SUMMARY OF ALTERATION OF SILICATES. 353 



transition class are tiiose of the muscovite group. The ferromagiiesian 

 class includes the pyroxene, arnphibole, chrysolite, biotite-phlogopite, and 

 clintonite groups. 



(1) In the upper physical-chemical zone, that of katamorphism, the 

 more common alteration products of the feldspathoid class are the kaolins, 

 the zeolites, the epidotes (including both zoisite and epidote proper), diaspore, 

 gibbsite, and quartz. Probabl}- all of these minerals form in both belts of 

 the zone, but the development of the zeolites and the epidotes is more 

 characteristic of the belt of cementation than of the belt of weathering. 

 Indeed, as pointed out under these minerals, under long-continued condi- 

 tions of the belt of weathering these minerals break up into carbonates 

 of the alkalies and alkaline earths, hydrous and anhydrous, oxides of 

 aluminum and iron, quartz, and, probably, also kaolin. In the zone ot 

 anamorphism the more common alteration products of the feldspathoid 

 class are muscovite (damourite) and scapolite. The nephelite, sodalite, and 

 leucite groups alter into the feldspars. 



(2) In the zone of katamorphism muscovite alters into serpentine, 

 talc, and vermiculite (hydromuscovite). In the belt of weathering the 

 serpentine and vermiculite may break up into simpler compounds of the 

 same character as those which form from the zeolites and epidotes. 



In the lower physical-chemical zone, that of anamorphism, muscovite 

 is one of the minerals commonly produced, and therefore does not usually 

 alter. But by profound and deep-seated metamorphism, the material of 

 muscovite may pass into the heavy ferromagiiesian minerals, such as garnet, 

 staurolite, etc. 



. (3) The ferromagiiesian silicates may be divided into two great 

 divisions — those which are nonaluminous, and those which are aluminous. 

 In the zone of katamorphism the most common alteration products of the 

 nonaluminous ferromaguesian silicates are talc and serpentine. The 

 nonaluminous pyroxenes and amphiboles ordinarily pass into talc; the 

 chrysolites ordinarily pass into serpentine. The transformations in these 

 directions are explained by the fact that the pyroxenes, amphiboles, and talc 

 are metasilicates, while the olivines and serpentines are orthosilicates. The 

 metasilicates naturally jaass into metasilicates, and the orthosilicates into 

 orthosilicates. In the zone of katamorphism the heavily aluminous ferro- 

 magnesian silicates alter into chlorites and epidotes. The pyroxenes and 

 amphiboles which are not heavily aluminous frequently split up into a com- 

 mon xlvii — 04 23 



