THE MINERALOGRAPHY OF THE FELDSPARS 



299 



small specimen received from Professor Barbier verified his optical 

 determinations, but was returned before any chemical tests could 

 be made. Dr. Olaf Andersen sent the writer a specimen labeled 

 "albite" from Kragero, Norway. This proved to be ^ peculiar 

 feldspar (see Plate II, G). The indices are: alpha, 1.527; beta, 

 1.534; gamma, 1.537. These agree well for albite, AbggAna. The 

 extinction angles, however, failed to be commensurate with the 



TABLE IV 



Second Recast of Barbierite 



values of the indices. (001): 1.4°;^ (010) 123.9°.' Thus it is quite 

 possible that this is a nephelite bearing albite of the following approx- 

 imate composition : OryAbssAuaNeg. 



Foerstner's^ investigations of the anorthoclases of the island of 

 Pantelleria took the form of measuring their physical properties at 

 various temperatures. He reached the conclusion that the potash- 

 soda feldspars constituted two series, one asymmetric and the other 

 monosym metric. "Both of these [systems] under consideration 

 show isodimorphism of the corresponding alkali silicates [KAlSijOg 

 and NaAlSi308], first emphasized by Professor P. Groth, one 

 'molecule' being in stable and the other in labile^ equilibrium. A 



' Determined by using the biquartz wedge after Wright. 



^ H. Foerstner, "tjber kiinstliche physikalische Veranderungen der Feldspate 

 von Pantelleria," Zeitschr.f. Kryst., IX (1884), 333. 



3 It seems to the writer that "metastable" is preferable to "labile." See G. H. 

 Gulliver, Metallic Alloys, 1913, 164-65. Also Miers and Isaac, Jotir. Chem. Soc, 

 LXXXIX (1906), 413; Proc. Roy. Soc, LXXIX, A (1907), 322; Phil. Trans., CCIX, 

 A (1909), 337- 



