A r . L. Bowen — The Composition of Nephelite. 49 



Art. YIII. — The Composition of Nephelite ; by N. L. Bowen. 



The composition of natural nephelite is a much discussed 

 question. The earliest formula, expressing the composition as 

 that of a simple orthosilicate (Na,K)AlSi0 4 , is now generally 

 recognized as incorrect since analyses always show somewhat 

 higher silica than this formula requires. The formula (JN"aK) 8 

 Al 8 Si 9 34 , which is now commonly given in text-books of min- 

 eralogy, obviously attempts to account for this excess silica. 

 Several analyses of excellent material have, however, been 

 advanced by different investigators as evidence of the incor- 

 rectness of this formula. These analyses demonstrate that no 

 definite formula can be assigned to nephelite since the excess 

 of silica is variable in amount. 



Moreover, the preparation of ' artificial soda-nephelite ', 

 NaAlSi0 4 , by Doelter and others, showing crystallographic 

 and optical properties essentially those of natural nephelite, 

 demonstrated long since that this compound is the ' essential 

 constituent ' of natural nephelite. 



The writer recently prepared this artificial soda-nephelite 

 and determined its properties. The marked degree of corre- 

 spondence with the natural mineral is set forth in the follow- 

 ing table : 



Opt. 

 Cryst'n char. we G habit 



NaAlSi0 4 (artif.) hex. .. 1*537 1*533 2*619 prism and base 

 Nephelite (nat.) hex. _- 1-541 1*537 2*55-2-65 prism and base 



commonest 



Several different investigators recognized this essential rela- 

 tion and sought to explain the composition of nephelite by 

 imagining it to be a ' mixture ' of NaAlSi0 4 with a potash 

 molecule, either KAlSi 2 6 , or K 9 Al s Si,O 10 , or K 4 Al 4 Si 5 18 . 



Each of these potash compounds has a silica ratio higher 

 than that in an orthosilicate, and its presence in variable 

 amount was intended to explain the excess of silica and its 

 vaiiable nature. 



The preparation of soda nephelite by the writer, just men- 

 tioned, was accomplished by fusing together Na 2 C0 3 ,Al 2 3 and 

 Si0 2 in the proper proportions. It was, however, found that a 

 • pure ' product was obtained only with some difficulty on 

 account of the partial volatilization of soda at high tempera- 

 tures. When the 'impure' mixture first obtained was crys- 

 tallized at a low temperature the microscope revealed nephelite 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 193. —January, 1912. 

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