MINERALOGICAL AND CHEMICAL CHARACTERS. 



27 



chemical constitution for the "micaceous" minerals like the vermicu- 

 lites, chlorites, margarite and the clintonite group. 



The four chief mica species — muscovite, lepidolite, phlogopite 

 and biotite — fall into two groups chemically as they do when classified 

 by their physical properties. The first two may be conveniently known 

 as alumino-alkaline micas, already placed together on p. 16 according 

 to their physical properties (Group I), whilst the last two may be dis- 

 tinguished as ferromagnesian micas, referred to before as Group II. 



Of the alumino-alkaline group, muscovite is characterised by its 

 high content of alumina and potash, whilst lepidolite contains lithia 

 accompanied by a decided quantity of fluorine. In the ferromagnesian 

 group, phlogopite is more essentially magnesian, whilst biotite is 

 distinctly ferriferous. The variations are too great to permit the 

 representation of a species by definite types ; but the following- 

 formulae and corresponding percentage compositions represent the 

 most frequent type of each species : — 



Muscovite. 

 2 H 2 0. K ; 0. 3 A1 2 3 . 6 SiO : 



Lepidolite. 



H 2 0. K 2 0. Li a O. 3 A1 2 3 - 6Si02 

 3 K..O. 3 Li 2 C 4 Al F,. 

 2 Al, 3 . 18 Si0 2 . 



Biotite. 



(H, K) 2 0. 2 (Mg, Fe)0. 

 (Al, Fe) 2 O,. 3 Si0 2 . 



SiU 2 . 



• • 



. 452 



Al a 3 . 



. 



. 38-5 



K 2 . 



• • 



. n-8 



H 2 . 



• • 



• 4'S 

 1000 



Si0 2 . 





• 5''3 



A1 2 3 . 





. 18-4 



A1F 3 . 





. I2 - 



K s O . 





• 13 + 



Li s O . 





. 4-2 



H 2 . 





• 07 



IOO'O 



SiO, . 



s 



• 39'9 



Al 2 3 +F 2 O 3 



. 22 - 9 



FeO . 



• • 



. i6 - o 



MgO . 



• » 



. 8-8 



K.O . 



. * 



• io"4 



H 2 . 



• ■ 



2'0 

 IOO'O 



( 17 ) 



