PAIGE AND steiger: sericitization 237 



a search for another source was demanded, and sericite was 

 chosen as the mmeral most likely to contain it. 



It is well known that muscovite mica contains fluorine. It is 

 now recognized that sericite is a form of muscovite, but analyses 

 proving that sericite contains fluorine are surprisingly few. 

 Spurr-* has argued that fluorine is necessary for the formation of 

 sericite, but his tests were not convincing as to the fluorine con- 

 tent of sericite, though its presence in a specimen (which con- 

 tained no sericite) to the amount of 0.12 per cent indicated that 

 the waters that altered the rock contained it. 



To throw more light on the matter therefore, a part of a narrow 

 replacement vein traversing quartz monzonite porphyry was 



TABLE I 

 Analysis of Sericite Vein 



SiOs ..68.11 NaaO 0.44 



AI2O3 16.84 K2O 4.08 



FeiOa 0.80 FeS2 6.68 



CaO 0.44 F *0.09 



MgO 0.50 



97.98 



chosen for analysis. The specimen contained about equal 

 amounts of sericite and quartz and a little pyrite. No fluorite 

 was visible with the high powers of a microscope, and no other 

 minerals than those mentioned were noted. 



The results of the analysis obtained are given in table 1. 



The mineral composition of the vein, as computed from the 

 analysis, is quartz, 45.30 per cent, sericite, 46.00 per cent, and 

 pyrite, 6.68 per cent: sum, 97.98 per cent. The analysis was 

 carried out by standard methods. The fluorine, however, was 

 indirectl}^ determined by the colorimetric method, depending 

 on the bleaching effect of fluorine on the color produced by 

 hydrogen peroxide with titanium solution. A qualitative check 

 was made by the old Berzelius method, and the presence of 

 fluorine was proved by its etching effect on glass. All the alkalies 

 and the alkali-earth metals were computed into the sericite. 

 Thus a maximum amount of sericite was figured. The percent- 



* Spurr, J. E., Geology of the Ton6pah Mining District. U. S. Geol. Survey 

 Prof. Paper 42: 232-3. 1905. 



