14 THE MINBEALS CONSTITUTING BOCKS 



through impurities or decomposition; and lastly, a constant 

 intergradation in composition, as already noted on p. 9. 



Nine varieties of feldspar are commonly recognized, which 

 on crystallographic grounds are divided into two groups: the 

 first, crystallizing in the monoelinic system, including ortho- 

 clase and hyalophane; and the second, crystallizing in the tri- 

 clmic system, including microcline, anorthoclase, and the albite- 

 anorthite series albite, oligoclase^ andesine, labradorite, and 

 anorthite. 



The Monoelinic Feldspars: OrtJwclase {Sanidm), Potash Feld- 

 spars. — Compos%tion : K^ Al^SigOig = silica, 64.7 % ; alumina, 

 18.4% ; potash, 16.9%. 



This is one of the commonest and most abundant of feldspars, 

 and forms an essential constituent of the acid rocks, such as gran- 

 ite, gneiss, syenite, and the orthoclase and quartzose porphyries ; 

 more rarely it occurs as an accessory in the more basic erup- 

 tives. Under the name Sanidin is included the clear glassy 

 variety of orthoclase occurring in Tertiary and modern lavas, 

 such as trachyte, phonolite, and the liparites. 



As a rock constituent the potash feldspars are of primary im- 

 portance, imparting by their preponderance, not merely color 

 and important structural features, but on their decomposition 

 yielding the potash, valuable for plant food, and the material 

 kaolin so essential for porcelain ware. In the thin sections, 

 under the microscope, the orthoclase of the older rocks is often 

 quite opaque, or at least muddy, through impurities or incipient 

 kaolinization. In many eruptives it has been one of the first 

 minerals to separate out from the molten magma, and shows, 

 therefore, more or less well-defined crystallographic boundaries 

 — is idiomorphiCy to use a more technical term. A well-defined 

 zonal structure is frequently observed, which is due to inter- 

 rupted periods of growth, and to a gradual change in the char- 

 acter of the magma, whereby the outer zones are more or less 

 translucent or opaque from impurities. Twin structure is very 

 common after what is known as the Carlsbad law, and when 

 the crystals are of sufficient size is easily recognized by the 

 unequal reflection of the light from the two sides of a crystal 

 on a cleavage surface. 



The TricUnic Feldspars, — The chemical relationship exist- 

 ing between the triclinic feldspars is shown in the following 

 table : 



