Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/437

Rh MINERALOGY 419 rnak and most mineralogists now take a similar view, regarding orthoclase, albite, and anorthite alone as true species, of which the others are mixtures. Those consisting essentially of potash and soda only are mechanical mixtures of orthoclase and albite, the distinct lamellse being visible by the microscope ; those again that contain essentially lime and soda together are, sometimes at least, chemical, being isomorphous compounds of albite and anorthite in various proportions, and with corresponding transitions in crystallographic and physical properties. Notwithstanding this, these intermediates must be regarded as independent mineral species, inasmuch as they are severally typical of certain rocks, and have characteristic forms differing from each other in angular inclination. 589. ORTHOCLASE, AlSi 3 + KSi 3. Oblique prismatic, C = 63 57. P (T and 1} 118 47 ; Po (a;) 65 46 ; 2P c oo (n) 90 71 ; 2Poo (?/)3545 . The commonest and simplest forms are oP, OP, POOO, and ooP c oo (M), ooP, OP (P), 2Poo (figs. 520 to 527). When ooP predominates the crystals Fig. 520. Fig. 521. Fig. 523. Fig. 524. Fig. 525. are short rhombic prisms ; when ooP c oo predominates they are tabular; when oo P and o&amp;gt;P c oo predomi nate they are short hexa gonal prismatic, when OP and ooP c oo they are rect angular prismatic, often much lengthened. Twins are very frequent, and occur according primarily to four laws. First, Fi g- 526. Fig. 527. through revolution of one half or of a whole crystal, then forming interpenetrating twins round a vertical axis (fig. 195). In the case of this hemitropic revolution one of the external faces becomes a face of union. According as the right or the left half (or whole crystal) is conceived to be that which has been revolved the crystals are termed right and left, as in figs. 188, 189. Second, by revolution of one half around an axis normal to M; in such twins the com position is not evidenced externally except by sutures. Third, Fig. 528. Fig. 529. 530. through revolution round an axis normal to P, forming orthorhombic prisms which show a herring-bone lineation, through the meeting of strire commonly present upon the face M parallel to the intersection of its edge with the face T (fig. 528). Fourth, by revolution round an axis normal to 2P c oo (71) ; this also forms a prism the section of which is nearly square (fig. 529). Compound twins on this last type are formed of 3 to 4 and 8 crystals (fig. 530). Occurs also massive, and coarse or fine granular. Cl. basal (P), very perfect ; clinodiagonal (M), perfect (P : J/=90) ; fracture conchoidal or splintery H. ==6 ; G. =2 53 to 2 58. Transparent to translucent on the edges ; vitreous but pearly on cl. ; and also opalescent, with bluish or changing colours. Occasionally colour less but generally red, yellow, grey, or green. B. B. fuses with difficulty to an opaque vesicular glass. Not affected by acids. C.c. : 64 6 silica, 18 5 alumina, and 16 9 potash, but generally 10 to 14 potash, 1 to 4 soda, to 1 3 lime, to 2 iron peroxide. Varieties are (1) Adularia and Ice-spar, transparent or translucent, splendent, and almost colourless. Some with bluish opalescence are named Moonstone ; St Gotthard, Mont Blanc, Dauphine, Areudal, Green land, and Ceylon. (2) Common Felspar, generally white or red, especially flesh-red, is a common constituent of many rocks. Crystals at Baveno on Lago Maggiore, Lomnitz in Silesia, Mourne Mountains and Wick- low in Ireland, Aberdeenshire (at Rubislaw 6 or 8 inches long) in Scotland, and at Carlsbad and Elnbogen in Bohemia. Amazon Stone, verdigris-green, from Sutherland, Lake Ilmen, and Colorado, and Murchisonite, golden or greyish yellow, from Arranand Dawlish, are varieties. (3) The Glassy Felspar or Sanidine (C 64 1, ooP 119 16 ) con tains 3 to 12 potash and 3 to 10 soda. Crystals imbedded ; vitreous, translucent, and often much cracked ; Arran, Eigg, and other parts of Scotland, Drachenfels, Auvergne, and other countries. Orthoclaso occurs in granite, gneiss, and porphyry in many countries. It is commonly associated with quartz ; sometimes, as in the Graphic Granite of Sutherland, Harris, and Portsoy, in letter-like combinations of the latter. It is very liable to decom position, when it is converted especially into kaolin, used for manufacturing porcelain and stoneware. The adularia or moon stone and the green amazon stone are cut as ornamental stones. Leelite, from Biddean nam Bian in Argyllshire and Grythyttan in Sweden, is a somewhat siliceous horny-lustred flesh-coloured com pact variety. Pctuntze and Hornstonc are similar but more impure. Microcline is a variety with angle distorted by interstitial penetra tion, by oligoclase (Sutherland), and by albite (Frederiksvaru, &c.). 590. ALBITE, AlSi 3 + NaSi 3. Anorthic. OP(P):oopoo (3/)8624 ; ooP (I) : oo P (T) 122 15 ; but angles variable. Crystals, generally like those of orthoclase, are tabular or prismatic (fig. 197). Hemitropes common, especially united by a face of ooPoo (figs. 531, 532) the re-entering angle be- et Fig. 531. tween the faces of OP (P and P) 172 48 being very cl.; :acte:istic. Fig. 198 is another common hemitrope. Also massive, an 1 in i ing plates. Cl. basal and brachy diagonal, almost equally ] fracture conchoidal or uneven. H. =6 to 6 5 ; G. =2 J Rarely transparent; vitreous, pearly on the cl. Color.:;,: generally white, grey, green, red, or yellow ; streak white, difficultly fusible, tinging the flame yellow, to a white semiopaque glass. Not affected by acids. C.c.: 68 6 silica, 19 Galumina with O l to 1 iron peroxide, and 11 8 soda, with 0-3 to 4 lime, to 2 5 potash. Hence albite and orthoclase both contain soda and potash, only in different proportions. Albite is most easily recognized by its frequent re-entering angles, its readier fusibility, and the obliquity (93 36 ) of its cl. planes, often marked with stria;. Pcridine is a variety of which fig. 533 is a typical form. Albite is a constituent of many &quot;greenstones,&quot; as at Corstorphine (Edinburgh), and of granite, syenite, gneiss, porphyry, and trachyte. Crystallized at Murdoch s Cairn, Aberdeenshire, being the colourless felspar of the red granites of Scotland. Dauphine, St Gotthard, Tyrol, Salzburg, and Arendal. Adinole is a compact variety similar in appearance to Leelite. 591. ANORTHITE, AlS i + CaSi. Anorthic. OP (P) : ooPoo (.17) 85 50 ; &amp;lt;xP (7) : oo P(T) 120 30 . Hemitropes common on both M and P. Angle between P and P 180 24. Cl. basal and brachy diagonal, perfect. H. =6 ;