36 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
Scolecite. Mesolite. 
,Si0 4 =Al .Si0 4 ^Al 
Al - Si0 4 3H 3 Al - Si0 4 ^H 3 
\siO 4 =0a \si0 4 =Na 2 
i I 
Oa +2H 2 Oa +H 2 
I I 
,Si0 4 =H 2 ,Si0 4 =H 2 
Al - Si0 4 =H 3 Al - Si0 4 ^H 3 
\si0 4 =5Al \si0 4 =Al 
the two species being possibly raiscible. On the other hand, the scole- 
cite formula may be halved and correlated with that of natrolite, thus : 
Natrolite. Scolecite. 
/ Si0 4 =NaH 2 / Si ° 4 >Ca 
Al-SiO,-NaH 2 Al-Si0 4 =H 2 +H 2 
\si0 4 =Al \si0 4 =Al 
Both formulae for scolecite satisfy the evidence, but between them it 
is difficult to decide. The existence of mesolite may perhaps turn the 
scale in favor of the anorthite structure. 
By the action of barium chloride solution upon an artificial natrolite, 
Leinberg* obtained a barium compound which may be the equivalent 
of scolecite. It is also near the tetragonal edingtonite in composition. 
The evidence as to the constitution of edingtonite, however, is very 
imperfect, and dehydration data are entirely wanting. It may also be 
represented as the barium equivalent of the monoclinic gismondite, the 
difference in form being due to a difference of combination on the part 
of the water. At all events, it is highly probable that edingtonite is a 
derivative of a barium feldspar. 
In the rhombohedral zeolites, gmelinite, levynite, and chabazite with 
its varieties, the ratios are approximately metasilicate, but somewhat 
variable. In this respect they resemble the feldspars, and the pseudo- 
metasilicate character is ascribable to mixtures of ortho- and tri- 
silicates, with sometimes one and sometimes the other predominating. 
Gmelmite is essentially a sodium zeolite, with comparatively little or 
no lime in its composition, and when lime is present it can be 
attributed to a hydrous anorthite with 18 molecules of water. Apart 
from the lime, gmelinite is easily interpreted as a mixture of two sili- 
cates of precisely similar chemical type; one an albite, and the other 
its orthosilicic equivalent, thus: 
/Si 3 8 ~Na 3 ,Si0 4 =Na 3 
Al— Si 3 O s =Al +9H 2 Al— 8i0 4 =Al +9H 2 
\Si 3 8 ^Al \si0 4 =Al 
* Zeitsch. Geol. Gesell., XXVIII, p. 553, 1876. 
