MICAS AND CHLOEITES. 
53 
foliated, all or nearly all are nionoclinic, and to eacli of the definite 
ferromagnesian micas a chlorite, higher in magnesia and water, seems 
to correspond. In former publications* I have sought to show that the 
chlorites may be interpreted as derived from magnesium orthosilicate 
in the same way that the micas are derived from the normal aluminum 
salt; but fuller consideration leads me to a more satisfactory general- 
ization, which unites both series under one set of fundamental formulae. 
Returning to the normal micas and ignoring the replacements of 
magnesium and aluminum by other equivalent bases, we have the 
following formuhie already developed: 
Biotite. 
,Si0 4 EEMgK 
Al-Si0 4 =MgH 
\siQ 4 =Al 
Phlogopite. 
.Si0 4 =MgK 
Al-Si0 4 =MgH 
\siQ 4 =MgH 
Clintonite. 
/ >Mg 
Al-O * 
Si0 4 =R' 
By admixtures of these types all of the ferromagnesian micas are 
completely explained, and for the orthochlorite series the following 
parallel expressions are similarly sufficient: 
Biotite-chlorite. 
.Si0 4 ^(MgOH) 2 H 
Al-Si0 4 H(MgOH) 2 H 
^SiO.EEAl 
Phlogopite-chlorite. 
.Si0 4 =(MgOH) 2 H 
Al-Si0 4 ~(MgOH) 2 H 
^Si0 4 =(MgOH) 2 H 
Chloritoid type. 
/ >Mg 
Al-O * 
\si0 4 =R' 
An average pennine consists of the first and second of these types 
in the ratio 1:1, and an average clinochlore has the same molecules in 
the ratio 2 : 1. From these ratios the following composition is deduced: 
SiO,. 
A1,0 : 
MgO 
H,0, 
Pennine. 
34.35 
14.60 
38.17 
12.88 
100. 00 
Clinochlore. 
34.97 
16.51 
36.27 
12. 25 
100. 00 
In both minerals, as among the corresponding micas, admixtures of 
molecules of the third type occur, giving a range of variation which 
is shown in the published analyses. Thus, for leuchtenbergite, we 
have, with the molecules given above, a large admixture of the 
compound 
°>Mg 
Al-O 
Si0 4 v (MgOH)H 2 
* Bull. U. S. Geol. Survey, No. 78, p. 22 ; Bull. No. 90, p. 17 ; Bull. No. 113, pp. 11 and 27. 
