32 Fool, and Bradley — Solid Solution in Minerals. 



own specimen, albite was associated with the nephelite, and 

 Mcrozewicz states that albite was present in the specimens con- 

 taining- the nephelites of analyses II and III and microcline- 

 microperthite, which would have a similar effect, in analysis 

 1Y. The ratio for silica in these four cases is 2 - 23, 2*21, 2*2] 

 and 2-21, which is as nearly constant as could be desired. In 

 analysis V, where the ratio for silica is only 2T2, the mineral 

 is stated to be exceptionally pure, with biotite crystals on the 

 outside. In VI or VII, where the ratios are 2 : 11 and 2-15, 

 sanidine was present which might have the effect of albite, 

 tending to raise the ratio to the saturation point, but in just 

 these two cases (from Vesuvius) the nephelite appears to be a 

 later growth on the sanidine and not intimately mixed with it. 

 In these cases, then, where albite or its equivalent was not 

 formed with nephelite, the ratio of silica to alumina shows that 

 the nephelite has not taken up the maximum amount of silica. 

 The most basic rock containing nephelite with which we are 

 acquainted is an iolite described by Hackman.* This rock 

 contains essentially pyroxene and nephelite with smaller 

 amounts of titanite, apatite and ivaarite. There is no albite, 

 quartz or feldspar present. The nephelite in this rock had the 

 following composition and ratios : 



Ratio 



SiO, 43-98 2-13 



Al„0 3 34-93 1-00 



CaO 0.36 



Na,0 16-76 ^ 0-94 



K.O 3-83 



99-86 



Here, again, the silica is below what we may call the 

 "saturation ratio" of 2-21. 



It would be of considerable interest if nephelites could be 

 found which closely approximated the formula !NaAlSi0 4 . 

 From what has been said above, such an occurrence could only 

 be expected where crystallization had taken place from a 

 magma so deficient in silica that albite did not form. 



In conclusion, the authors consider that the arguments 

 advanced in the present article may be applicable to other 

 minerals. Work has already been begun on the mineral 

 pyrrhotite with the hope that similar deductions may be 

 applied to this mineral. 



Chemical and Mineralogical Laboratories of the Sheffield Scientific School 

 of Yale University. 



New Haven, Conn., October, 1910. 



*Bull. de la Commis. Geol. de Finlande, 1900, p. 9. 



