30 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
On this basis anortliite becomes the calcium equivalent of nephelite, 
which latter species is also alterable into kaolin. Again, garnets are 
known to alter into feldspars and scapolite; and, according to Brauns,* 
in the alteration of diabase, prehnite and epidote are sometimes derived 
from anorthite. These species, therefore, are all connected by numer- 
ous cross relations, all emphasizing one another and pointing to a com- 
munity of molecular type. I have already called attention to the facts 
that anorthite and meionite occur among the products of fusion of 
garnet, and these data fit in well with the others. So far the formula 
are highly suggestive, but as yet they do not indicate the mechanism 
of the reaction by which a trisilicate feldspar breaks down into kaolin, 
and they need development in that direction. 
Closely allied to the feldspar in its petrographic relations is the 
isometric mineral leucite, AlKSi 2 6 . Empirically it seems to be a 
metasilicate, and is commonly so regarded, but it may easily be con- 
ceived as a mixed salt, containing ortho- and trisilicate molecules. By 
alteration it yields orthoclase, nephelite, muscovite, and kaolin, and the 
pseudo-leucite of Magnet Cove has been shown by J. F. Williams to 
consist of orthoclase and eheolite intimately commingled. t This case 
probably represents the typical breaking up of leucite, the formation 
of kaolin or of muscovite in other instances being due to secondary 
reactions. On the other hand, C. and G. Friedel J have obtained leucite 
synthetically from muscovite as a starting point, orthoclase and nephe- 
lite being produced at the same time j and Lemberg, § in his 
experiments, has transformed leucite into sanidine, anorthite, and 
microsommite, and also into andesine. In a later paper || Lemberg 
describes the action upon various silicates of the salt Na 2 Si0 3 , 8H 2 0, at 
200° under pressure, kaolin, albite, eheolite, leucite, and analcite all 
yielding a silicate-cancrinite containing Si0 3 in place of C0 3 . These 
facts connect the several species together, but to their explanation the 
empirical expression AlK!Si 2 6 gives no clew. A formula for leucite, 
to be satisfactory, must be a multiple of this, and several such multi- 
ples fulfill the conditions of the problem. 
The isometric form of leucite suggests at once a relation with garnet 
and the sodalite group, and this can be indicated by the quadrupled 
formula Al 4 K 4 Si 8 24 . We then have, as a distinct possibility, the fol- 
lowing series of molecules: 
Garnet. Sodalite. Leucite. 
/Si0 4 ^ ) „„ Si0 4 =Na 2 Si0 4 = K 2 
ai mo- r 3 >A1-C1 / >Al-Si0 4 =Al 
Al-biu 4 - Al-Si0 4 =Na 2 Al-Si 3 8 =K 2 
\Si0 4 5EAl \ S i0 4 ;Al ^Si 3 8 -Al 
*Neues Jahrbuch, 1892, II, p. 1. 
» t Ann. Rept. Geol. Survey of Arkansas, 1890, II, p. 267 et seq. 
:Bull. Soc. Min., XIII, p. 134, 1890. 
§ Zeitsch. Geol. Gcsell., XXVIII, pp. 611-615, 1876. 
|| Zeitsch. Geol. Gesell., pp. 961-2, 1885. 
