294 



HAROLD L. ALLING 





constants leads to the conclusion that the composition was about: 

 Mi64.3 Abas-p An3.8 Qzj.o . A chemical analysis of the same material 

 was made for the writer by H. B. Croasdale'^ with the following 

 results : 



TABLE I 



Hypoperthite, San Diego County, California 



It was found when the material was recast that it was necessary to 

 calculate an appreciable amount of "nephelite" or "carnegieite,"^ 

 as can be observed from Table II. 



Thus it has turned out to be a very different feldspar than was 

 formerly supposed. The presence of this component may well 

 account for the development of twinning and for the failure to deter- 

 mine by optical means the actual composition. Carnegieite is 

 reported to twin according to the albite and pericline laws, often 

 simultaneously. 



The heating of normal feldspars, lacking the nephelite-carne- 

 gieite component, leaves the matter of the isomerism of orthoclase- 

 microcline in doubt, although long heating at 900° C. in some 

 instances seemed to indicate a possible inversion of microcline into 

 orthoclase, and long heating of orthoclase at 700° C. did produce a 

 slight increase in the amount of twinning. 



The lack of success of these thermal studies is offset by the results 

 of plotting the values of the specific gravities of the potash-soda- 

 lime feldspars. The data employed consist of about 80 feldspars, 

 most of them natural, a few artificial. It is a regrettable fact that 



"^ Of the Fraser Laboratories, New York City. 



=> Na2Al2Si208. H. S. Washington, Jour. Geol., XVI (1908), 10; H. S. Washington 

 and F. E. Wright, Amer. Jour. Sci. (4), XXVI (1908), 187, XXIX (1910), 52-70, and 

 XXXIV (1912), 555- 



