94 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
theory, if we represent Si 3 8 and SiC) 4 groups indiscriminately by the 
general symbol X, all of the ainphiboles are covered by the following 
typical symbols, in which R" stands for any dyad metal, and R' for K, 
Na, H, AlO, or Fe^O: 
R" 4 X 2 
JX 3X2XVP 2 
R" 2 X 2 R' 4 
or, structurally, 
R" K" R' 
\b" 
w/ \r 
x/ 
Nx/ 
II 
II 
R" 
R' 2 
II 
R' 2 
In a similar way, all the pyroxenes, except the acmite-spodumene 
group, which has the special formula discussed previously, may be 
represented as formed by mixtures of 
R" 4 X 4 R" 4 
R" 4 X 4 (E"'0) 8 
which is in accordance with the theory developed by Tschermak except 
as to the molecular magnitude of the compounds — that is, the pyroxenes 
are essentially bipolymers of the amphiboles, and the character of the 
structure is the same for both groups. The olivine minerals are repre- 
sented by similar orthosilicate formula?, R 4 (Si0 4 ) 2 being the constitu- 
tional equivalent of R 4 (Si0 4 )(Si 3 8 ). Pseudomorphs of pyroxene 
(fassaite) after monticellite, have been found at Monzoni, and are well 
known. Furthermore, Becke* has described pseudornorphs of antho- 
phyllite and actinolite after olivine, so that a connection between the 
two groups is clearly indicated. The tracing of this connection in a 
more general way would seem to offer a profitable field for investiga- 
tion. 
By the hydration of pyroxene or amphibole either serpentine or talc 
may be generated. The latter species has the composition H 2 Mg 3 Si 4 12 , 
and may be written structurally like amphibole either 
Mg H 2 
II • 
/Si0 4v /Si0 4X 
Mg< >Mg or Mg< >Mg 
II II 
H 2 Mg 
Both expressions are in accord with the fact recorded by Schueider 
and myself.* that upon the ignition of talc one-fourth of the silica is 
*Min. Pet. Mitth. (X. F.). 4, pp.355 and 450. 
t Bull. U. S. Geol. Survey Xo. 78. p. 13. 
