352 A TREATISE ON METAMORPHISM. 



KAOLIN GROUP. 



Kaolinite is the only important rock-making mineral of this group. 



Kaolinite: 



H 4 Al 2 8i 2 9 . 

 Monoclinic. 

 Sp. gr. 2.6-2.63. 



occurrence. — Kaolinite is a secondary product in all classes of igneous 

 rocks and occurs as an important constituent in all sedimentary rocks except 

 the pure sandstones and the pure limestones. Kaolinite and quartz are the 

 chief constituents of the clays, and kaolinite is a very abundant constituent 

 of muds and grits. 



Lvaolinite is a product which forms extensively in the zone of katamor- 

 phism in the belt of cementation and in the belt of weathering. It is likely 

 to be produced as a result of the decomposition of any of the aluminous 

 minerals. It has been noted as having' been produced from the following 

 minerals: Andalusite, anorthoclase, biotite, cyanite, epidote, leucite, micro- 

 cline, nephelite, orthoclase, plagioclases, scapolites, sillimanite, sodalite, 

 topaz, and zoisite. Of these, undoubtedly the most important are the feld- 

 spars, and especially the acid feldspars. 



Alterations. — No alterations of kaolinite are recorded. It is certain, how- 

 ever, that where the kaolin-bearing sediments are deeply buried the mineral 

 becomes dehydrated, that such bases as the alkalies and alkaline earths and 

 iron replace the hydrog'en, and that various anhydrous silicates or silicates 

 low in hydrog'en are produced. It is certain that in the zone of anamor- 

 phism the minerals which in the upper physical- chemical zone have broken 

 up into kaolinite as one of the products may recombine to a large extent 

 and reproduce the original minerals. 



SUMMARY OF ALTERATION OF SILICATES. 



While the important groups of the rock-forming silicates have been 

 treated separately, it may be well in closing the section to class together 

 the groups of the original minerals which have a somewhat similar chemical 

 composition and therefore alter into somewhat similar products. 



These classes are called by the petrographers (1) the feldspathoid 

 class, (2) the transition class, and (3) the ferromagnesian class. The felds- 

 pathoid class includes the feldspar, nephelite, sodalite, leucite, and 

 wernerite groups. The only rock-forming minerals belonging to the 



