24 HOLLAND: MICA DEPOSITS OF INDIA. 



Striated phlogopite from Waltair, Madras Presidency. 



Fig. 6. Percussion-figure with 

 hading line parallel 

 to the striation. 



Fig. y. Asterism viiih prominent ray 

 trave'/sing the striation per- 

 pendicularly' 



numerous lath-shaped or acicular inclusions in the mica crossing one 

 another at angles of about 6o°. These inclusions have been variously 

 attributed to kyanite (G. Rose), rutile (Sandberger, Lacroix), and 

 tourmaline (Rosenbusch). It is almost certain, however, that the 

 striation producing this form of diffraction is much more minute 

 than that caused by rods of recognisable inclusions. 



The larger inclusions in mica are generally in the form of very 

 thin plates lying between the cleavage sheets, though sometimes they 

 cut through the bundles obliquely. The commonest of these inclu- 

 sions are black, brown or red plates of iron-oxide, possibly magnetite, 

 forming long strips or dendritic growths, having a regular crystallo- 

 graphic disposition with regard to their host the mica. Thin films of 

 quartz, needles of black tourmaline, feathery, radiating plumes of 

 red tourmaline, stout plates of garnet and stumpy crystals of green 

 apatite have been observed in Indian muscovites. 



Intergrowtks of two varieties are quite common, especially 

 intergrowths of biotite and muscovite. The junction line between 

 the two varieties may be quite irregular, but the muscovite appears 

 to be generally, if not always, outside and surrounding the biotite. 



The hardness of mica varies between degrees 2 and 3 of Mohs' 

 scale, being distinctly harder than talc, from which it can thus 

 be readily distinguished. Mica itself varies considerably in hardness ; 



( 14 ) 



