22 



HOLLAND: MICA DEPOSITS OF INDIA. 



In other cases a well formed, six-sided crystal, apparently simple, may 

 be found on examination in polarised light to be composed of two or 

 more individuals, having an irregular junction-line, but with their 

 lateral axes (determined by the position of the optic-axial plane) dis- 

 posed at angles of about 6o° to one another. Some fine examples of 

 this form of twinning have been obtained near Kangayam in the 

 Coimbatore district, Madras. One such is represented by fig. 5, which 

 recalls the intergrowths sometimes found in apparently simple hexa- 

 gonal crystals of quartz. 





Fig. 5. Six-sided crystal of Muscovite formed by the intergroisith of two 

 individuals, from near Kangayam, Coimbatore district, Madras. 



All micas are negative in the kind of double refraction which they 

 exhibit, but the angle between the optic axes varies from 70 in musco- 

 vite to about io° in phlogopite and still less in biotite ; so that the 

 character of the figure obtained by examining cleavage plates in con- 

 vergent polarised light forms a ready means of distinguishing between 

 the common members of groups I and II (p. 16). 



Muscovite is practically devoid of pleochroism, whilst in biotite 

 there is always a great contrast between the colour of rays vibrating 

 parallel to, and that of the rays vibrating across, the basal cleavage 

 ( 12 ) 



