Washington and Wright — Feldspar from Linosa. 65 



The possibility of the existence of this mineral seems to have 

 been first pointed out by Lemberg,* though he did not succeed 

 in preparing it, and he remarks on its probable instability 

 under ordinary conditions. 



Soda anorthite seems to have been actually formed by S. J. 

 Thuguttf by heating artificial " nephelite hydrate " to a white 

 heat, followed by rapid cooling. A crystalline melt was 

 obtained which contained lath-shaped sections of an apparently 

 triclinic mineral, which showed numerous polysynthetic twin- 

 ning lamellae, with extinction angles of about 36°. The pho- 

 tomicrographs in Thugutt's paper show clearly sections of this 

 twinned plagioclase-like substance. 



E. Esch^: describes a nephelite in the nephelinite from the 

 Etincle volcano in German Kamerun. This nephelite shows 

 extraordinary optic properties, and in all probability is tri- 

 clinic, the crystals being intricately twinned so as to resemble 

 an apparently simple nephelite crystal. The individuals are 

 biaxial, with small optic axial angle, optically negative, and so 

 twinned that basal sections are often divided into sextants, one 

 of which may be normal to a negative acute bisectrix, while 

 the opposite sextant is then about normal to the positive obtuse 

 bisectrix. From this behavior, combined with large extinction 

 angles, Esch considers the mineral to be triclinic. 



It may also be noted, as germane to the present subject, that 

 the corresponding potassium alumino-silicate, K 2 Al 2 Si 2 8 , has 

 been produced artificially in several modifications which differ 

 crystallographically from the natural mineral kaliophilite.§ 

 One of these, formed by Lemberg and examined by Lagorio, 

 was in aragonite-like twins, resembling those of the Etinde 

 nephelite. Another was isometric, while those formed by 

 "Weyberg were prismatic and possibly tetragonal. 



In the Geophysical Laboratory, soda anorthite was first 

 obtained in 1905 by Dr. Allen by fusing together the com- 

 ponent oxides in proper proportions. The resulting glass was 

 clear and brilliant|| but contained bubbles here and there, and 

 although it softened gradually at high temperatures it was not 

 as viscous as albite glass. The power of crystallization of 

 this snbstance from the pure melt is not so great as that of 

 many silicates, owing to its high viscosity. The glass w r as 

 crystallized by heating it to 1080°, and was examined by 



*Zeitschr. deutsch. geol. Ges., vol. xl, p. 641, 1888. 



fNeues Jahrb., Beil. Bd. ix, p. 561, 1894. 



JSitzb. Berl. Akad., vol. xviii, p. 400, 1891. 



% Cf. Z. Weyberg, Centralblatt Min. etc., p. 395, 1908. 



I For a determination of the refractive indices of this glass, the writers 

 are indebted to Mr. E. S. Larsen, of the Geophysical Laboratory. His 

 values were obtained by the minimum deviation method with a polished prism 

 of the glass. The results were : n Na = 1*5181, n u = 1*5148, n T] = 1-5208. 



Am. Jour Sci.— Fourth Series, Vol. XXIX, No. 169.— January, 1910. 

 5 



