FELSPATHOID GROUP I J 



even thin sections under the microscope are commonly hazy. 

 Sanidine is a glassy, transparent variety of orthoclase, which is 

 found in lavas of late geological date. Its clearness is due to the 

 absence of the decomposition products, which render ordinary 

 orthoclase turbid. 



2. T?iclinic Felspars 



The minerals of this series are grouped together under the com- 

 prehensive term of Plagioclase, because of the difficulty of distin- 

 guishing them from each other under the microscope ; they are 

 very generally characterized by polysynthetic twinning, which 

 makes fine parallel lines on the basal cleavage planes. Chemi- 

 cally, they are soda, lime, or lime-soda felspars, of which the 

 latter are isomorphous mixtures of albite and anorthite. The fol- 

 lowing table (from Levy and Lacroix) gives the composition of 

 the various members of this series, representing the soda-felspar 

 constituent, or albite, by Ab, and the lime-felspar constituent, or 

 anorthite, by An : — 



Name. Composition. Specific Gravity. 



Albite Ab 2.62 



Oligoclase AbioAn 3 2.65 



Andesine Ab2Ani 2.67 



Labradorite .... Ab 2 Ans 2.70 



Anorthite An 2.75 



It will be observed that the specific gravity increases with the 

 lime constituent, and the fusibility diminishes in the same propor- 

 tion. Anorthite is decomposed by hydrochloric acid, labradorite 

 is slightly attacked by it, while the other members of the series 

 are not affected. 



Anorthoclase is a triclinic potash-soda felspar (AbaOrj), but is 

 not a common constituent of rocks. 



II. The Felspathoid Group 



These minerals are very closely allied to the felspars in chemi- 

 cal composition, but differ from them in crystalline form and 

 c 



