44 T. L. Walker — Crystalline Symmetry of Torhernite. 



4. Optical properties. — Thin basal cleavage plates when 

 viewed under the microscope between crossed nicols in con- 

 vergent polarized light give a biaxial interference figure with a 

 very small optical angle — much like those observed in many 

 biotites. Though nearly uniaxial the biaxial character is so 

 constant in all parts of the thin plates that by revolving till 

 the cross is perfect it is regularly seen that the directions of 

 the two cleavage systems and the cross hairs agree. Though 

 the optical angle is quite small the biaxial nature of torbernite 

 is plain — much more so than in the majority of biotites. But 

 even if at times the interference figures were practically uni- 

 axial, that would not be sufficient ground for retaining torber- 

 nite as a tetragonal mineral any more than the very small 

 optical angle for biotite would justify one in maintaining that 

 biotite is hexagonal. The case of biotite, pseudo-hexagonal in 

 form and with very small optical angles, at times practically 

 uniaxial and torbernite, pseudo-tetragonal in form with a very 

 small optical angle, are quite analogous. The minerals also 

 resemble one another in offering only poor crystals for gonio- 

 metric determinations and in being plainly monoclinic when 

 examined by means of corrosion figures. 



Indian Museum, Calcutta. 



