CHAPTER X 



SILICATES, TITANATES, ETC. 



FELDSPARS AM) LEUCITE 



THE FELDSPARS 



THE feldspars constitute a most important group of rock-form- 

 ing mint-nils, so important that not only is nearly 60 per cent, 

 of the igneous rocks composed of feldspars, but their classifica- 

 tion depends to a great extent upon the quantity and species of 

 feldspar that is present. They are isomorphous in the fullest 

 sense of the term, and even though they belong to both the mono- 

 clinic and triclinic systems and are also salts of two silicic acids, 

 yet they mix in all proportions, forming solid solutions. One 

 species grades gradually into the others. The various mixtures 

 which seem to be more constant in nature have been given special 

 names, and in the past were considered as distinct species. 



From a chemical standpoint there are four species, two of which, 

 orthoclase and albite, are salts of the trisilicic acid (H 4 Si 3 O 8 ) while 

 the other two, anorthite and celsian, are salts of orthosilicic acid 

 (H4Si0 4 ). 



Since they form a compact isomorphous group, they are very 

 similar in all their physical and crystallographical properties. 

 . The following table will show very clearly these relations and 

 will also serve for their microscopic indentification. 



ORTHOCLASE 



Orthoclase. Potassium aluminium trisilicate ; KAlSi 3 O 8 ; 

 K 2 = 16.9, A1 2 O 3 =18.4, Si0 2 =64.7 ; Monoclinic; Type, Digonnl 

 Equatorial; a : b : c = .658 -A : .555 ; p = 116 3' = 001 A 100 ; 

 100 A 110 = 30 36'; 001 A 101 = 50 17'; _OOU011 = 26 31'; 

 Common forms, c(001), b(010), m(110), x(101), y(201), n(011), 

 o(lll); Twinning common, composition face b twinning axis 

 4 (Carlsbad twins), composition face n with twinning axis _L n 



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