THE PLAGIOCLASE FELDSPARS. 259 



changes take place are identical with the corresponding changes of ortho- 

 clase and microcline — i. e., alterations represented by equations (1) and (2) 

 take place in the zone of katamorphism, and especially the belt of weathering. 

 Alterations (3) and (4) occur in the zone of anamorphism, and that of 

 equation (5) is known for the belt of cementation. One general point is 

 clear from the above, that in the anorthoclase rocks we have a source for 

 paragonite in the paragonite-schists and paragonite-gneisses. 



TSICLINIC. 



The plagioclase feldspars are a group of triclinic feldspars which range 

 from sodium-aluminum silicate to calcium-aluminum silicate. The former 

 is a polysilicate and the latter an orthosilicate, hence there is great variation 

 both as to composition and as to acidity. The names, compositions, and 

 specific gravities of the species, as given by Tschermak and Dana, are as 

 follows : 



ALBITE, OLIGOCLASE, AXDESIXE, EABRADOKITE, BYTOWXITE, AXD AXORTHITE. 



Albite: 



NaAlSi 3 8 . 



Triclinic. 



Sp. gr. 2.62-2.65. 

 Oligoclase: 



Ab to Ab 3 Aiii. 



Triclinic. 



Sp. gr. 2.65-2.67. 

 Andesine: 



Ab 3 An, to AbjAn^ 



Triclinic. 



Sp. gr. 2.6S-2.69. 

 Labradorite: 



AbiAri, to Ab,An 3 . 



Triclinic. 



Sp. gr. 2.70-2.72. 

 Bytownite: 



AbjAiij to An. 



Triclinic. 



Sp. gr. 2.72-2.74. 

 Anorthite: 



CaAl 2 Si 2 8 . 



Triclinic. 



Sp. gr. 2.74-2.76. 



occurrence. — The plagioclases are probably the most important rock- 

 making constituents, being approached in abundance only by the orthoclase 

 feldspars and by quartz. The plagioclases are present as pyrogenic con- 

 stituents in the great majority of igneous rocks. They also occur very 



