Chap. V] 
WIKCHITE. 
153 
With regard to the variable extinction angles and marked zoning 
exhibited by winchite I will quote what I have 
Extinction angles. ... , . . , ^ , 
said in the original account' : — 
' By polarized light it is seen that the mineral shows irregular zoning, due pro- 
bably to varying composition. As the inner zone seems to have the presumed 
manganese- ore dust developed in it more readily than the outer shell, it seems 
probable that there is a gradual change from the most basic composition inside to 
the most acid outside. This change in composition is accompanied by a change 
in position of the elasticity axes. For the basic portions of the amphibole 
the u elasticity axis is the one nearest to the vertical crystallographic axis c. 
The angle may be as low as 16°, but as one passes from the interior, most 
basic, portion to the outer, most acid, portion in a zoned individual, the axis it 
gradually swings away from the vertical crystallographic axis c till it makes an 
angle sometimes as large as 70° with c. The elasticity axis c has of course now 
rotated into proximity with the crystallographic axis c, making with it an angle 
of 20°, so that cAc = 20'. 
' -4s will be seen from the pleochroism scheme, this rotation of the elasticity 
axes is accompanied by a change from lilac to blue, as the elasticity axis nearest 
the vertical crj-stallographic axis changes from ;i to c ; consequently these 
compound crysl^als do not show even colouring, but are in irregular patches 
of blue and lUac. Though some of the ci-ystals in the microscope slide are 
almost entirely of one composition, either basic or acid, it is easy to distin- 
guish them at once by the colour corresponding to the elasticity axis nearest c 
thus' : — 
Chemical character. 
-\xis nearest c is 
Corresponding colour. 
Basic 
a 
Pinkish lilac. 
Acid 
c 
Blue. 
The assumption made above as to the chemical character of the 
different zones is of course difficult to prove or disprove. I have not 
been able to separate for analysis portions of the acid and basic varieties, 
and I doubt if this is possible. As the analysis to be given below shows 
such a small proportion of manganese in the mineral, it seems difficult to 
believe that the small differences in the amoimts of this element possible 
in the different zones of the mineral could produce such striking varia- 
tions in the extinction angles ; and that, even if this be so, the small 
proportion of manganese present could, by separating out in the oxide 
form, give rise to such considerable clouds as are referred to above. For 
convenience sake, however, I shall continue to refer to these two portions 
I Rec. Gaol. Surv. IhcI., XXXI, p. 235, (19(4). 
