CLAKKE.] 
MICAS AND CHLORITES. 
49 
probably occurs, commingled with others of the ordinary lepidolite and 
muscovite types. In short, all the lithia micas contain compounds of 
the type 
Al— F 
\si 3 8 =R'3 
with members of the normal mica series.* The fl uoriferous phlogopites 
may contain molecules of the same character, especially AlF 2 .Si 3 8 .MgK, 
or they may be of the normal type with univalent — Mg—F among 
the monad bases. Between these alternatives it is difficult to decide, 
and more evidence is necessary. 
By alteration lepidolite passes into cookeite, a mineral which well 
illustrates the equivalent character of hy droxyl and flu or ine. Penfi eld's 
analysis gives for cookeite a general formula, 
OH 
Al— OH 
X Si0 4 =R' 3 
and in detail a mixture of molecules, 
10Al(OH) 2 SiO 4 .Li 3 
+14Al(OH) 2 Si0 4 .H 3 
+33Al(OH) 2 Si0 4 .Al 
The identity of type between cookeite and lepidolite is perfectly clear, 
and the analogy with the clintonite micas is still more apparent. 
In a similar way the other micas alter by hydration, yielding a great 
variety of products which are known in general as vermiculites. This 
is especially true as regards the ferromagnesian micas, which lose alka- 
lies and take up water with the greatest ease, in accordance with what 
seems to be a well-defined law. Thus we have 
Biotite. 
.Si0 4 =MgK 
Al— Si0 4 =MgH 
Hydrobiotite. 
.Si0 4 =MgH 
Al— Si0 4 =MgH +3H 2 
\si0 4 =Al 
\si0 4 =Al 
Phlogopite. 
.Si0 4 HMgK 
Hydrophlogopite. 
.Si0 4 =MgH 
Al— Si0 4 =MgH 
^SiO.EIMgH 
Al— Si0 4 =MgH +3H 2 
\si0 4 =MgH 
CUntonite. 
Al— O/ 
HydrocUntonite. 
/°)Mg 
Al— (K -f3H 2 
\Si0 4 =R' 3 
^SiO^R'a 
v For fuller discussion, see Bulletin U. S. G. S. Xo. 113, p. 22. 
Bull. 125- 4 
