37 



since they do not occur in such portions of the crystals as 

 are undoubtedly fresh. 



The colour of the mineral is wax-yellow passing into light 

 greyish yellow or hair-brown. The projecting central portion 

 of the larger crystals is almost always of a light ash-grey 

 colour with a faint tinge of yellow or brown. Only in thin 

 plates is the mineral translucent, otherwise only subtranslucent. 

 The crystal planes that are not dull have a vitreous or, owing 

 to the striation, almost a silky lustre, approaching on the base 

 to adamantine. The fracture shows a greasy lustre, or some- 

 times only a waxy glimmer. 



A section of a small homogeneous crystal, oriented parallel 

 to the c-axis, shows under the microscope a straw-yellow colour 

 without any perceptible pleochroism. Even if the section is 

 very thin, the interference colours are vivid, indicating strong 

 double refraction. In sections from a crystal with the central 

 portion projecting, oriented as before, the end portions per- 

 fectly agree with sections of the small crystals. The middle 

 part is also straw-yellow faintly inclining to green. Weak pleo- 

 chroism is observable here; the ray vibrating parallel to the 

 c-axis is most strongly absorbed. A reddish brown pigment is 

 arranged in zones parallel to the base and to the traces of 

 the rhombohedral faces. These zones alternate with nearly 

 colourless ones. Sections parallel to the base through the end 

 portions of the crystal are straw-yellow and homogeneous, showing 

 a regular positive axial image with the coloured rings close 

 to one another. A similarly oriented section through the cen- 

 tral portion of the crystal, on the other hand, shows the reddish 

 brown pigment in hexagonal zones. The axial image is less 

 distinct, and the coloured rings lie less closely, though the 

 plate has the same thickness as the preceding one. 



For the determination of the indices of refraction of the 

 mineral a prism was ground with the refracting edge parallel 



