16 THE CONSTITUTION OF THE SILICATES. r bull. 125. 
sustained also by the position of aluminum in the periodie classifica- 
tion of the elements. 
This much admitted, the general relation of which I have spoken is 
as follows : Many, perhaps all, of the orthosilicates of aluminum are 
most simply represented as substitution derivatives of the normal salt 
Al 4 (Si0 4 ) 3 . To illustrate this rule for present purposes, the following 
examples will suffice: 
Aluminum orthosilicate Al 4 (SiQ 4 ) 3 
Eucryptite Al 3 (Si0 4 ) 3 Li 3 
Nephelite Al 3 (Si0 4 ) 3 Na 3 
Kaliophilite Al 3 (Si0 4 ) 3 K 3 
Muscovite Al 3 (Si0 4 ) 3 KH 2 
Paragonite Al 3 (Si0 4 ). J NaH 2 
Andalusite Al 3 (Si0 4 ) 3 (AK)) 3 
Topaz Al 3 (Si0 4 ) 3 (AlF 2 ) 3 
Katrolite Al 2 (Si0 4 ) 3 Na 2 H 4 
Biotite Al 2 (Si0 4 ) 3 MgKH 
Grossularite Al 2 (SiO 4 ) 3 0a 3 
Prehnite , Al 2 (SiO 4 ) 3 0a 2 H 2 
Phlogopite Al(Si0 4 ) 3 Mg 3 KH 2 
These formulae express not only the composition of the minerals, but 
also many facts concerning their relations, such as their association, their 
alteration one into another, and so on. Thus, topaz and andalusite are 
crystallographically akin; both minerals, as well as others in the series, 
alter easily into muscovite, and these facts become intelligible in the 
light of the formulae given. In the use of the formulae, however, one 
possible misconception must be avoided. They express a relationship 
of constitution, but do not imply that Xature first generated the normal 
salt and then actually developed the other compounds from it. To 
emphasize this point an analogy may be drawn from organic chemistry. 
Alizarin, derived constitutionally from anthracene, was originally 
obtained from a glucoside contained in madder root. But nobody sup- 
poses that the madder plant took anthracene as a starting point from 
which to produce the dye. The constitutional or structural derivation 
is one thing; the natural origin is quite another. 
Whether aluminum orthosilicate as such exists in nature is still 
a matter of doubt. The rare mineral xenolite has the composition 
Al 4 (Si0 4 ) 3 , but it needs further study before its integrity as a species 
can be fully recognized. At j>resent the weight of evidence seems to 
be in its favor, but its occurrence is that of a secondary mineral. 
Among the products of primitive magmas it has not been observed. 
As regards its ultimate constitution or chemical structure, that ques- 
tion must also be left open. It can be written structurally in several 
ways ; either with each aluminum atom linked with all the Si0 4 groups 
or with only one atom so connected. In a sense this problem is analo- 
