FELDSPAR GROUP. 273 



tained as follows. Adding together the anorthite and albite 

 formulas, we have CaiNa 2 Al 2 24 Si s ; then dividing by 2, the 

 formulas becomes £Ca£Na 2 Al 12 Si 4 , which expresses the 

 composition of andesite. With 3 parts of the Ca unisili- 

 cate, and 1 of the Na 2 tersilicate, the composition is that 

 of labradorite. So it is for other combinations, that is for 

 other species between anorthite and albite in composition. 



The quantivalent ratio for the R, Al, Si, in the several 

 species of the group, is as follows : V means triclinic in 

 crystallization, and IV monoclinic. 







SYSTEM 



OF 







SYSTEM OP 





RATIO. 



CRYSTALLIZE 





RATIO. 



CRYSTALLIZA- 







TION. 









TION. 



Anorthite, 



1:3:4 



v, 





Oligoclase, 1 



:3:9 



V. 



Labradorite, 



1:3:6 



V, 





Albite, I 



:3:12 



V. 



Hyalopliane, 



1:3:8 



IV, 





Microcline, 1 



:3:12 



V. 



Andesite, 



1:3:8 



v, 





Orthoclase, 1 



:3:12 



IV. 



These are the normal ratios ; but there is some variation 

 from them in the analyses, part of which is variation in 

 actual composition, and part a result of interlamination or 

 mixture of two feldspars together. Thus, orthoclase occurs 

 mixed with microcline, albite, or oligoclase. But while such 

 mixtures account for the soda found in some analyses of 

 orthoclase, it does not for that in all, since soda does occur 

 in many specimens of pure orthoclase, replacing part of the 

 potash. It is the same with the triclinic feldspar micro- 

 cline, which has the composition of orthoclase, and may 

 have the alkali portion all potash or part soda, one analysis 

 of typical microcline giving only 0*48 of soda. It is, hence, 

 not safe to calculate the percentage of orthoclase present 

 in a feldspar from the percentage of potash. Moreover, 

 potash is present in much albite. 



The above ratios show that anorthite has for the ratio 

 between R + R and Si, 4 : 4, or 1 :1, as in true unisilicates ; 

 while in albite and orthoclase, the same ratio is 4 : 12 or 

 1 : 3, that of a tersilicate, as above stated. 



C. In physical characters : the hardness being between 6 



