JV. L. Bow en — The Composition of Nephelite. 53 



For a great many nephelites, then, mere expression of com- 

 position in terms of three components may be accomplished in a 

 number of different ways, but for a systematic method which 

 we may hope to apply to all possible nephelites we must use 

 the three molecules JNaAlSi0 4 , KAlSi0 4 , and jSTaAlSi 3 8 , that 

 Schaller has suggested. The artificial nephelite A enables one 

 to come to this decision. 



The ability to come to this definite decision also makes it 

 possible to predict when nephelite may be expected to be 

 4 saturated ' with silica. The presence* of orthoclase in a 

 rock may be considered as having a l silicifying ' effect on 

 nephelite, but this effect would be limited, for the resultant 

 composition could never pass above the orthoclase-soda-nephe- 

 lite line. The artificial nephelite A shows that the composi- 

 tion may lie above that line. It is, therefore, only in the 

 presence of albite itself that nephelites may be expected to be 

 4 saturated ' with silica. 



It would appear that the presence of jadeite should bring 

 about a like result, but this possibility may be disregarded in 

 nature. The presence of the jadeite molecule in dilute solu- 

 tion in a hornblende or pyroxene would not have a comparable 

 effect. 



Indeed this remark applies equally well to albite itself. If 

 the albite molecule were present as oligoclase or andesine it 

 would, theoretically, not have an equal 'silicifying' effect on 

 nephelite, for a nephelite saturated with albite would be in 

 equilibrium only with a plagioclase saturated with albite and 

 the only plagioclase saturated with albite is albite itself. On 

 the other hand, a nephelite found associated with anorthoclase 

 saturated with albite should itself be saturated with albite. 

 The conditions necessary for the saturation of nephelite with 

 albite are so unlikely to occur that it may be safely said that 

 natural nephelites are probably never saturated. 



Summary. 



1. A diagram is given which shows that the composition 

 of nephelites cannot be explained by assuming mutual solid 

 solution of any two components. It is necessary to imagine 

 solid solution among three components. 



2. Mere expression of the composition of nephelites in 

 terms of three components may be accomplished in a number 

 of different ways. 



3. From the results of synthetic work it becomes possible 

 to decide upon one of these methods as the only one applica- 



*By " presence" it is necessary to understand intimate association during 

 the process of crystallization, not mere proximity. 



