THE DECOMPOSITION OF FELDSPABS' 17 



been derived a large share of the salts of potash, lime, and soda, 

 as well as aluminons silicates which form so essential a portion 

 of the soils. The method of feldspathie decomposition as com- 

 monly understood is given on p. 223. 



This decomposition nsnally manifests itself by a whitening 

 of the mass, accompanied by opacity and a general softening, 

 whereby it falls away to loose powder unless confined. As seen 

 in thin sections nnder the microscope, the decomposition goes 

 on most rapidly along lines of cleavage, naturally attacking the 

 outer portions first, so that the crystals show fresh unaltered 

 cores surrounded by opaque and ^' muddy" borders. In cases 

 where the feldspars carry iron this usually makes its presence 

 known by a reddening or browning of the mass, due to oxida- 

 tion. In presence of abundant carbonic acid, the liberated iron 

 may be carried away in solution and the color remain unchanged. 



Daubree, who submitted feldspathie fragments to trituration 

 in revolving cylinders of stone and iron, found that in all such 

 cases not merely were the particles worn down to the condi- 

 tion of fine silt, but that an actual decomposition took place, as 

 well, as shown by the presence of alkalies in the form of soluble 

 silicates in the water with which the cylinders were partially 

 filled. 



The production of kaolin through feldspathie decomposition 

 has become so well recognized that it is customary to speak 

 of this form of decomposition as kaolinizatmi, a term which we 

 shall have frequent cause to use.^ 



It should be noted that orthoclase, though so frequently found 

 muddied and impure, apparently in an advanced stage of de- 

 composition, does not in reality decompose so readily as the 

 plagioelase (soda-lime) varieties. This fact has been noted by 

 Lemberg,^ who states that the apparent decomposition may be 

 due to physical causes, as disintegration, inclusions of some 

 easily decomposable silicate, or to originally water-filled cavities 

 the contents of which have been absorbed through the formation 

 of secondary hydrous silicates. 



^The statement by Rosier (Neues Jahrbueli fur Min. Geol. u. Petrog., 

 Beilage Band, Vol. 2, 1902) to the effect tliat kaolinization is never due 

 to weathering, "but is a deep seated process, finds little confirmation in 

 America. 



2 Zeit. Deut. GeoL Gesellscliaf t, 35, 1883. 



3 



