40 THE CONSTITUTION OF THE SILICATES. [bull, 125. 
temperatures, and may therefore be regarded as constitutional. On 
this basis the formula of the mineral becomes 
x Si 3 8 =Al 
Al— Si 3 8 ^H 2 .AlH 2 2 
\si 3 8 =Ca 
I 
Ca + 14H 2 <3 
I 
Si 3 8 =Ca 
1— Si 3 8 =H 2 .AlH 2 2 
\si 3 8 =Al 
which, in type, resembles that of chabazite, although the two species 
are crystallographically dissimilar. 
To heulandite and epistilbite, two isomeric compounds, essentially the 
same formula belongs, but with less water of crystallization. Calcu- 
lated with Al 6 (Si 3 8 ) 6 Ca 3 .16H 2 0, we have for their composition 
Si0 2 .... 58.63 
A1 2 3 16.61 
CaO 9.12 
H : *. 15.64 
100. 00 
in close agreement with analysis. Hersch's work on heulandite shows 
about three-fourths of the water, or 12 molecules, to be easily 
expelled, leaving 4 molecules, as in stilbite, constitutional. Hence 
its structural formula may be written like that of stilbite, with 12 
instead of 14 H 2 0; or, Al 4 (Si 3 8 ) 6 Ca 3 H 4 (AlH 2 2 ) 2 .12H 2 0. Brewsterite 
appears to be like lieulandite, but with barium and strontium replacing 
calcium. 
According to Doelter, stilbite and heulandite, on fusion, yield a pyrox- 
enic mineral with silica and an amorphous ground mass, and sometimes 
also anorthite. The same chemist* also effected the synthesis of heu- 
landite from anorthite by digesting the latter with freshly precipitated 
silica and carbonated water at a temperature of 200°. These data are 
but partially interpreted by the constitutional formulae here given, and 
for which alternative expressions are possible. A connection with 
anorthite is established, but the isomerism between heulandite and 
epistilbite, and the formation of a substance resembling pyroxene from 
stilbite and lieulandite, remain to be accounted for. Possibly epistilbite 
" Xeues Jalirb., 1890, I, p. 128. 
