Washington and Wright — Feldspar from Linosa. 69 



Experiments on the melting temperature of soda anorthite 

 have been made, and also on the relation between soda 

 anorthite and nephelite, whether they are monotropic or enan- 

 tiotropic, but the results are not yet decisive, and mention of 

 them will be deferred until more definite information is at 

 hand. 



In one of the preparations crystallized at about 1100°, single 

 grains free from twinning were observed, which proved to be 

 uniaxial and optically negative, and similar to nephelite in 

 other properties, except that the refractive indices were very 

 slightly lower. Artificial nephelite has been produced by 

 several workers.* 



The effect of an impurity, or of the presence of other sub- 

 stances, on the stability of soda anorthite has not yet been 

 determined. It is, however, of interest to note that, while soda 

 anorthite crystallizes invariably out of the pure Na 2 Al 2 Si 2 8 

 melt, crystals obtained by melting down natural nephelite from 

 Magnet Cove, Arkansas, and then allowing it to crystallize, 

 were uniaxial and optically negative, and agreed in optic 

 properties with the original nephelite. Natural nephelite is 

 not the pure sodium salt, contains but notable but varying 

 amounts of potassium as ever-present and essential constitu- 

 ent, and from its melt not a trace of the triclinic form was 

 observed to crystallize out. 



It is to be hoped that eventually well-developed crystals of 

 soda anorthite will be obtained suitable for precise optic and 

 goniometric work, because then its relation to the plagioclase 

 feldspars, whether isomorphous or not, can be positively 

 determined. The fact of its notable solid solution in the Linosa 

 plagioclase, the similarity in twinning phenomena, refrac- 

 tive indices, birefringence, specific gravity, triclinic symmetry 

 relations, and also in the chemical formulas, are strong argu- 

 ments in favor of close crystallographic resemblance and 

 probable isomorphous relations between soda anorthite and 

 the plagioclase feldspars. 



In the preceding pages the name soda anorthite has been 

 applied to the triclinic phase of Na 2 Al 2 Si 2 8 , this having been 

 used previously by Lemberg and Thugutt. While this name 

 has some justification by analogy, and might serve as a pro- 

 visional designation, yet it is open to serious objections. In 

 the first place it is not in harmony with the usual nomencla- 

 ture of the feldspars, soda orthoclase, for instance, signifying 



*Fouque and Michel Levy, C. R., lxxxvii, p. 961, 1878 ; xc, p. 698, 1880 ; 

 and Bull. Soc. Min. Fi\, ii, p. 116, 1879 ; and iii, p. 118, 1880 ; Hautefeuille, 

 Ann. de l'Ecole Monn. Super., ix, 1880 : Bourgeois, Ann. Phys. Chim., 1883, 

 p. 19: Doelter, Zeitschr. Kryst., ix, p. 321, 1884; C. and G. Friedel, Bull. 

 Soc. Min. Fr., xiii, p. 129, 1890. 



