CLARKE.] 
DIORTHOSILICATES. 
83 
and the magnesian spadaite may be similar, with the composition 
,Si 2 7 .MgH 3 
Mg< 
>Si 2 7 .MgH 2 
• Mg< 
\Si 2 7 .MgH 3 . 
Another magnesian silicate, saponite, is perhaps normally H 4 (MgOH) 2 
Si 2 7 , although the analyses all show admixtures of some aluminous 
compound. 
Among the silicates of aluminum, salts of diorthosilicic acid are 
very rare. The only one which seems to be at all well defined is iolite, 
which agrees best with the formula 
Si 2 7 .AlMg 
Al_Si 2 7 .AlMg 
Nhwmaioh), 
Al— Si 2 7 .AlMg 
\si 2 7 .AlFe 
which requires 
SiO* 49.26 
A1 2 3 33.50 
MgO 9.85 
FeO 5.91 
H.,0 148 
100. 00 
in close concordance with the best recorded analyses. By alteration, 
iolite passes into mica, going through an intermediate stage, however, 
known as chlorophyllite. This substance may be regarded as formed 
by hydration, in which the linking group of Si 2 7 in iolite is split into 
two orthosilicic radicles, yielding two molecules of the type 
.Si0 4 E£H 2 . A10 2 H 2 
Al— Si 8 7 =AJMg 
\si 2 7 eAlMg 
from which the final transition into a mica is easy. If we take Kam- 
melsberg's analysis of chlorophyllite, recalculate the ferric oxide into 
alumina and lime into magnesia, reducing afterwards to 100 per cent, 
we get the following comparison between observed fact and the com- 
position of chlorophyllite computed from the foregoing formula : 
Si0 3 . 
A1,0 3 
Fe 2 3 
MgO. 
CaO. 
H..0 . 
Found. 
46.31 
25.17 
10.99 
10.91 
.58 
6.70 
100. 66 
Reduced. Calculated. 
47.99 
33.34 
11.74 
6. 93 
48.39 
32.90 
12.90 
5.81 
100. 00 
100. 00 
