clahke.] MICAS AND CHLORITES. 45 
Type AkXsR'^ 
Natrolite Al 2 (Si0 4 ) 3 Na 2 H 4 
Prehnite Al 2 (Si() 4 ) 3 Ca 2 H 2 
Scolecite Al 2 (Si0 4 ) 3 CaH 4 , H 2 
Type Al 4 X 6 Ca 6 . 
Mesolite Al 4 (Si0 4 ) 6 CaNa 2 H 8 ,H 2 
Foresite Al 4 (SiO 4 ) 6 CaH 10 , H 2 
Some of these formulae are conditioned by the replacements and 
alternatives cited in the previous pages and in my former paper upon 
the zeolites, and they are further modified by the character of the 
hydration. The expressions for analcite and faujasite, as based upon 
the leucite type, need not be repeated. 
The morphological characteristics of the zeolites probably depend in 
great part upon their mode of hydration, but this point needs to be 
developed. So also does the relation between zeolites and kaolin, into 
which the minerals of this group sometimes alter. Furthermore, zeolitic 
substances of indeterminate nature are believed to exist in soils and 
clays, and it is conceivable that such bodies may be intermediately 
formed during the transition from feldspar into kaolin. In studying 
the mechanism of that change this possibility ought to be considered. 
V. THE MICAS AND CHLORITES. 
On account of their wide distribution, their variety of composition, 
and their genetic relations to other species, the micas and chlorites 
form one of the most instructive and interesting families of minerals. 
Two of the micas, muscovite and biotite, have already been noted 
among the members of the first and second of the preceding groups; 
and we have seen how frequently they arise from the alteration of 
other silicates, while these in some cases have been synthetically 
derived from micaceous material. 
As regards the substitution theory, the minerals of this family are 
peculiarly suggestive, for the reason that they form a series of the most 
complete character. Thus, starting from the normal aluminum ortho- 
silicate, we have 
Normal orthosilicate Al 4 (Si0 4 ) 3 
Muscovite Al 3 (Si0 4 ) 3 KH 2 
Normal biotite Al 2 (Si0 4 ) 3 Mg 2 KH 
Normal phlogopite Al(Si0.i) 3 Mg;,KH 2 
No further substitution of the same order is possible, for the reason 
that its formation would remove the linking atom of aluminum, and 
break up the fundamental molecule. 
Muscovite, the first species in the foregoing series, occurs in nature 
as an independent mineral, and also as an alteration product of 
nephelite, eucryptite, topaz, andalusite, the feldspars, the scapolites, 
and various other natural silicates. All of ^hese alterations become 
