148 A. F. BUDDINGTON 



If now the quantity of silica is present in the solution in large enough 

 excess and the effectiveness of hydrolysis is relatively stronger, the 

 reaction will produce pyrophyllite: 



2 KH 2 Al 3 Si 3 I2 +6Si0 2 +H 2 -> 6HAlSi 2 6 +K 2 Si0 3 



(sericite) (pyrophyllite) 



while if K,Si0 3 or potash in some other form is present in large 

 enough quantity the alternative reaction will take place and sericite 

 will be produced: 



6HAlSi 2 6 + K 2 Si0 3 -> 2KH 2 Al 3 Si 3 I2 -f-6Si0 2 -fH 2 



(pyrophyllite) (sericite) 



CHEMICAL PHENOMENA CONNECTED WITH ORIGIN OF PYROPHYLLITE 



From a comparison of the analysis (No. 2) of the silicified rhyo- 

 lite with that of the pyrophyllite (No. 5), it will be seen that the 

 change in composition has been such as might have been brought 

 about essentially through three processes: (1) the introduction of 

 alumina, (2) the replacement of the alkalies by hydroxyl, and (3) the 

 solution of silica. The analyses 2, 3, 4, and 5, recalculated into 

 their mineral composition, show a direct transition from the country 

 rock (the silicified rhyolite) into pyrophyllite through a decrease 

 in the quantity of quartz, feldspars, and impurities and a simul- 

 taneous increase in the content of pyrophyllite. In order that the 

 original rock may be so altered as to give the mineral analyses 

 shown by the transitional rocks, it is necessary that metasomatic 

 replacement of both the quartz and the feldspars should have pro- 

 ceeded synchronously and at a much faster rate with respect to the 

 quartz than with respect to the feldspars. This process would 

 involve the introduction of large amounts of alumina, the gradual 

 replacement of the alkalies by hydroxyl at a more rapid rate in the 

 case of the soda than of the potash, and the solution of portions of 

 both the silica existing in combination with other elements in the 

 rock and that present as free quartz. 



The heterogeneous, blotchy character of the quartz-pyrophyllite 

 rock when seen in thin section suggests that the rock may have 

 been in the condition of a more or less homogeneous glass when 

 acted upon by the silicifying and pyrophyllitizing solutions, in view 

 of the fact that its chemical composition and mineral arrangement 



