390 



WASHINGTON AND MERWINI NEPHELITE CRYSTALS 



Somma (Vesuvius), the only other nephelite for which crystal- 

 lographic constants have been established; and also 4 angles 

 which are fairly consistent with each other measured by Striiver^ 

 on two crystals from the Alban Hills. 



Monte Ferru 

 obs. calc. 



c(OOOl) AP(lOTl) = 44°10' 44°11' 





A q(10T2) = 62°51' 62°47' 

 A z(202l) = 25°50' 25°55' 

 c = 0.841 



Differences in c are not surprising in view of the differences 

 in composition shown later. 



Refractive indices were determined on fragments of several 

 crystals of the nephelite. e = 1.529, co = 1.532-3. From in- 

 dependent measurements on two crystals w - e = 0.0026. Com- 

 parison witti other nephelites of determined composition is made 

 in Table 1. The refracti^^e indices are considerably lower than 

 have been observed in other nephelites. A systematic investi- 



TABLE 1 



Monte Ferru 



Artificial Soda Nephelite^ 



Artificial Soda-Lime Nephelite^. 



Monte Somma^ 



Magnet Cove, Ark.^ 



1.532-3 



1.537 



1.537 



1.5421 



1.5468 



gation of the relation of composition to optical properties is 

 being undertaken. Available results are as yet too few for 

 discussion. 



A complete chemical analysis was made on a batch of 60 

 crystals weighing in all 0.1865 gram. These were dried at 110° 



2 Zeitschr. Kryst. 1 : 240. 



' Koksharov, quoted by Dana, gives 0.8389; Baumhauer gives 0.8383, Zeitschr. 

 Kryst., 18: 613. 



^ N. L. Bowen, Am. J. Sci. 33: 566. 1912. 



'Average of 4 accordant determinations. See "Rock Minerals," Iddings; 

 and Zeitschr.. Kryst., 22: 333; 53: 426. 



« See "Rock Minerals," Iddings. 



