270 ORTHOCLASE. 



lour white, reddish-white, flesh-red, and 

 greyish. Lustre vitreous, sometimes pearly 

 on the cleavage-faces. Semi-transparent o'r 

 translucent. Streak greyish- white. Frac- 

 ture conchoid al to uneven and splintery. 

 H. 6. S.G. 2-5 to 2*6, increasing as the pot- 

 ash is replaced by soda or lime. 



Fig. 319. 



Comp. K Si + A:l SiS = silica 64-8, alumina 

 18-4, potash ] 6-8= 1 00 ; but a portion of the 

 potash is frequently replaced by lime, soda, 

 magnesia, &c. 



Analysis, from Baveno, by Ahich 



Silica 

 Alumina 

 Potash . 

 Soda 

 Lime . 

 Magnesia 



65-72 

 . 18-57 

 . 14-02 

 . 1-25 

 . 0-84 

 . 0-10 



100-00 



BB fuses with great difficulty to a blis- 

 tered, turbid glass ; in borax dissolves slowly, 

 forming a transparent glass. 



I^ot acted on by acids. 



Localities. — English. Cornwall, in large 

 opaque-white crystals, in the granite ge- 

 nerally: in irregular crystalline mass, St. 

 Stephens; near the old Lizard Head, in 

 green Talc; St. Michael's Mount; Huel 

 Coates, near St. Agnes, in twin crystals, 

 converted into Cassiterite (Jigs. 317, 319) — 

 Scotch. Rubislaw, Aberdeenshire; Drumi- 

 doon, or Drimadoon, in Arran, in claystone 

 porphyry (fig. 318). — Irish. Slieve Corra, 

 MourueMountains, in opaque-white crystals 

 (figs. 316, 317), and in twins like those from 

 St. Gotthard. — Foreign. Ekatherinenburg, 

 in Siberia, and in many parts of the Ural ; 

 Arendal, in Norway; Carlsbad and Eln- 

 bogen, in Bohemia ; Baveno, on Lago Mag- 

 giore. in Piedmont; Lomnitz, in Silesia; 

 St. Gotthard (figs. 316, 317); United 

 States, at Rossie, in New York ; Brazil, &c. 



OSTEOLITE. 



N'ame. From k^o;^ straight, and ^^Aaw, to 

 cleave. 



Orthoclase, or Potash Felspar, enters into 

 the composition of many rocks, and is one 

 of the ordinary ingredients of granite, of 

 which it commonly constitutes about 45 per 

 cent. 



This mineral is very liable to decomposi- 

 tion. In that process the potash enters into 

 combination with a portion of the silica, 

 and is carried off in a soluble form ; while 

 the residue, consisting of a white earth, 

 composed of silicate of alumina, is the 

 Kaolin or China-clay, which is used in the 

 manufacture of porcelain and earthenware. 



For varieties, see Adularia, Amazon" 

 Stone, Aventurine Felspar, Chester- 

 lite, Erythrite, Glassy Felspar, Hel- 



LEFLINTA Or LeELITE, IcESPAR, LoXO- 



CLASE, Microcline, Moonstone, Mur- 

 CHisoNiTE, Perthite, Rhyacolite, Sani- 



DINE, SUNSTONE, VaLENCIANITE. 



Brit. Mus., Case 29. 



M.P. G. Horse- shoe Case, Nos. 944, 955— 

 959, 965, 972, 973, 1039. Upper Gallery, 

 Wall-case 6, No. 3, &c. &c. 



Orthotomous Kouphone Spar, Mohs. 

 See Thomsonite. 



Oserskite, Breithatipt. A variety of 

 Aragonite, from Nertschinsk. 



Osmelite, Breithaupt. Occurs in thin 

 pri,5matic concretions, radiated or stellar. 

 Colour greyish-white, inclining to smoke- 

 grey. Lustre between pearly and vitreous. 

 Translucent. Feel greasy. Fracture fibrous 

 and conchoidal. H. 4 to 5. S.G. 2 8 to 2-83. 

 Ajialysis; by Adam : 



Silica 52-91 



Lime 32-96 



Soda 8-89 



Potash 4-01 



Oxide of iron . . .0-54 



100-75 



Locality. In trachyte, at Niederkirchen, 

 near Wolfstein, on the' Rhine, on Calc Spar. 



Name. From oV^t--^, smell, in allusion to 

 the argillaceous smell given out by it when 

 breathed upon. 



OsMiDE op Iridium, ) o t 



OSMIUM-IRIDIUM. 'jSeelRIDOSMINE. 



Osteocolla. (From oa-nov, a bone, and 

 y.oXXx., glue.) Calc Spar incrusted on sticks, 

 stones, and other bodies. See Ostreocolla. 



Osteolite, Bromeis. An earthy form o' 

 Apatite, occurring in altered dolomite, be- 

 tween Ostheim and Eichen, near Hanau ; 

 and at Amberg, in the Erzgebirge. 



