l6 HOLLAND: MICA DEPOSITS OF INDIA. 



II.-MINERALOGICAL AND CHEMICAL 

 CHARACTERS. 



The minerals of the mica group, though differing considerably in 

 chemical composition and though exhibiting variations in certain 

 physical properties, possess one common striking characteristic— a 

 highly perfect, basal cleavage, by which the crystals can be split into 

 the thinnest films. 



The acute bisectrix of the optic axes being very nearly at right 

 angles to the basal plane of the mica-crystal, these lamellae, so easily 

 obtained, form a convenient means for classifying the different varieties 

 of mica according to their optical properties. 1 Two groups are distin- 

 guished according to whether the optic axial plane is approximately 

 parallel, or approximately at right angles, to the equivalent of the 

 plane of symmetry- These are as follows : — 



Group I. Group II. 



{Optic axial plane perpendicular {Optic axial plane parallel to 

 to the plane of symmetry y* the plane of symmetry ) % 



Muscovite. Phlogopite. 



Lepidolite. Lepidomelane. 



Paragonite. Zinnwaldite. 



Margarite. Biotite (Meroxene 



Anomite (a variety of variety.) 3 



Biotite.) 3 



1 As stated in the introduction, this paper is primarily intended for the use of 

 those in India interested in the mica industry. The writer's acquaintance with 

 most of these leads him to suppose that an account of the properties of the minerals, 

 as worked out by mineralogists, will not be unappreciated, and although many 

 unfamiliar terms are, for the sake of precision, necessarily employed without expla- 

 nation, they represent properties concerning the nature of which an empirical 

 knowledge at least is easily acquired. For the satisfaction of the trained mineralo- 

 gist it is necessary to state that, whilst many of the facts recorded in this chapter 



( & ) 



