CHAPTER III. 
THE ORTHOSILICATES OF ALUMINUM. 
I. THE NEPHELITE TYPE. 
If, in accordance with the ideas developed in the preceding chapter, 
we start out from the normal salt Al 4 (SiG 4 ) 3 , the first and simplest 
replacement possible is that of a single aluminum atom by three 
monads, giving a compound of the general formula Al^SiO^R^. This 
formula represents several well-known minerals, and I propose to des- 
ignate it as the nepheiite type. At first sight it seems to be reducible 
to the simpler expression R'AlSiC^, but that expression, as will be seen 
later, does not indicate all the known relations of the group. 
The first three representatives of this type are as follows : 
Eucryptite Al 3 (Si0 4 ) 3 Li 3 
Nepheiite Al 3 (Si0 4 ) 3 Na 3 
Kaliophilite Al 3 (Si0 4 ) 3 K 3 
These species are all hexagonal, are nearly equal in density, and all 
gelatinize wich hydrochloric acid. The second and typical member of 
the series has been made synthetically, and is then found to have the 
composition indicated by the formula. The natural nepheiite, however, 
has a composition which is more exactly represented by the complex 
formula R / 8 Al li Si 9 3 4, in which a little potassium appears among the 
components of R', while there is an excess of silica over the amount 
required by theory. The potassium is doubtless due to an isomorphous 
admixture of kaliophilite, and the excess of silica can be explained by 
the presence of a salt isomeric with albite and having the composition 
Al 3 (Si 3 8 ) 3 Na 3 . This replacement of Si0 4 by Si 3 G 8 appears to be com- 
mon among the silicates, and its recognition clears up many discrep- 
ancies. In this case one molecule of the trisilicate commingled with 
fifteen of the ortho-salt will produce the divergence from normal com- 
position shown in the analyses of natural nepheiite. 
By direct alteration nepheiite passes easily into two zeolitic min- 
erals, having the subjoined formuhe: 
Hydronephelite Al 3 (Si0 4 ) 3 Na 2 H.3H 2 
Natrolite Al 2 (Si0 4 ) 3 Na 2 H 4 
In natrolite, two atoms of aluminum have been replaced by R' 6 = 
Ka 2 H 4 , and this species, with others of like origin, falls therefore into 
another group. Still it is desirable now to call attention to the fact, 
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