336 A TREATISE ON METAMORPHISM. 



MICA GROUP. 

 MUSCOVITE, PARAtSONITE, BIOTITE, AND PHLOGOPTTE. 



The mica group includes the following - rock-making species: 



Muscovite: 



( H, K) AlSi0 4 . ( Normal muscovite KH 2 Al 3 Si 3 12 . ) 



Monoclinic. 



Sp. gr. 2.76-3.0. 



Paragonite: 



H 2 NaAl 3 Si 3 12 . 



Monoclinic. 



Sp. gr. 2.78-2.90. 



Biotite: 



(H,K) 2 (MgFe) 2 Al 2 Si 3 0i 2 . (Dana.) (Proportion of Mg:Fe varies widely. Normal 



biotite: KHMg 2 Al 2 Si 3 I2 . (Clarke.)) 

 Monoclinic. 

 Sp. gr. 2.7-3.1. 



Phlogopite: 



KH. 2 Mg 3 AlSi 3 12 . 



Monoclinic. 



Sp. gr. 2.78-2.85. 



MUSCOVITE. 



Muscovite, as already noted, is hydrogen-potassium-aluminum silicate. 



occurrence. — Muscovite is an abundant constituent in the plutonic rocks, 

 but is rather rare as a constituent in the volcanic rocks. It is one of the 

 most abundant constituents of the metamorphosed rocks, being a chief 

 mineral in many metamorphosed sedimentary and many metamorphosed 

 igneous rocks. As a secondary constituent, it is derived from many other 

 minerals. The more important of these are feldspar, including both ortho- 

 clase and plagioelase, nephelite, sodalite, leucite, the scapoiites, spodumene, 

 topaz, andalusite, and cyanite. It is also recorded as a pseudomorph after 

 tourmaline, garnet, beryl, and cordierite. There is little doubt also that 

 muscovite in the metamorphosed rocks is largely formed from the materials 

 of the zeolites. Some of the minerals, such as nephelite, sodalite, and 

 leucite, from which the muscovite is derived, occur only in the igneous rocks. 

 Others of them, such as the zeolites, occur only in rocks of altered or sec- 

 ondary nature. Others of the minerals from which muscovite is derived, 

 such as topaz, cyanite, and andalusite, are chiefly metamorphic constituents. 



