Constitution of the Natural Silicates. 413 



are treated side by side with hydrochloric acid, the Edwards 

 phlogopite decomposes much more rapidly than the Burgess 

 mica. 



After very prolonged ignition the Edwards phlogopite and 

 the Port Henry iron mica were, still completely decomposable 

 by aqueous hydrochloric acid. There was, therefore, no split- 

 ting np of their molecules which could be determined by this 

 method. The Burgess phlogopite, on the other hand, showed 

 a small amount of change. After eight hours of ignition over 

 the blast, treatment with strong hydrochloric acid for three 

 days, and subsequent leaching with soda solution to remove 

 free silica, 2 - 45 per cent of insoluble residue remained. This, 

 analyzed, gave 



Si0 2 30-94 



R 2 3 _ 48-06 



MgO 19-01 



Alkalies undet. 



98-01 



This agrees quite nearly with the formula MgAl 2 Si0 6 , which is 

 the composition of a possible member of the clintonite group. 



Now from the analyses of the micas we get the subjoined 

 molecular ratios ; in which titanic oxide is thrown in with the 

 silica, the alkalies are united as potash, and only the essential 

 water, stable above 300°, is retained. 



Burgess. Edwards. Port Henry. 



Si0 2 -668 -751 -609 



R 2 3 -169 -110 -177 



RO -669 -736 -464 



K 2 . -116 -098 ' -094 



H 2 -110 -258 -187 



F -118 -018 



Hence we have the following empirical formulae : 



Burgess. 1VR 2 3 . 67RO . 12K„0 . 11H„0 . 67Si0 2 . 12F. 



Edwards 11R.O, . 74RO . lOl^O . 26H 2 . 75Si0 2 . 



Port Henry... 18R 2 3 . 46RO . 9K 2 . 19H 2 . 6lSi0 2 . 2F. 



Deducting oxygen equivalent to the fluorine we have — 



Burgess R'" s ^ 67 K 24 H 22 Si 67 269 F 12 . 



Edwards R'",.R' T4 K, H M Si„O„ i . 



Port Henry R"' 36 R' / 46 K 18 H 38 Si 61 219 F 2 . 



In all of these micas the silicon and oxygen are present in 

 almost the exact orthosilicate ratio ; but in order to discuss the 

 expressions further it is necessary to recall the theory of the 

 mica group which has already been cited. Upon that theory, 



