal, uneven, or splintery. H. = 7 ; G. =2 5 to 2 8; 2 65 in the purest varieties. Colourless, but more often white, grey, yellow, brown, red, blue, green, or even black. Lustre vitreous, inclining to resinous; transparent or translucent ; when impure almost opaque. B. B. infusible alone; with soda effervesces, and melts into a clear glass. Insoluble in acids, except the hydrofluoric; when pulverized, slightly soluble in solution of potash. C.c. : Fig. 309. Fig. 311. Fig. 312. Fig. 313. 48 05 silicon and 51 95 oxygen; but frequently a small amount of the oxides of iron or titanium, of lime, alumina, and other substances. The following are varieties : Rock-crystal : highly transparent and colourless ; Dauphine, Switzerland, Tyrol, Hungary, Madagascar, and Ceylon. Amethyst : violet-blue (from iron peroxide or manganese), and often marked by zigzag or undulating lines, and the colour dis posed in clouds ; Siberia, Persia, India, Ceylon, Brazil (white or yellow, named false topaz), Hungary, Ireland (near Cork), and Aberdeenshire. Wine-yellow (Citrin and Gold Topaz) ; the brown or Smoky Quartz (coloured by a substance containing carbon and nitrogen); and the black or Morion, from Siberia, Bohemia, Pennsylvania, and other places. Cairngorm Stone, black, brown, or yellow, from Aberdeenshire mountains. The above are valued as ornamental stones ; those which follow are less so. Rose Quartz : red inclining to violet-blue; Clashnaree Hill (Aber deen), and Rabenstein in Bavaria. Milk Quartz : milk-white, and slightly opalescent; Greenland. Prase: leek and other shades of green; Saxony and Cedar Mountain in South Africa. Cat s-ey, inclosing asbestos: greenish white or grey, olive-green, red, brown, or yellow ; Ceylon and Malabar. Avanturine, enclosing mica : yellow, red, green, or brown; India, Spain, and Scotland. Siderite: indigo or Berlin blue ; Golling in Salzburg. Common Quartz, crystallized or massive, white or grey, also red, brown, &c., is a frequent constituent in many rocks. Some impure varieties are properly rocks, as: (1) Fcrruyinous Quartz, or Iron Flint: red, yellow, or brown; often associated -with iron ores. (2) Jasper : red, yellow, brown, also green, grey, white, and black ; alone, or in spots, veins, and bands (Jlibbo)i or Egyptian Jasper); Urals,Tuscan Apennines, Harz, and many parts of Scotland. (3) Lydian Stone, or Flinty Slate : black, grey, or white ; has a splintery or conchoidal fracture, breaks into irregular fragments, and passes by many transitions into clay-slate, of which it is often merely an altered portion, as in Scotland; used as a touchstone for gold, and at Elfdal (Sweden) manufactured into ornaments. (4) Hornstone or Chert : compact, conchoidal, splintery fracture ; translucent on the edges; dirty grey, red, yellow, green, or brown; passes into flinty slate or common quartz; common in the Mountain limestone, Oolite, and Grct-nsand formations; and often contains petrifactions, as shells, corals, and wood. Other siliceous minerals seem intimate mixtures of quartz and opal, as: Flint : greyish white, grey, or greyish black, also yellow, red, or brown ; sometimes in clouds, spots, or stripes ; semitrans- parent; lustre dull; fracture flat conchoidal; occurs chiefly in the Chalk formation, as in England, Ireland, Aberdeenshire, France, Germany, and other countries; sometimes in beds or vertical veins, often in irregular lumps or concretions, inclosing petrifactions, as sponges, echinoids, shells, or siliceous Infusoria. The colour is partly derived from carbon, or organic matter. Used formerly for gun-flints, and still for the manufacture of glass and pottery; ami cut into cameos or other ornaments. Chalcedony : semitransparent or translucent ; white, grey, blue, green, yellow, or brown ; stalactitic, reniform, or botryoidal, ami in pseudomorphs or petrifactions ; Iceland, Faroes, Trevascus in Cornwall, Scotland, Hungary, Bohemia, Oberstein. Carnelian : chiefly blood-red, but also yellow, brown, or almost black ; India, Arabia, Surinam, and Siberia; also Bohemia, Saxony, and Scotland (Fifeshire). Plasma: leek- or grass-green, and waxy lustre; Olym pus, Black Forest, India, and China. Chrysoprasc: apple-green; Silesia, and Vermont in North Americn. Moss- Agate and Heliotrope : dark green and dendritic (called Blood stone when sprinkled with deep red spots); India, Siberia, Bohemia, Fassa Valley, island of Rum and other parts of Scotland. 136. TRIDYMITE, Si0 2. Hexagonal ; P middle edge 124 4, polar edges 127 35. Single crystals, very minute hexagonal tables of OP, ooP, but with the edges replaced by P and ooP2, are rare (fig. 314). Mostly twinned Fig. 314. Fig. 315. Fig. 316. in double or (oftener) triple combinations (figs. 315 to 317). Cl. basal, indistinct; fracture con choidal. H. = 7 ; G. =2-282 to 2 326. Colourless and transparent; vitreous, pearly on the base. Fig. 317. B.B. like quartz. C.c.: 96 silica, with some alumina, magnesia, and iron peroxide, probably from the matrix. Discovered by Von Rath in the trachyte of San Cristobal, near Pachuca, in Mexico; also in the trachyte of Mout-Dore (Puy-de- Dome), the Drachenfels, and Hungary. Many opals, treated with solution of potash, leave crystals, as those from Zimapan, Kaschau, Silesia, and the cacholong from Iceland. Where such crystal; are abundant, the opal becomes opaque or snow-white. Jcnzsch regards these as still another variety of silica.