MICAS. 83 



MUSCOVITE MICA (H 2 KAl 3 (SiO0 8 ). 

 (Curve a, fig. 68.) 



Cleavage piece, clear white; a thick piece shows a slight brownish 

 color. The micas are the only minerals examined in which the maxima 

 do not agree with those found by previous observers. From the fact 

 that quartz was examined at the same time, and no disagreement was 

 observed with other observers, the discrepancy is probably due to the 

 kind of mica used. 



Rubens and Nichols (loc. cit.) found reflection bands at 8.32 and 

 9.38 /*. 



600/0 



6 



769 

 FiG. 68. Muscovite (a); Biotite. 



!O 



II 



In the present curve there are bands at 9.2, 9.7, and 10.2 p.. The 

 absence of the 8.32 /* band was also noted by Rosenthal, 1 whose reflec- 

 tion curve does not show any sharp maxima. 



BIOTITE MICA, (H,K) 2 (Mg,Fe) 2 Al(SiOO. 

 (Curve b, fig. 68.) 



The biotite curve has maxima at 9.3, 9.6, and 9.85 p., the latter being 

 unusually sharp. The reflecting power of the micas is high in compar- 

 ison with some of the silicates. The position of the maxima is variable, 



1 Rosenthal: Ann. der Phys. (3), 68, p. 791, 1899. 



