338 A TREATISE ON METAMORPHISM. 



pentine and gibbsite as compared with the muscovite is 88.44 per cent, and 

 of the talc and gibbsite 46.69 per cent. 



The change of muscovite to paragonite merely requires the substitution 

 of sodium for potassium, and may be written as follows : 



(3) 2H 2 KAl 3 Si 3 12 -|-Na 2 C03=2H 2 NaAl 3 Si 3 12 +K,CO :! +k. 



The decrease in volume is 2.67 per cent. 



Muscovite under deep-seated conditions is a mineral which is practi- 

 cally permanent. In fact, under these conditions, as already indicated, it is 

 produced by the alteration of other minerals. The above alterations of 

 muscovite, resulting in the formation of vermiculite, serpentine, and talc, 

 with gibbsite all occur in the zone of katamorphism, and especially in the 

 belt of weathering*. Even under the conditions of the surface belt the proc- 

 esses of change are exceedingly slow. Corresponding with this position, 

 the changes take place with increase of volume and liberation of heat. 



PARAGONITE. 



Paragonite is hydrous sodium-aluminum silicate. 



occurrence. — Paragonite is not certainly known as an original pyrogenic 

 constituent in igneous rocks. It is found especially in the metamorphosed 

 igneous rocks and in the semimetamorphosed and completely metamor- 

 phosed sedimentary rocks. In many so-called sericite rocks it is probable 

 that a portion of the micaceous mineral is paragonite rather than muscovite. 

 Paragonite is especially likely to occur in the metamorphic rocks, instead 

 of muscovite, where the original rocks, either igneous or sedimentary, bear 

 a considerable amount of sodium. Very frequently associated with para- 

 gonite are the heavy metamorphic minerals, such as cyanite, staurolite, 

 garnet, tourmaline, etc. In certain places muscovite has been noted as pass- 

 ing to parag-onite, and thus the potassium mica is a source for the soda mica. 



In summary, paragonite as a metamorphic mineral is derived from 

 anorthoclase, muscovite, and plagioclases. 



Alterations. — Alterations of paragonite are not recorded in the standard 

 text-books. However, there can be little doubt that this mineral undergoes 

 a set of alterations in the zone of katamorphism, and one would expect 

 that these alterations would be analogous to those which take place with 

 muscovite. 



