498 NEW YORK STATE MUSEUM 



The orthoclase or potasli feldspar lias a composition of silica 

 64. Y, alumina 18.4, potasli 16.9, while in the plagioclase group the 

 composition of the different members is given as follows.-^ 



Silica Alumina Potash Soda ''I Lime 



Albite. 68 20 ... 12 12 



Oligoclase 62 24 ... 9 5 



Labradorite 53 30 ... 4 13 



Anorthite 43 37 20 



In treating the decomposition or kaolinization of feldspar, most 

 writers are apt tO' give the impression that it is the orthoclase which 

 furnishes kaolinite by its decomposition, whereas both groups may 

 produce it, and indeed the plagioclase varieties decompose much 

 more readily than the orthoclase. This fact was noted by Leim- 

 berg. (Z. d. d. G. G. 35,1883) The same fact was observed by the 

 writer in the kaolin at Eonne, Denmark, which is produced by 

 the decomposition of a granite containing both plagioclase and 

 orthoclase. In partially weathered specimens the plagioclase was 

 the more extensively affected. As a rule the orthoclase feldspar is 

 much more common than the plagioclase. 



Aside from the kaolinization of feldspar by the ordinary processes 

 of weathering it seems possible and even probable that its decom- 

 position may be brought about by the action of mineralizing vapors, 

 that is, vapors whose presence seems to be necessary to the forma- 

 tion of certain minerals, as at Cornwall, Eng., where it was foimd 

 that the feldspar of the granite* on both sides of the tin veins had 

 been altered to kaolin. This change is attributed to the action of 

 fluoric vapors whose presence is pretty well proven. That such 

 a process is possible is shown by J. H. Collins (Min. mag. 1887. 

 7:213, in the " l^ature and origin of clays and the composition of 

 kaolinite ") who exposed feldspar to the action of hydrofluoric acid. 

 The feldspar, according to IVCr Collins, was converted into hydrated 

 silicate of alumina, mixed with soluble fluorid of potassium, while 

 pure silica was deposited on the sides of the tube. 



' G. P. Merrill. Rocks, rock-weathering and soils, p. 15. 



