318 A TREATISE ON METAMORPHISM. 



While for the sake of simplicity wollastonite is supposed to form, the 

 more frequent association of the aluminum-silicate group is with tremolite, 

 actinolite, and diopside. For the first and last of these minerals the freed 

 silica unites with the calcium and magnesium together, and for the second 

 with the calcium, magnesium, and iron. The equations representing the 

 changes are analogous to (2), and the volume changes are in the same 

 direction. 



Alterations. — The standard stated alterations of the aluminum-silicate 

 group are to talc (steatite) (massive; sp. gr. 2.75) and to muscovite 

 (damourite) (monoclinic; sp. gr. 2.88). It is recorded also that topaz and 

 andalusite alter to kaolin (monoclinic; sp. gr. 2.615). Occasionally also 

 andalusite may alter into the heavier mineral cyanite (triclinic; sp. gr. 

 3.56-3.67). 



The alterations of the minerals into talc require an entire change of 

 base; that is, from aluminum silicates to magnesium silicates. The reac- 

 tions being those of the zone of katamorphism, the most probable source 

 of the magnesium is doubtless the carbonate, which may be derived from 

 the decomposition of magnesium rocks such as the pyroxenites, olivinites, 

 etc. The process, however, requires the separation of aluminum either as 

 corundum (rhombohedral ; sp. gr. 4.025), coruudophilite (monoclinic; sp. 

 gr. 2.90), diaspore (orthorhombic; sp. gr. 3.40), gibbsite (monoclinic; sp. 

 gr. 2.35), or some other form. Since the reaction takes place in the upper 

 physical-chemical zone, gibbsite will be regarded as the product formed. 

 The change of the aluminum-silicate minerals to muscovite requires the 

 addition of potassium. This is doubtless derived from the liberation of 

 potassium during the decomposition of the potash feldspars, and will there- 

 fore be regarded as added as a carbonate. The change from andalusite to 

 cyanite is simply a molecular one, the result being a mineral of great 

 specific gravity. It has already been seen that andalusite is a product of 

 less intense metamorphism, and that more intense inetamorphism produces 

 sillimanite and cyanite. The change of andalusite to these heavier minerals 

 is therefore one which requires deep-seated conditions, and is characteristic 

 of the zone of katamorphism. 



The equations representing the change of andalusite, sillimanite, and 

 cyanite to talc with gibbsite may be written as follows: 



(1) 4Al 2 3i0 5 +3MgC0 3 +13H 2 0=H 2 Mg 3 Si 4 12 +8Al(OH) 3 +3C0 2 +k. 



