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MANGANESE DEPOSITS Oi)- INDIA : MINERALOGY. [ PaRT 1 : 
and has its surface lustre-mottled to a certain extent owing 
to numerous small inclusions of the usual manganese-ore 
(braunite). The reflections from three of them, however, 
,, are sufficiently good for measurement. The mean of the 
values obtained by making use of the two edges thus available 
w?,s 55° 30', and then 55° 28|' for a re-determination, giving 
a mean of 55° 291'. 
Thus we see that the mean value for crystal II is 55° 27J' and for III 
is 55° 291'. Hence 55° 281', or for simplicity 55° 28', can be taken as 
the value of the angle ?«m'" for winchite. Considering the fact to be 
noticed below, that there is a great variation in the extinction angles 
measured in different individuals and even in different parts of the 
same individual, and that this variation is probably an expression of the 
variable composition of the mineral, we should expect a certain variation 
in the interfacial angles of different specimens. Hence the value for the 
angle mm"' as measured on crystal I, 55° 17', may be that correspond, 
ing to a specimen of slightly different composition. 
Hence the value of the angle >/im"' may be taken varying from 55° 
17' to 55° 28'. For the sake of comparison the values of this angle mm'" 
for the various amphiboles to which winchite may be related are given 
below. They are taken from Dana's Mineralogy : — 
Amphibole . , 55° 49' 
Richterite . . 56° T 
''• Astochite . . 56° 27' (cleavage angle). 
Under the microscope the mineral is found to exhibit the following 
Pleochroism. pleochroism : — 
ii =z jjinkisli lilac, 
b =r paler lilac, 
c = blue. 
In one case, in a thick slide, the following was the pleochroism 
observed : — 
a = amethystine- rose, 
b = pale, 
c = bright sky-blue, 
the b axis colour being the palest of the three. 
