154 MANGANESE DEPOSITS OF INDIA: MINERALOGY. [ PaRT I: 
of the mineral as acid and basic. There is little to add to the above 
account of the extinction angles of this mineral. In those examples in 
which zoning is absent the whole mineral seems to be as a rule com- 
posed of the basic variety, and the most usual value of the angle i\ ^ c 
in these unzoned crystals is 20" to 26°, usually nearer the latter 
figure. In one case where I found an entirely acid example without 
zoning the extinction angle ('■ was 38°. 
In the first account of this mineral I said that the polarization 
^ , , , ^. colours rarely rise above the first order. But 
Double refraction. . . . , i <• • 
now, after exammmg a large number of sections, 
I find that this is not exactly true. Sometimes the colours are 
as high as green and yellow of the second order. In the zoned 
crystals the interior more basic portion exhibits somewhat higher 
colours under polarized light than the outer more acid portions. It is 
possible that this difference is due to the section of a crystal being slightly 
thinner at the edges, so that a small thinness of the adjoining mineral 
passes under it at this point. From a careful examination of the sec- 
tions, however, I do not think this is the case, but that the difference 
in birefringence is a true one. I have not been able to measure the bire- 
fringence, except in the chance sections one gets under the microscope. 
But from these, by comparing the colours exhibited with those of the 
quartz usually present in the same slide, I have deduced the values of 
the birefringence given below. In one section of a zoned individual, y - « 
worked out as 0 030 for the most basic portion, and 0'024 for the most 
acid. In another slide, the value of y-« worked out as 0'020 to 0'022, 
as deduced from several sections irrespective of their basicity. In one 
section giving straight extinction and showing the ■> and b axis colours, 
and consequently to be regarded as the basic variety, the polarization 
colour, being the same as that of quartz, indicated the value of 0'007 to 
0'008 for 3 - a. It must be noticed that sections of winchite often ex- 
hibit the polarization colour usually referred to as ultra blue, together 
with various other allied colours of curious tint not belonging to 
Newton's scale. These are particularly prominent in well-zoned 
individuals, so that it seems probable that it is due to the outer acid 
zone of a crystal compensating the double refraction of the inner basic 
zone. From the above it will be seen that the general values for 
the birefringence of this mineral may be given as follows : — 
y a = 0 020 to 0'0:jO, 
/3-a = 0 '007 to 0 '008. 
