254 A TREATISE ON METAMORPHISM. 



cimolite (amorphous; sp. gr. 2. 24), allophane (amorphous; sp. gr. 1.87), and 

 perhaps others. The chief differences are in the amounts of water added 

 and the amount of silica which separates. Pyrophyllite (H 2 Al 2 (Si0 3 ) 4 ) 

 differs from kaolinite in that less silica is removed and less basic water is 

 added; it therefore might be considered as an intermediate stage in the 

 alteration. Halloysite (H 4 Al 2 Si 2 9 . Aq.) differs from kaolinite only in having 

 water of hydration. Newtonite (H 8 Al 2 Si 2 0n.Aq.) differs from kaolinite in 

 containing twice as much basic water as that mineral, and in being hydrated. 

 Cimolite (H 6 Al 4 (Si0 3 ) 9 .3H 2 0) differs from kaolinite in containing more 

 silica, more basic water, and water of hydration. Allophane (Al 2 Si0 5 .5H 2 0) 

 differs from kaolinite in containing less silica and much water. It would 

 be easily possible to formulate equations along the line of that given for 

 kaolinite for each of these minerals and to calculate the volume relations. 

 However, this hardly seems necessary since these minerals as secondary 

 products to orthoclase and microcline appear to be very subordinate in 

 amount. 



Another alteration of orthoclase and microcline of some little impor- 

 tance is into gibbsite (monoclinic; sp. gr. 2.3-2.4). The reaction is: 



(2) 2KAlSi 3 8 4-3H 2 0+C0 2 =2Al(OH) 3 +6Si0 2 +K 2 C0 3 +k. 



The decrease in volume of the gibbsite and quartz as compared with the 

 orthoclase is 6.61 per cent. 



Another of the very important alterations of orthoclase and microcline 

 is to muscovite (monoclinic; sp. gr. 2.76-3.0) and quartz (rhombohedral; 

 sp. gr. 2.653-2.654). The reaction is: 



(3) 3KAlSi 8 8 +H 2 0+C0 2 =KH 2 Al 3 Si 3 0, 2 +6Si0. 2 +K 2 C0 3 +k. 



Provided the silica separates as quartz and the potassium unites with 

 carbonic acid and the potassium carbonate be removed in solution, the 

 decrease in volume is 15.58 per cent. 



While this reaction may take place under exceptional conditions, it is 

 believed, as explained below, that where muscovite forms from orthoclase 

 one of the rich aluminous minerals often unites with the orthoclase to 

 produce the mica. Supposing the aluminous mineral to be gibbsite, the 

 reaction is : 



(4) KAlSi 3 8 +2Al(OH) 3 =KH 2 A1 3 Si 3 12 +2H,0-k. 



The decrease in volume of the muscovite as compared with the orthoclase 

 and gibbsite is 20.^1 per cent. 



