PAIGE AND steiger: sericitization 239 



silicon tetrafluoride (SiF4), a gas soluble in water. Silica would 

 thus be attacked and carried off. It is known that 

 SiF4 + 4H2O = Si(0H)4 + (4HF) 

 2SiF4 + (4HF) = 2HoSiF« 



3SiF4 + 4H2O = Si(0H)4 + 2H2SiF6 

 Thus silicon hydroxide and hydrofluosilicic acid are formed. 

 This latter compound is stable only in water, and is inert so far 

 as attacking quartz is concerned. In the rocks under discussion 

 here, it would probably form alkaline salts. On evaporation 

 of the water, however, silicon tetrafluoride (SiF4) would again 

 be formed and the more or less insoluble alkaline salts deposited. 

 Whether the silicon tetrafluoride set free would, on recombining 

 with water, again produce the active agent hydrofluoric acid is 

 not known. More likely the process outlined above would be 

 repeated, a certain portion of silicon hydroxide, Si (OH) 4, being 

 deposited, and the inert hydrofluosilicic acid going again into 

 solution, to again combine with alkaline bases. 



But it is to be expected that these salts will yield to decomposi- 

 tion by acidified waters, as does sericite. And there is reason to 

 believe that fluorine will be again and again set free, so long as 

 sulphuric acid waters are present. 



summary 



The intense sericitization common in regions where large 

 deposits of secondary chalcocite occur may be in large measure 

 due to the fact that the primary mineralizing waters contained 

 appreciable amounts of fluorine. The further decomposition of 

 the rocks by descending sulphate waters with the attendant 

 formation of kaolin-like substances may also be accelerated by 

 the presence of the fluorine in the sericite, which is set free by 

 reaction with sulphuric acid. The removal of quartz from veins 

 consisting of kaolin-like matter which apparently has metasomati- 

 cally replaced porphyry is thus explained. 



The presence of fluorine in veins and the determination by 

 chemical analysis of 0.20 per cent fluorine in the sericite of the 

 Tyrone district, New Mexico, support this thesis. 



