clarke.] META- AND DIMETASILICATES. 97 
gelatinizes with hydrochloric acid, while danburite is attacked by the 
reagent only after ignition. Possibly the difference may be as follows : 
y SiO=Al /Si0 3 — B=0 
Ca< Ca< 
\SiO,=Al \Si0 3 — B=0 
danburite being a metasilicate. Other formulae are also possible, and 
more data are needed before any conclusion can be reached. The asso- 
ciations of danburite with feldspars, mica, pyroxene, etc., suggest that 
it may be a pseudometasilicate, allied in structure to the aluminous 
constituent of augite. 
Chrysocolla is probably a metasilicate, and perhaps, empirically, 
CuSi0 3 .2H 2 0. It can not be well regarded as impure dioptase,for the 
latter gelatinizes with hydrochloric acid, while chrysocolla does not. 
The species, which may be a mixture of compounds, needs careful 
investigation. 
Among the titanosilicates, the recently described neptunite* appears 
to be a meta compound, 
SiO s — R' 
/SiOs 
Ti< >R" 
^Si0 3 
Si0 3 — R' 
with R'=Na, K, and R /7 =Fe, Mn. The analogy between this formula 
and the formulas of the titanium orthosilicates is its sole justification. 
The ratios given by analysis, however, are sharply in accord with the 
proposed expression. 
The salts which are fairly ascribable to dimetasilicic acid, H 2 Si 2 5 , 
are few in number and comparatively rare. Among them : petalite, 
AlLi(Si 2 5 ) 2 , is the best known and most characteristic. Its formula, as 
here written, is closely similar to the metasilicate formula for spodu- 
mene, and the two species are commonly associated. Petalite, how- 
ever, has far the lower 'density of the two, and is therefore presumably 
composed of smaller molecules. An alteration product of petalite, 
hydrocastorite, approximates roughly to 
Al/° H)2 
\si 2 5 — H 
which requires 
SiO, 60.60 
A1 2 3 25.75 
H 3 13.65 
100. 00 
The actual hydrocastorite contains about 4.3 per cent of lime, and 
is doubtless impure. By Doeltert petalite is interpreted somewhat 
* Mink. Geol. Foren. ForliamTl., XV, p. 196. 
t Min. Pet. Mitth., 1878, I, p. 529. 
Bull. 125 7 
