Washington — Constitution of Some Salic Silicates. 557 



minerals under discussion. A brief recapitulation will suffice 

 here. Some of the feldspars and scapolites, as orthoclase, 

 albite, microcline, and marialite, are entirely unattached. Leu- 

 cite is soluble, with production of pulverulent silica, while 

 nephelite, anorthite, and many zeolites are easily soluble, 

 giving gelatinous silica. In general, resistance to the attack of 

 acid increases with the silica content, but the bases seem to 

 have little influence, leucite, nephelite, and anorthite being 

 about equally soluble. 



Polymorphism . — A very marked characteristic common to 

 the feldspars, lenads, and zeolites is the frequent dimorphism 

 or polymorphism of their molecules, though certain cases (the 

 monoclinic feldspars) may rather be attributed to polysym- 

 metry. Examples are: orthoclase and microcline; albite and 

 barbierite,* the recently discovered monoclinic soda-orthoclase ; 

 nephelite and the triclinic carnegieite ; anorthite, celsian (the 

 monoclinic barium salt), and the presumably hexagonal form 

 present in calcic nephelite ; kaliophilite (phacolitc) and the 

 artificial triclinic, isometric and tetragonal (?) forms of the 

 same molecule ;f dimorphous leucite, which is enantiomorph- 

 ously transformable at about 500° ; heulandite and epistilbite, 

 and possibly laumontite and levynite ; and dimorphous analcite. 



The existence of pseudo-leucite, an intimate mixture of 

 orthoclase and nephelite with the crystal form of leucite, is of 

 interest and importance in this connection. There are good 

 grounds for rejecting the usual view that it is a pseudomorph 

 in the ordinary sense, by which soda has partially replaced the 

 potash of an original leucite, and for considering it as repre- 

 senting an original definite and distinct, sodi-potassic, leucite- 

 like mineral, which is only stable at high temperatures and 

 which breaks down on cooling into a microscopic mixture of 

 (K,]Sra)AlSi 3 8 and Na^Al^Si^Og, crystallizing as a sodic ortho- 

 clase and nephelite. This view has been previously suggested.:}: 

 The composition of pseudo-leucite from various localities varies 

 somewhat, but the amount of nephelite is always subordinate 

 to that of sodic orthoclase. The most recently described§ cal- 

 culates out about 52*9 orthoclase, 27"8 albite, and 19'3 nephe- 

 lite, neglecting the small amounts of biotite and scapolite which 

 are present. This is approximately (f or ^ ab) 4 ne l5 which cor- 

 responds to the formula KNaAl 2 Si 6 O l4 , or a molecule each of 

 potassium and sodium leucite with one extra of silica. 



*Barbier and Piost, Bull. Soc. Chim.. vol. iii, 1908, p. 894. Gonnard, 

 Bull. Soc. Geol. Fr., vol. xxxi, 1908, p. 303. W. T. Schaller, this Journal, 

 vol. xxx, 1910, p. 358. 



fZ. Weyberg, Centralblatt Min.. 1908, p. 395. 



X C. W. Knight, this Journal, vol. xxi, 1906, p. 71. H. S. Washington, 

 Jour. Geol., vol. xv, 1907, p. 387. 



§C. W. Knight, op. cit., p. 292. 



