98 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
differently, being given the empirical formula Al 2 Li 2 Siio024 ; but the 
formula here adopted is the one most generally received. 
Milarite, HKCa 2 Al 2 (Si 2 5 ) 6 , is, like beryl, hexagonal j and its formula 
is analogously to be written 
/Si 2 5 
Al-Si 2 O 5 > 0a 
^Si.Os 
I 
Ca 
I 
Al— Si 2 5 — H 
\si 2 5 — K 
For this species the only evidence is that of its composition. Its gene- 
sis and its possible alterations are unknown. 
The two closely allied zeolitic minerals, mordenite and ptilolite, are 
also to be classed as dimetasilicates. Their constitution, which I have 
fully discussed elsewhere,* is easily expressed by regarding both min- 
erals as mixtures of the two molecules 
/ >Oa / 
Al-Si 2 5 Al-Si 2 5 -R' 
\si 2 5 + 6H 2 and \si 2 5 + 6H 2 
Al-Si 2 5 -H Al-Si 2 5 -H 
\si 2 5 -H \si 2 5 -H 
in which H' = Na or- K. In one-occurrence of 'ptilolite the water is lower 
than is required by these formulae, and it seems probable that a tri- 
hydrate may exist. 
The metasilicate zeolite, laubanite, is the precise equivalent of ptilo- 
lite and mordenite, and is easily interpreted thus: 
/ S, %Oa 
Al-Si0 3 
\si0 3 +6H 2 
A1-SK) 3 
\ S i0 3 >Ca 
Pilinite, a similar mineral, seems to be Al 2 (SiO 3 ) 5 0a 2 .H 2 O, a monohy- 
drate corresponding to the hexhydrated laubanite. Unlike laubanite, 
pilinite is undecomposed by hydrochloric acid ; but physically all four of 
the species here grouped together resembie one another very closely. 
The rare mineral hyalotekite, which contains, boron, agrees very 
closely with the empirical formula R'^BFSieOn, if we regard the water 
* Am. Jour. Sci., August, 1892. 
