92 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
has been named griqualandite, is very near Fe //7 HSi20 6 , the equivalent 
of acmite in general type. This last compound can be written 
H 
I 
*Si0 4 x 
Fef >F e 
X Si 3 8 ^ 
I 
H 
and so adjusted as an amphibole -hydrogen-acmite to the remainder of 
the group; but glaucophane and crocidolite are best formulated as 
follows : 
Glaucophane. Crocidolite. 
(AlO^j (Fe"'0) 8 
\\ ' .11 
/Si 3 8 \ /Si^Os. 
Mg( >Mg Fe< >Fe 
X Si 3 o/ x si 3 o/ 
II II 
Na 2 Na 2 
which makes them, as trisilicates, precisely equivalent in structure to 
the normal amphiboles. These compounds, and their corresponding 
orthosilicates, commingled with salts like tremolite or actinolite, give 
mixtures which conform in composition to the aluminous hornblendes. 
Closely allied to crocidolite and glaucophane is the recently described 
amphibole crossite;* in which the ratio of Si:0 is distinctly less than 
1:3. This species may be written as a mixture of the two molecules 
R (Fe"'0) 2 
II II 
/Si 3 8x /S^Osv 
1 R< >R + 1 R< >R 
X Si 3 o/ X Si 3 0/ 
II II 
Na-2 Na 2 
with some Al in place of Fe'", and R being=Fe, Mg, Oa. Gastaldite 
and' riebeckite are similar species, but the analyses are not perfectly 
conclusive. It is, perhaps, necessary with these minerals to assume 
the presence of acmite like molecules, riebeckite being empirically 
near 2Fe'"NaSi 2 6 +FeSi0 3 . 
Arfvedsomte, in which R" is mainly Fe, may be represented quite 
closely as a mixture of this order: 
R R 
II II 
/SiO, x .SiO. 
7 R< >R + 3 R< >R 
X Si 3 0/ X Si 3 o/ 
II II 
Na 2 (AiO) 2 
* Palache, Geol. Bulletin, Univ. of California, I, p. 181. 1894. 
