4 8 



INFRA-RED TRANSMISSION SPECTRA. 



A1(OH) 2 are present. 1 In the present examination there is no indica- 

 tion of a band at 3 //,, which is a characteristic of the OH radical. 







FIG. 34. Clinochlore(a); Peninite. 



TOURMALINE 



(Cut parallel to optic axis. Fig. 35.) 



These curves are due to Merritt (loc. cit.), and show the variation in 

 transmission for the ordinary ray (plotted o-o-o), for the extraordinary 

 (x-x-x), and for unpolarized light. The important absorption bands 

 are at 1.28 and 2.82 /,. The latter is to be noticed in considering the 

 effect of SiO 2 , SiO 3 , and SiO 4 groups. 



MICA; MUSCOVITE, HiKAU(SiO); BIOTITE, (H,K)i(Mg,Fe)AU(SiO),. 



(Cleavage parallel to c; curve a, muscovite; * 0.02 and 0.04 mm; curve b, 



biotite, = 0.03 mm. Fig. 35.) 



The constitution of the micas is involved in a greater or less degree 

 of uncertainty. They are all silicates of aluminium, and either K, Na, 

 Li, or of Fe and Mg. All the micas yield water upon ignition, but it is 

 uncertain whether this water of constitution is due to the presence of 



1 Clarke and Schneider : American Journ. Sci., 40, 405, 1890. 



