142 



of the twin lamellae. In the majority of cases this is impossible. The fact 

 that only a small number of sections of the larger felspars are usually 

 available for examination renders the statistical method of M. LfivY of little 

 use in the precise determination of these felspars. Nevertheless, if a section 

 can be found in which twinning on the albite plan occurs, and in which the 

 extinctions are symmetrical with reference to the trace of the twinning 

 plane; then, if the extinctions between two lamellae exceed 37, the existence 

 of labradorite, bytownite or anorthite may be inferred, and if the extinctions 

 exceed 62 one of the two latter minerals must be present/ 1 * 



In the case, however, of the larger felspars it is in most cases easy to 

 obtain cleavage flakes, and then a precise determination may be effected by 

 the method of Des CLOISEAUX. To obtain cleavage flakes a portion of the 

 rock may be roughly powdered, grains of the felspar picked out and these 

 grains again broken on a small anvil. From the powder thus produced 

 cleavage flakes may be selected and examined in parallel and convergent 

 polarised light. In most cases the felspars are twinned on the albite plan 

 and this allows the flakes parallel to the basal plane (P or 001) to be easily 

 recognised. Flakes parallel to the brachypinacoid (M or 010) show no 

 twinning when the albite type is the only one present, and they are often 

 rhomboidal in form, the edges being determined by the perfect cleavage 

 parallel to P, and the less perfect cleavage parallel to a prism face of the 

 crystal. These latter flakes are the most useful for the purpose of 

 determining the nature of the felspar by optical methods, as they enable 

 one to ascertain the sign of the extinction in SCHUSTER'S sense as well as the 

 amount. 



The optical characters of the plagioclase felspars of the soda-lime group 

 have been investigated in great detail by DES CLOISEAUX (2) and SCHUSTER. (8) 

 The accompanying figure taken from Dr. SCHUSTER'S paper will explain the 

 nomenclature which he has introduced, and which is now adopted by common 

 consent in referring to the optical characters of the felspars. 



(1) The following arc the extinctions in the zone perpendicular to the brachypinacoid 

 (010) for the principal felspars according to Mm. FOUQUE and LEVY. Sections in this zone 

 are recognised by the fact that the extinctions of the two adjacent lamellae of a felspar twinned 

 on the albite plan are symmetrical with reference to the trace of the plane separating the 

 lamellae : 



Extinctions Extinction angle 



referred to the between two 



trace of 010. adjacent lamellae. 



Microcline ... ... to 18 ... ... to 36 



Albite to 1545' to 3130' 



Oligoclase ... ... to 1830' ... ... to 37 



Labradorite ... ... to 3115' ... ... to 6230' 



Anorthite ... ... to 3721' (and over) ... to 7442' (and over) 



(2) Sur les proprietes optiques birefringentes caracteristique des quatre principaux felspaths 

 tricliniques. Ann. d. Chimie et d. Physique. 5 serie. t. IV. 1875. 



(3) Die optische Orientirung der Plagioclase. T.M.M. Neue Folge, vol. III. p. 117. 



