SII.ICATKS, TITAN ATI -, ETC, 



193 



of symmetry, 010, in muscovite, paragonite, lepidolite, or the alkali 

 micas, ami some Montr-, ti-rmrd aimmites, to parallel to the plane 

 of >ymmetry in most biotites, phlo^opite, zinmvaldite, and lepi- 

 domelaiie. The former are the micas of the fii>t da--, while the 

 latter are the micas of the second class. The angle between the 



Fio. 508. Biotite Crystals from Franklin, New Jersey. 



optic axes in some cases is so near zero that biotite for a long time 

 was thought to be hexagonal, but 2V may vary from 0, to in some 

 cases nearly 60. 



Biotite is probably the most common of all the micas, occurring 

 as a primary mineral in such rocks as granite, syenite, diorites, and 

 the basaltic igneous rocks, in which latter muscovite is rare. It 

 separates from the magma early, crystallizing before the feldspars, 

 and in some cases is intergrown with other species forming crystals 

 of zonal structure, in which the central portion will be the more 

 magnesian varieties of phlogopites, while the alkali micas form the 

 outer zone. In this way lepidolite often forms the outer zone of 

 muscovite. Biotite is also of metamorphic origin, as in the gneisses 

 and schists, as well as in the sedimentary rocks. As a secondary 

 mineral it is not formed from the material furnished by the altera- 

 tion of a single mineral, as is the case of the feldspars and muscovite, 



