494 



MINERALOGY 



but by double decomposition and precipitation from solution of 

 materials furnished by magnetite and feldspars or amphibole as 

 an illustration. 



Owing to its iron content, biotite is much easier decomposed by 

 weathering and oxidation than muscovite, the first step in which 

 seems to be the absorption of water, or hydration, and loss of alkali, 

 forming hydrobiotite or vermiculite, H 2 Mg 2 Al 2 (SiO 4 ) 3 . 3 H 2 O. The 

 second step is the formation of chlorite, H 2 (MgOH) 4 Al 2 (Si0 4 )3 by 

 further hydration, in Which the water becomes more firmly attached 

 to the molecule. When the biotite is rich in iron, magnetite is a 



FIG. 509. Biotite and Muscovite in a Section of Augite-syenite, between Crossed 

 Nicols. The Dark Areas are Dark Brown Biotite, Parallel to the Cleavage. The 

 Fine Striations are Traces of the Micaceous Cleavage. 



product. A large number of minerals have been described as 

 derived from biotite, as epidote, serpentine, kaolinite, gibbsite, 

 etc. 



Biotite when fused loses its water and breaks down, forming 

 olivine, leucite, spinel, and glass. Its synthesis is as in muscovite. 



PHLOGOPITE 



Phlogopite. Magnesian mica, H 2 KMg 3 Al(SiO 4 )3 ; K 2 O = 

 8.40, MgO = 28.90, A1 2 O 3 = 10.87, SiO a = 44,81, 'H 2 = 5.42, 

 with a little iron, sodium, and fluorine; Monoclinic; Type, 



