866 STUDIES FOR STUDENTS 



The chemical processes involved in this feldspathic decom- 

 position are of sufficient importance to warrant further discus- 

 sion, even though it may involve a certain amount of repetition of 

 what has gone before. 



Berthier, Forschammer, Brogniart,^ Fournet,^ and others 

 explained more than iifty years ago the process of feldspathic 

 disintegration through the breaking up of its complex mole- 

 cule into alkaline silicates soluble in water and aluminous sili- 

 cates which are insoluble. The loss in silica, as noted above, 

 was supposed to be due to the removal, by solution, of these 

 alkaline silicates. Ebelman,^ however, subsequently showed that 

 silicate minerals poor or quite lacking in alkalies lost a portion 

 of their silica with equal facility. He accounted for this on 

 the supposition that the silica set free — in a nascent state — 

 was soluble either in pure water, or water containing carbonic 

 acid. Bischof states that when meteoric waters containing car- 

 bonic acid filter through rocks containing alkaline silicates, the 

 first action consists in the partial decomposition of these sub- 

 stances by the carbonic acid, and the formation of alkaline car- 

 bonates, which are dissolved. 



If the water thus impregnated, on penetrating further below 

 the surface, comes in contact with calcareous silicates, another 

 change will take place consisting of a decomposition and replace- 

 ment of these calcareous silicates by the alkaline silicates, and a 



chemical changes incidental to the alteration are as shown in the following tables, I 

 being the fresh oligoclase and II the decomposition product. 



I. II. 



Silica, ------- 59.51 45.29 



Alumina, ------ 25.10 25.68 



Iron sesquioxide, - - - - - 1.08 12.29 



Lime, ------ 4.03 0.52 



Magnesia, ------ trace 2.98 



Potash, ------ 2.10 3.00 



Soda, ------- ^.26 2.14 



Water, - . - . - - 0.92 12.49 



'Arch du Museum, Vol. I, 1839 (cited by Ebelman). 

 ^Annales de Chimieet de Physique, Vol LV, 1833. 

 3 Annales des Mines, Vol. VII, 1845. 



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