JV. L. Bowen — The Binary System. 571 



existence of anhydrous orthorhomic crystals of like composition 

 is to be expected. The orthorhombic mineral barsowite may 

 represent one of these anhydrous thomsonites. The best 

 analyses of that mineral give it a composition close to that of 

 anorthite, with some soda and small quantities of other oxides. 



No orthorhombic mix-crystals were encountered in the 

 present work, but it is quite possible that such a series may 

 be capable of existence at low temperatures where changes are 

 so slow as to be obtainable only with the efficient fluxes of 

 nature. 



That nephelite will hold 35 per cent anorthite in solid solu- 

 tion, whereas carnegieite, which, like anorthite, is triclinic, will 

 hold at most 5 per cent, comes rather as a surprise. That 

 miscibility in the solid state does not always follow from 

 crystallographic similarity is, however, well exemplified in a 

 number of cases. 



The very limited solubility of Na 2 Al 2 Si 2 8 in anorthite has 

 some importance in the light of the Linosa feldspar of abnor- 

 mal composition recently described by Washington and 

 Wright. 8 These writers came to the decision that the com- 

 position of the feldspar might be written Ab 8 An 10 Cg a . The 

 writer has found that solid solution of Ne in anorthite extends 

 only up to 2 per cent, while, if the explanation of the Linosa 

 feldspar offered by Washington and Wright is correct, solid 

 solution of Ne in the plagioclase Ab 8 An 10 extends to 5*5 per 

 cent. 



Carnegieite. — The mineral carnegieite has .never been 

 definitely identified in nature. Most natural nephelites prob- 

 ably crystallized as such, for, if formed by inversion from car- 

 negieite, the crystals would be unlikely to show continuous 

 orientation. 



In order that carnegieite might occur in a rock it is necessary 

 that its crystallization should have taken place at a temperature 

 above that at which the crystal form would change to that of 

 nephelite, otherwise nephelite would occur. The inversion 

 temperature 1248° applies only to the pure compound 

 NaAlSi0 4 . The presence of other substances affects this in- 

 version point only when those substances can be taken into 

 solid solution by the nephelite or carnegieite. In this work 

 the effect of anorthite has been determined, and is such that 

 crystallization would have to occur at a temperature still 

 higher than 1248° in order that carnegieite might form, the 

 maximum raising being about 100°. 



The effect of the other substances which nephelite is capable 

 of holding in solid solution is, however, still unknown, but the 

 evidence is that the aggregate effect is not sufficient to carry 

 the inversion point below the temperature at which the crystal- 



