clarke.] NORMAL ZEOLITES. 39 
with the subjoined comparison between observation and theory: 
Found. 
Calculated. 
Si0 2 
A1,0 3 
CaO 
Na 2 
K 2 . 
H 2 . 
47.76 
21.91 
11.12 
1.34 
.21 
18.65 
99. 99 
46.55 
21.98 
12.07 
19.40 
100. 00 
Here, as with the preceding minerals of this group, the accordance 
is quite satisfactory. 
One more point with regard to chabazite remains to be noted. 
According to Kenngott,* a crystal of this mineral, heated to nearly 
the melting point of glass, retains its form and luster, but loses trans- 
parency. If, after cooling, such a dehydrated crystal be moistened 
with a drop of water, the latter is taken up with evolution of much 
heat. This observation may possibly imply that the constitutional 
hydroxyl of chabazite is united wholly or in part with calcium, in 
which case the formula as written by me should be correspondingly 
modified. OaOH would appear in it in place of A1H 2 2 , but the funda- 
mental type need not be thereby changed. The question thus raised 
suggests profitable lines for experimental investigation, which ought 
to shed much light upon the constitution of this and other allied 
minerals. 
The supposition that the minerals of the chabazite group may be 
partly derived from a trisilicate anorthite is strongly sustained by the 
composition of stilbite, which, most typically, is represented by the 
empirical formula Ca 3 Al 6 (Si 3 8 )fi + 18H 2 0. This requires 
Si0 2 57. 51 
ALO3 16. 29 
CaO 8. 94 
ILO 17.26 
100. 00 
which agrees with many analyses. Some varieties of stilbite show less 
silica, which may be due to the presence of orthosilicate molecules, and 
in most caSes a little soda or potash replaces lime. The latter fact rep- 
resents the presence of albite, eheolite, or orthoclase among the parent 
minerals of the zeolite, and involves no change in the structure of the 
molecule. So far as can be determined from the observations of Hersch, 
about two-ninths of the water in stilbite is retained at moderately high 
Quoted by Hersch, p. :i6. 
