HYDRON SILICATES IN ROCKS OF CONCEPTION BAY 147 



Cole (1886) describes pinite occurring as an alteration product 

 of spherulitic rhyolites, in conjunction with silicined spherulites. 

 He suggests that thermal waters are responsible for the origin of 

 both the pinite and quartz. 



From these references it is evident either that there is a differ- 

 ence of opinion as to the conditions under which pinite forms, or 

 that it is stable under widely variant temperatures and pressures. 

 It is probable, however, that it demands higher temperatures and 

 pressures than exist at the surface as conditions for its most favor- 

 able development, and such is doubtless the case with respect to 

 the pinite of Conception Bay. 



ALTERNATIVE DEVELOPMENT OF PINITE OR PYROPHYLLITE 



Since sericite or pinite is the usual product of hydrothermal 

 alteration it is pertinent to inquire if any reason can be found why 

 in certain cases pyrophyllite should be the product formed. A 

 possible equation (1) representing the formation of sericite from 

 orthoclase is quoted from Clarke (191 1), and a similar possible 

 equation (2) representing the formation of pyrophyllite from ortho- 

 clase is given below: 



(1) 6 KAlSi 3 8 +2H 2 0^2KH 2 Al3Si 3 Oi2+2K 2 Si03+ioSi0 2 



(sericite) 



(2) 6 KAlSi 3 08+3H 2 0^6HAlSi 2 6 +3K 2 SiO-r-3Si0 2 



(pyrophyllite) 



From these equations three factors are suggested as the possible 

 elements influencing the alternative development of sericite and 

 pyrophyllite: (1) the effectiveness of hydrolysis, (2) the mass 

 action of the excess silica in solution, and (3) the mass-action effect 

 of excess potash in solution. The dominance of the first two 

 factors would be conducive to the formation of pyrophyllite, and 

 the dominance of the third factor would be favorable to the pro- 

 duction of sericite. This may be illustrated more graphically, 

 without however implying anything as to the actual mode of opera- 

 tion, by writing the equation for the formation of pyrophyllite from 

 sericite as a balanced reaction: 



2 KH 2 Al3Si 3 I2 +6Si0 2 +H 2 0±?6HAlSi 2 06+K 2 Si03 



(sericite) (pyrophyllite) 



