clarke.1 ORTHOSILICATES OF DYAD BASES. 71 
probably never pure and definite, are not altogether satisfactory; but 
they indicate in a general way a transformation into the compound 
Fe'"OH 
II 
/Si0 4x 
Gl< >G1 
\Si(V 
II 
(T0 2 H 2 ) 2 
to which the alteration product very roughly approximates. 
It will be observed that all of the foregoing structural formulae in 
this group of compounds are rings or'series of rings. From them, how- 
ever, chain-like molecules are derivable, and these seem to exist in the 
cases of friedelite, pyrosmalite, and dioptase. The last named mineral, 
simply written, is 0uH 2 SiO 4 ; but it is morphologically related to the 
two other species, which have a much greater complexity of composi- 
tion. The following expressions derived from the polymer K 8 (Si0 4 ) 4 
are probably the best to represent existing evidence: 
Friedelite. 
Dioptase. 
/SiO^HzfMnCl). 
Mn< 
/Si6 4 _H 3 
Cu< 
>Si0 4 =H 2 
>Si0 4 =H 2 
Ou< 
Mn< 
>Si0 4 =H 2 
>Si0 4 =H 2 
Cu< 
Mn< 
X Si(V_:HMu. 
\SiO 4 =H0u, 
Pyrosmalite is like friedelite, but with a large part of the manganese 
replaced by iron. Possibly karyopilite may be similar, having the 
formula 
/Si0 4 £EH 2 (MnOH) 
Mn< 
>Si0 4 =H 2 
Mn< 
\Si0 4 =HMn. 
These formuhe are purely tentative and need additional support. By 
synthetic and genetic investigations they may be supported or over- 
thrown. That they sustain one another, and fit in well with the formuhe 
of the preceding species, is all that can be said in their favor. 
For serpentine, H 4 Mg 3 Si 2 9 , several formuhe are possible, and concern- 
ing them there has been much discussion. The species commonly origi- 
nates in nature from the alteration of olivine on the one hand and 
from pyroxene or amphibole on the other, and it is therefore conceivable 
that it may include two or more isomeric compounds. In favor of this 
supposition there is no direct evidence, but still it should not be left 
entirely out of account. 
By some authorities serpentine is regarded as an orthosilicate, and 
by others as a salt of the acid H (i Si 2 () 7 . On the latter supposition it 
