260 A TREATISE ON METAMORPHISM. 



abundantly as allogenic constituents in the sedimentary rocks. In such 

 rocks the more siliceous plagioclases are more plentiful than the less siliceous 

 plagioclases, because of the more ready decomposition of the latter. The 

 plagioclases develop abundantly as autogenic constituents in the metamor- 

 phic rocks of both sedimentary and igneous origin. The plagioclase albite 

 is recorded as being derived from analcite, heulandite, laumontite, plagio- 

 clases (with orthoclase), sodalite, spodumene, and stilbite. The plagioclase 

 anorthite is not recorded as being derived from other minerals. But it is 

 seen in the zone of katamorphism that anorthite passes into various zeolites 

 by simple hydration. It can hardly be doubted that when such zeolites 

 pass into the zone of anamorphism by dehydration they are sources of anor- 

 thite. Doubtless also anorthite is produced in different ways from the com- 

 binations of various minerals, just as it passes into different combinations of 

 minerals. The intermediate feldspars, which are intermolecular mixtures of 

 albite and anorthite, may be derived from any of the minerals from which 

 albite and anorthite are formed. 



Alterations. — In treating the alterations of the plagioclases the only prac- 

 ticable plan is to calculate equations and volume reactions separately for 

 albite and for anorthite. For any of the intermediate feldspars the corre- 

 sponding equations may be written by multiplying the albite and anorthite 

 equations by the number of molecules of these compounds, respectively, 

 and adding the products. However, the alterations of the plagioclases are 

 so complicated that I have not been able to make the treatment more than 

 very partial. 



The species belonging to the more siliceous half of the plagioclase feld- 

 spars — i. e., albite, oligoclase, and andesine — frequently undergo alterations 

 similar to those of the monoclinic feldspars, producing kaolin (monoclinic; 

 sp. gr. 2.615), gibbsite (monoclinic; sp. gr. 2.35), and quartz (rhombohe- 

 dral; sp. gr. 2.6535). These alterations may be considered as coming 

 from the albite molecule. 



But the more common alterations of the plagioclases are into the zeo- 

 lites, epidote (monoclinic; sp.gr.3.38), quartz, the scapolites, and paragonite 

 (monoclinic; sp. gr. 2.84), and the less siliceous feldspars into more siliceous 

 plagioclase feldspars. The plag'ioclases are also recorded as altering into 

 prehnite (orthorhombic; sp. gr. 2.875) and albite. By pyrochemical 

 methods plagioclase and sodium carbonate at 220° C. produce the zeolite 

 analcite (isometric; sp. gr. 2.255), and this process is more rapid in propor- 



