20 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
sible formula 1 have been tripled, for otherwise the relationships which 
exist could not be structurally shown. The correctness of this pro- 
cedure will appear still more definitely in the consideration of the 
groups which follow. 
The species sillimanite is isomeric with andalusite, but the structural 
character of the isomerism is not clear. The minerals kornerupine and 
kryptotile, which will be considered later, are possibly also representa- 
tives of the nephelite type. 
II. THE GARNET-BIOTITE TYPE. 
By this title I propose to designate the second series of derivatives 
from the normal salt Al 4 (Si0 4 ) 3 , in which two atoms of aluminum have 
been replaced. The general formula of the type, obviously, is 
Al2(Si0 4 ) 3 E / 6, and in this series bivalent elements or radicles frequently 
appear. In natrolite, R / 6 =Na 2 H 4 ; in biotite, B / 6 =R // 2 R / 2 ; and in garnet 
R' 6 are entirely replaced by R 7/ 3 . There are, therefore, three subtypes 
to consider; one in which all the replacing atoms are univalent, one 
in which all are bivalent, and one intermediate between the other two. 
Under the first subtype three species seem definitely to occur, viz: 
Natrolite Al 2 (Si0 4 ) 3 Na 2 H 4 
Dumortierite Al 2 (Si0 4 ) 3 (A10) 6 
Zunyite Al 2 (Si0 4 ) 3 B' 6 * 
Natrolite, as we have already seen, may be directly derived from 
nephelite by alteration. Dumortierite is analogous to andalusite, and, 
like the latter, is orthorhombic. In zunyite R' 6 is composed of the 
groups — A1=F 2 , — A1=G1 2 , and — Al=(OH) 2 , with possibly a little 
— Al =0, and the mineral thus seems to be chemically related to dumor- 
tierite in the same way that topaz is related to andalusite. But topaz 
and audalusite have crystallographic analogy, while zunyite, being 
isometric, is quite unlike its chemical congeners. The species evidently 
needs further study, but at present, if we unite the chlorine in it with 
the fluorine, it may be provisionally represented by the expression 
,Si0 4 =(A10 2 H 2 ) 2 .AlF 2 
Al-Si0 4 =(A10 2 H 2 ) 2 .ALF 2 
\si0 4 ; 
A\ 
This formula expresses the facts which are now available, but is not 
conclusive. Its isometric character, however, helps to connect zunyite 
with the garnet and sodalite groups, as has been shown by Brogger.f 
In the second subtype, when R' G is partly composed of bivalent and 
partly of univalent atoms, two species may be placed, thus : 
Prehnite Al 2 (SiO 4 ) 3 0a 2 H 2 
Biotite Al 2 (Si0 4 ) 3 Mg 2 HK 
* Groth's formula, generalized. t Zeit. Kryst. Min., XVIII, p. 209. 
