70 
THE CONSTITUTION OF THE SIL T CAT3S. 
[BULL. 125. 
The three orthorhombic species, bertrandite, calamine, and ilvaite, 
may be most conveniently represented as derivatives of the bipolymer 
K 4 (Si0 4 ) 2 , like chondrodite. Crystallographically, ilvaite resembles 
humite, but bertrandite and calamine are related to each other. The 
simplest analogous formulae for the three minerals are as follows : 
Bertrandite. 
Gl 
Calamine. 
Ilvaite. 
Ca 
.SiO. .Si0 4X /SiO. 
Gl< >G1 Zn< >Zn Fe< >Fe 
>Si0 4 < \SiO/ \SiO/ 
H/ X G1— OH. || || 
(ZnOH) 2 . Al— OH. 
Calamine may also be written as a metasilicate, (ZnOH) 2 Si0 3 , with 
half of the formula indicated above, but then the analogy with ber- 
trandite disappears. The structure proposed is, therefore, preferable, 
at least until more evidence has been accumulated. 
To the datolite group a similar constitution is ascribable. The 
species, reduced to their simplest empirical expressions, are these : 
Datolite HCaBSiO s 
Homilite Ca 2 FeB 2 Si 2 Oi 
Euclase . . HGIAIS1O5 
Gadolinite Gl 2 FeY 2 Si 2 O 10 
By doubling the formulae of datolite and euclase, all four of the 
minerals become similar in constitution. Hydrogen, here, is evidently 
basic, and boron must play the same part as aluminum and yttrium. 
Assuming these elements to be present in the univalent groups BO, 
AlO, and YO, the subjoined formulae follow : 
Datolite. Euclase. 
H 2 H 2 
II II 
,sio 4X .SiO. 
Ca< >Oa Gl< >G1 
\SiO/ \SiO/ 
II II 
(BO) 2 (A10) 2 
Homilite. Gadolinite. 
Fe Fe 
Oa< 
/Si0 4 
x Si0 4 - 
II 
(BO) 2 
\Oi 
Gl 
x SiO/ 
II 
(YO), 
>G1 
Gadolinite alters with great ease, passing by hydration into a brown- 
ish-red earthy substance. The analyses of this substance, which is 
