MINERALOGICAL AND CHEMICAL CHARACTERS. 



21 



by the intersection of rays at 6o°. It occurred to the writer that 

 possibly at a higher temperature, muscovite, like some other minerals, 

 might present a higher grade of crystalline symmetry ; in other words 

 its percussion-figure might possibly be hexagonal instead of monocli- 

 nic in its symmetry at the temperature at which these natural figures 

 were produced. To test this idea the writer produced percussion- 

 figures on several muscovites heated to the temperature of melted lead 

 (about 30o°C), and found that the angle k Mas invariably larger when 

 produced at a high temperature than the corresponding angle obtained 

 on the same mica at ordinary temperatures. The following are the 

 figures actually obtained as averages of several determinations on each 

 sample of mica : — 



Locality. 



Angle k of percussion-figure 

 produced at 



ordinary tem- 

 peratures. 



about 300° C. 



Saidapuram, Nellore ..... 



Inikurti, Nellore 



Utukur, Nellore ...«,. 

 Koderma, Hazaribagh 



55° 

 54° 



53° 30' 

 53° 



57° 30' 

 57° 

 56" 

 56' 



In the commonest form of twinning the individuals are joined along 

 their basal planes, and the vertical faces then show re-entrant angles 

 as in fig. 4. Twinning of this type repeated on a very minute scale is 

 probably the cause of some micas behaving as monoclinic crystals to 



oP 



oo-P<5> 



*P 



Fig. 4. Muscovite crystal twinned parallel to the basal plane. 



optical tests, whilst their etch-figures indicate triclinic crystallization. 



( I' ) 



