2 A1 2 3 , 4 Si0 2 ) by An we have the following scheme for the plagioclase 

 felspars. 



Ab ... ... Albite 



Ab to Ab 3 An l ... Oligoclase, Anorthite rises to ... 26*2 percent. 



Ab a An l , to Ab^,An^ ... Andesine ... ... ... 51'5 ,, 



Abi, An-^ to Ab l , An a Labradorite ... ... ... 76-1 ,, 



Ab t , An 3 , to An ... Bytownite ... ... nearly lOO'O ,, 



An ... ... Anorthite 



The chemical compositions of the above mixtures are as follows : 



The extinction angles on the two principal cleavages referred to the edge 

 P/M, according to SCHUSTER'S researches, are : 



Ab, extinction on P + 430' on M + 19 0' 



Ab a An lt + 1 4' + 436' 



Ab lt An lf 510' 16 0' 



Ab,,An a , 1740' 2938' 



An, 37 0' 36 0' 



The observation of cleavage flakes in convergent light is often of great 

 use in the determination of felspars, and will sometimes yield valuable 

 information when the boundaries of the flakes are not sufficiently well 

 defined to enable a precise determination of the extinction to be made. 

 Examined with the ordinary arrangement for the use of convergent polarised 

 light, with a petrographical microscope, a flake of anorthite parallel either to 

 M or P shows an optic axis within the field of view, but near the margin ; a 

 flake of bytownite shows an optic axis close to, but not within the field. A 

 flake of labradorite parallel to M shows a positive bisectrix, veiy oblique to 

 the axis of the instrument, and one of the axial shadows. In oligoclase and 

 andesine the M cleavage is nearly at right angles to the positive bisectrix. 

 In albite it is oblique to the bisectrix. The observations in convergent light are 

 mainly useful in distinguishing between anorthite, bytownite and labradorite ; 

 a single glance at the interference figure is sufficient to show to which of 

 these any given felspar is most nearly allied. 



The determination of the felspar by optical methods may be checked by 



