Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/438

Rh 420 MINERALOGY G. = 27 to 278. Transparent or translucent; vitreous. Colourlessor white. B. B. fuses to a clear glass ; soluble without gelatinizing in con. h. acid. C.c. : 43 silica, 36 9 alumina, 201 lime, sometimes Fig. 534. with magnesia and soda. Fetlar in Shetland ; Lendalfoot in Ayr shire, in gabbro; Monte Somma, Iceland, Java. Lepolite and Am- phodelite are varieties. In Latrobite the greater part of the lime is replaced by potash. Glen Gairn and Labrador. At both rose-red. 592. OLIGOCLASE, 2AlSi 3 + (Na, Ca) 2 Si 3. Anorthic. OP : ooPoo 86 10 ; ooP : oo P 120 42. Hemitropes face m, with p:p 173 4 ; 1:1 120 20 ; y : y 179 9 ; x : x 175 59. Cl. basal, perfect; brachydiagonal, less so. H. =6; G. -&amp;gt;2 62 to 2 8 4. Vitreous, resinous on the cl. White, with a tinge of green, grey, or red. B. B. melts easier than orthoclase or albite to a clear glass ; not affected by acids. C.c. : 63 silica, 23 - 4 alumina, 8 4 soda, and 4 - 2 lime; thus nearly = 3 albite and 1 anorthite. Distinguished from orthoclase by the marked striae on the faces ; less readily from albite, but more fusible and G. higher. The common associate of orthoclase in the Scotch grey granites, especially in vein granite, as at Rispond and Ben Loyal (figs. 536, 537) in Sutherland, and at Fig. 536. Fig. 537. Rubislaw ; Scandinavia, Urals, Harz, and North America. The Sunstone, from Foinaven in Sutherland, Norway, Lake Baikal, and Ceylon, with a play of colour due to imbedded crystals of rubin- glimmer (gothite), belongs to this species. 593. LABRADORITE, A-lSi 3 + (Ca, Na)Si. Anorthic. OP : oof oo 86 40 ; OP : oo P 111; OP : ooP 113 34 ; ooP : oo P 121 37 ; o&amp;gt;Po : &amp;lt;xP 120 53 ; ooPoo : oo P 117 30 . Hemitropes of three types : (1) according to the first law of orthoclase as in fig. 538 ; that is, vertical revolution and face of union ooPoo ; (2) re volution of one half with reunion on the face ooPoo, M M Fig. 539. as in fig. 539 ; (3) with twin face P, as in fig. 540. Hemitropes of the last form also occur in which the lower half consists of a heini- trope formed according to the second method. Crystals imbedded in rocks consist generally of repeated twins affording an angle of 173 20. Cl. basal, perfect; brachydiagonal, less so ; both usually striated on account of the above twinning. H. = 6 ; G. = 2 68 to 2 74&quot;. Translucent ; vitreous, on the cl. resinous. Grey, passing into white, green, yellow, or red. The faces of ooPoo often exhibit very beauti ful changing colours blue, green, yellow, red, or brown some times bands intersecting at certain angles. B. B. fuses more readily than orthoclase to a compact colourless glass. Sol. in h. acid. C.c. : 52 - 9 silica, 30 3 alumina, 12 - 3 lime, and 4 5 soda. It is thus = 1 albite and 3 anorthite. Common constituent of dolerite, gabbro, and hypersthene rocks. In Scotland, Labrador, Finland, Harz, Tyrol ; also at Etna and Vesuvius. 594. ANDESIXE, Anorthic. Crystals similar to albite and anorthite. Twin face M. Crystals generally formed of repeated plates. G. = 2 67 to 2 7. Physical properties like albite; more easily fusible to a porous white glass ; h. acid sometimes dissolves out alternate lamina of crystals. C.c. : 597 silica, 25 6 alumina, 77 soda, and 7 lime, and thus nearly 1 of albite and 1 anorthite. Typical of the primary limestones and a granitic belt therein in Scotland, as at Shinness, Urquhart, Dalnaiu, &c. In the Andes, the Vosges, and Iceland. 595. HYALOPHANE, A- lSi 3, KSi 8 + AlSi, BaSi. Oblique prismatic ; resembles orthoclase ; crystals and angles nearly the same. Cl. OP, perfect. H. =6 to 6 5 ; G. =2 8 to 2 9. Transparent. Lustre vitreous. Colourless, white, and flesh-red. C.c. : silica 527, alumina 21, baryta 15 1, potash 7 8, soda 2&quot;1. B. B. difficultly fusible to a blebby glass, not acted upon by acids. Binnen in Valais, Jacobsberg in Sweden. 596. BARSOVITE, AlSi + C aSi. Right prismatic, or oblique prismatic. H. 5 - 5 to 6 ; G. =2 - 58. Snow-white; translucent. Fracture granular. Pearly. C.c. .-silica 42 2, alumina 36 4, lime 19 8. Gelatinizes in h. acid, difficultly fusible. A dimorphic form of anorthite. Barsovskoi in the Urals. 597. SAUSSURITE. A massive, granular, translucent, white or pale green felspathic mineral of the nature of anorthite mixed with labradorite. H. = 6 to 7; G. =3 26 to 3 4. Probably a mixture. Occurs in loose blocks near Geneva, and in Corsica. In China and in India is carved under the name of Oriental jade (nephrite). Seems to be confounded also with zoizite, and perhaps with yu (prehnite). Jadcite is similar. ZEOLITE GROUP. These crystallize in all the systems except the anorthic, and them selves present great variety of development. Mostly hyaline and white ; rarely red, grey, or yellow. Cl. generally distinct. All yield water in closed tube ; all fusible B. B. most easily, and often intumescing ; all sol. in acids, and mostly gelatinize or deposit silica. They are hydrated silicates of alkalies, or alkaline earths, mostly with silicates of alumina, but rarely contain magnesia. Some mineralogists regard the water as basic, in union with silica, and Kenngott gives the formula in that form, thus : Analcime, (NaAl) 2i + 2(ft, Si), Natrolite, (NaAl) 2Si + 2(fi, Si), Stilbite, C a, Al + 6(&, Si), and the others similar. They are generally found in amygdaloidal cavities or fissures of trap or plutouic rocks, apparently as deposits from water percolating into them, and are thus probably products of decomposing nepheline or felspars, or hydrated felspars them selves. They never form constituents of rocks. Natrolite, scolezite, thomsonite, and the connected varieties are marked by their needle-like radiating forms ; stilbite and heulandite by their broad, foliated, pearly cleavage. 598. PECTOLITE, 4CaSi + NaSi, + H. Oblique prismatic, C 84 37. oopoo (c) ; OP (u} 95 23 . Cl. c and u. Twin-face c ; chiefly spher oidal and radiating fibrous. H. =5; G. =274 to 2 88. Translucent; crystals pearly ; fibres silky. Pale green to yellowish white. Sol. in h. acid, leaving silica. C.c.: 5 i 2 silica, 337 lime, 9 &quot;4 soda, and 27 water. Ratho, Corstorphine, Castle Rock, and Arthur s Seat, Edinburgh; Kilsyth, Stirling ; Knoekdolian and Lendalfoot, Ayr shire; Skye ; Montebaldo; Monzoni Valley in Tyrol. 599. WALKEKITE, Like pectolite, but columnar. H. = 4 &quot;5 ; G. = 2 7. Flesh-coloured. Lustre pearly to greasy. C.c.: silica 537, lime 28 6, magnesia 5 1, soda 7 9, water 4 6. Corstorphine Hill, Burntisland. 600. XONOTLITK, iCaS i + fi. Massive. H. - 6 ; G. = 2 6 to 2 7. Pink, white, and grey. Tough ;