clarke] NORMAL ZEOLITES. 35 
Of these formulae, the last one, that of foresite, is uncertain, on 
account of great variations in the published analyses. It is therefore 
to be regarded as approximate only, and as peculiarly subject to revi- 
sion. For gismondite the composition is empirically that given by 
Dana, and the calcium is partly replaceable by potassium. Gismon- 
dite is said to give off one-third of its water at 100°, and in the formula 
that amount is represented as water of crystallization. For thomsonite 
the evidence is much fuller. Here four-sevenths of the water is 
regarded as constitutional, a proportion which is fairly in accord with 
the data by Damour and by Hersch, as cited in Dana.* In nature 
thomsonite often occurs as a derivative of elreolite, while, according to 
Doelter (loc. cit.), it yields anorthite upon fusion. In some cases thom- 
sonite, variety mesole, carries an excess of silica, which may be attrib- 
uted to an admixture of trisilicate groups; that is, to Si rj G 8 replacing 
Si() 4 . In a thomsonite derived from a plagioclase feldspar this should 
be expected, and so also should the almost invariable replacement of 
a part of the calcium by sodium. Nonsodic thomsonite exists, but is 
probably rare. A thomsonite containing Ca 2 Na 2 in place of Ca 3 would 
have the following theoretical composition : 
SiO, 37.27 
A1 2 0, 31.68 
CaO 11.59 
Na 2 6.42 
H,0 13.04 
100. 00 
}HzO (water of crystallization) 5. 59 
This agrees well with published analyses, so that the formula pro- 
posed satisfies all the conditions imposed by the evidence and also by 
the general substitution theory. 
In scolecite, Al4Ca2H 8 (Si04) 6 .2H 3 0, we find a zeolite with relations to 
natrolite on one side and to anorthite on the other. Approximately 
one-third of its water is given off at temperatures below 300°, to be 
taken up again in moist air, and on fusion, according to Doelter, it yields 
anorthite. By the slow action of soda solutions it is transformed 
into natrolite and mesolite, while natrolite, by means of calcium 
chloride, is convertible into scolecite.t Mesolite, which is intermediate 
between natrolite and scolecite, may be regarded as a mixture of the 
two, or it may be formulated as an independent compound. Written 
on the anorthite type, scolecite and mesolite become 
*Syat. Miri., Gth ed., p. G08. Compare Hersch, Der Wassergehalt der Zeolithe, Zurich, 1887. 
f Lemberg, Zeitsch. Geol. Gesell., XXVIII, p. 551, 187G. 
