Wright — Determination of Feldspars. 361 



Art. XXXII. — The Determination of the Feldspars by Means 

 of their Refractive Indices ; by Feed. Eugene Weight. 



Of the many methods which have been suggested for the 

 discrimination of the feldspars, perhaps none has received less 

 actual attention from penologists than those based on the refrac- 

 tive indices, notwithstanding the fact that the refractive power 

 of a mineral is one of its fundamental properties and can be 

 determined approximately with comparative ease. The appli- 

 cation of the refractive index methods to the feldspars was 

 tested in detail by F. "Wallerant,* who determined their refrac- 

 tive indices directly on a total refractometer attachment to his 

 microscope. His particular method is applicable to all minerals 

 except those of the highest refracting power, and would be in 

 general use at the present time were it not for the special 

 apparatus demanded and the fact that thin sections for his 

 purposes require special preparation. 



The method which is described below has proved serviceable in 

 actual practice, and is based on the ability to determine the aver- 

 age refractive indices of the feldspars under the microscope by 

 applying the principles developed by Schroeder van der Kolk 

 some years ago.f The conception of the method is therefore not 

 novel, and the following paragraphs are intended solely to sug- 

 gest a modus operandi which has been found convenient. 



Small mineral fragments when observed in obliquely incident 

 light show, on immersion in liquids of higher or lower or equal 

 refractive index, characteristic phenomena which are extremely 

 sensitive and can well be used to distinguish minerals whose aver- 

 age refractive indices vary only five points in the third decimal 

 place. As the theory of these phenomena, however, is given in 

 full in the papers cited, only the practical application of the 

 method to the study of the feldspars will be considered below. 

 Equipment. — Seven liquids are required whose refractive 

 indices for yellow light are equal respectively to the refractive 

 indices /9 for 



Orthoclase (Or) 1-523 



Albite (Ab^AnJ 1-536 



Oligoclase (AbjAnJ 1-545 



Andesine (Ab 3 An 2 ) 1-554 



Labradorite (Ab 2 An~) 1-563 



Bytownite (Ab 1 An 3 ) .. 1-572 



Anorthite (AbjAn^) 1-582 



*Bull. Soc. Min. Fi\, pp. 268-271, 1898. 



f Tabellen zur mikroskopiscken Bestimmung der Mineralien nach ihrem 

 Brechungsindex. Wiesbaden, 1900. Compare also Bosenbiiscli-Wulnng, 

 Mikroskopische Physiograpkie, 1904, I, 1, 259-261, also II, 345 ; and Fred. 

 E. Wright, Tschermak Miner. Petr. Mittheil., xx, 239, 1901 and this Journal, 

 xvii. 385-387, 1904. 



