202 Walker — Crystal Symmetry of the Mica Group. 



These observations prove that in phlogopite the optical plane 

 is neither parallel to nor at right angles to the clinopinacoid 

 (so called) and compels us to regard this mineral as tri clinic, 

 which agrees with the conclusions of Wiik, founded upon the 

 asymmetric etching figures on the basal cleavage plates, as 

 shown in fig. 2 [after Wiik]. The fact that thicker cleavage 

 plates often give no divergence accounts for the common view 

 as to the position of the optical plane being parallel to the 

 clinopinacoid. The freedom from divergence in the thicker 

 plates is to be accounted for by supposing the mica to be twin- 

 ned polysynthetically in very thin lamellae with (001) as com- 

 position face, which is the commonest recognised twinning law 

 for micas, the apparent optical plane in a thick crystal plate 

 representing the average for the different crystal individuals 

 which go to form the cleavage plate. The maximum diver- 

 gence then becomes the divergence for a sheet which consists 

 of a single individual ; those peculiar etching figures, which 

 are somewhat hexagonal in outline with two small inscribed 

 equilateral triangles with their points so arranged as to consti- 

 tute a six-pointed star, would also be explained on the suppo- 

 sition that one of the triangles represents the etching figures 

 for one very thin individual, and the other triangle the figures 

 for the individual immediately below. 



Biotite. 

 It is difficult to determine the exact position of the optical 

 plane in the biotites, for the optical angle is so small that the 

 hyperbolas of the interference figure scarcely separate when 

 rotated between crossed nicols in convergent polarised light. 

 This difficulty is also increased by the much darker color of 

 the biotites and by their giving much poorer percussion figures 

 against which to measure the divergence in the absence of 

 crystal edges on the plates. A few of the many biotites ex- 

 amined gave definite results. 



Locality. Divergence. Material used. 



India. 



Hazaribagh, Bengal 9° Cleavage fragments of biotite. 



Siberia. 



Lake Baikal 5 " » « 



Sweden. 



Wermland 9° 30' Cleavage fragments of lepidomelane. 



Prussia. 



Eifel 7 Crystal of rubellan. 



These observed divergences would refer biotite to the tri clinic 

 system which agrees with the etching figures obtained by Wiik 

 reproduced in fig. 3. 



Lithia Micas. 

 Only about half a dozen specimens of lepidolite and zinn- 

 waldite were available for examination. The optical plane is 



