44 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
character of its hydration, other than is shown in the formula as given, 
remains to he determined. The mineral is decomposable by acids with 
difficulty. 
From the evidence given in the foregoing pages the constitutional 
and genetic connection between the normal zeolites and the feldspars 
seems to be perfectly clear, and it ought to be easily emphasized by 
investigations along petrographic lines. Data of this kind, in addition 
to those already cited, are even now available, and many alterations of 
the most pertinent kind have been observed. Thus laumontite, heu- 
landite, stilbite, and analcite alter into albite or orthoclase; laumontite 
and stilbite into analcite; chabazite into natrolite; and gismondite into 
phillipsite. So also alterations into prehnite are recorded on the part 
of laumontite, scolecite, mesolite, natrolite, and analcite; and the 
identity of chemical type seems to be almost unquestionable. From 
the formula? here developed all of these alterations become intelligible, 
and the theory of substitution from normal salts is very emphatically 
sustained. For the group of zeolites it only remains to summarize the 
expressions which have been adopted. 
For starting points the following minerals are taken : 
Nephelite Al 3 (Si0 4 ) 3 Xa 3 
Albite and orthoclase Al3(Si 3 8 ) 3 K / 3 
Anorthite Al 6 (SiO 4 ) 6 0a 3 
Trisilicic anorthite Al 6 (Si 3 8 ) 6 Ga 3 
The last compound is purely hypothetical. To these may be added 
leucite, from which analcite and faujasite are derivable, and the barium- 
strontium feldspars which give rise to brewsterite, edingtonite, and 
harmotome. Brewsterite, as the equivalent of heulandite, and harmo- 
tome, as a barium phillipsite, need not be considered further; and as 
for edingtonite, its constitution is very uncertain. The remaining zeo- 
lites are as follows, with the difference between water of constitution 
and water of crystallization disregarded, except in the few cases in 
which the distinction affects the integrity of the molecular type. The 
symbol X is used to express Si0 4 or Si 3 O y indiscriminately : 
Type AUX ?J R'z. 
Hydronephelite Al 3 (Si0 4 ) 3 Na 2 H, 3H 2 
Offretite Al 3 (Si0 4 )(Si 3 8 ) 2 E / 3 , 8H 2 
Gmelinite Al 3 (Si0 4 )(Si 3 8 ) 2 Na 3 , 9H 2 G 
Type Al 6 XfiCaz. 
Thomsonite Al 6 (Si0 4 ) 6 Ca 3 , 7H 2 
Gismondite Ai 6 (Si0 4 ) 6 Ca 3 , 12H 2 
Laumontite Al 6 (SiQ 4 ) 3 (Si 3 Q 8 ) 3 Ca 3 , 12H 2 Q 
Phillipsite Al 6 (SiO 4 ) 3 (Si 3 O 8 ) 3 0a 2 K 2 , 14H 2 
Levynite - Al 6 X 6 Ca 3 , 15H 2 
Heulandite, epistilbite Al € (Si 3 O 8 ) 6 0a 3 , 16H 2 
Stilbite Al 6 (Si 3 8 ) 6 Ca 3 , 18H 2 
Ghabazite AL3(SiO 4 ) 3 (Si 3 O 8 ) 3 0a 3 , 18H 2 Q 
