HYDRON SILICATES IN ROCKS OF CONCEPTION BAY 149 



would involve the simultaneous replacement of the feldspars of 

 a crystalline rock by silica and pyrophyllite, and of its quartz by 

 pyrophyllite. 



ORIGIN OF PINITE AT MANUELS 



From a further study of the chemical analyses, it becomes 

 evident that while the silicified rhyolites, pyrophyllitized rhyolites, 

 and pyrophyllite have all decreased in their content of iron, mag- 

 nesium, potassium, and soda, the pinite analysis (7) shows a decided 

 increase in the first three of these elements. During the formation 

 of pyrophyllite vast quantities of potash must have been liberated 

 and carried in solution in the circulating waters. Is it not possible 

 that away from the main channels these waters deposited their 

 load as pinitic replacements of the rhyolite under the control of 

 lower temperatures and pressures and the mass-action effects of 

 the excess potash in solution ? It seems reasonable to suppose that 

 the pinite here was an essentially contemporaneous formation 

 with the quartz-pyrophyllite schists and pyrophyllite, receiving 

 some of the magnesia, potash, and iron released by the formation 

 of the pyrophyllite, as the quartz schists have received some of the 

 silica originating at the same time. 



ORIGIN OF THE PINITE SCHIST AT HARBOUR MAIN 



The chemical analysis of the pinite schist recalculated for 

 sericite gives the rock a mineral composition of about f sericite 

 and f quartz, feldspar, and other constituents. Examination of 

 thin sections shows that this result has been brought about through 

 the replacement of both the feldspar and quartz by sericite. If 

 we consider the rock previous to pinitization to have had the com- 

 position of the silicified rhyolite (No. 2), then the process cited 

 involves the substitution of potash for soda in the feldspars, the 

 addition of potash and alumina, and the subtraction of silica and 

 soda. 



There is considerable evidence that this rock was not formed 

 at the surface. There are no secondary products of decomposition, 

 such as kaolin or limonite, associated with the pinite. It occurs 

 in quantity only in certain zones and extends to a considerable 

 depth, as exposed in a prospect pit for talc, south of Harbour Main. 



