NO. 1867. CRYSTALLIZED VARISCITE FROM UTAH.— SCHALLER. 419 



though occasionally a rectangular one can be noted among the mass. 

 The particles still manifest themselves in the dehydrated lavender 

 crystals and show no difference, except in their orientation and 

 shape, from the rest of the crystal. 



PROPERTIES OF DEHYDRATED LAVENDER VARISCITE. 



The very remarkable color change undergone by the mineral on 

 dehydration at a relatively low temperatm'e (below 160°) also 

 markedly affects the optical properties. The most striking change 

 is in the pleochroism. The green crystals of variscite are only faintly 

 pleochroic (colorless to pale green), whereas the lavender ones are 

 strongly pleochroic with intense shades. 



The optical orientation of these lavender colored crystals could 

 not be determined, though numerous attempts were made, both on 

 loose crystals and on thin sections of the granular aggregate. The 

 birefringence of the mineral has become so low that no definite 

 effects could be obtained when tests for axial angles were made. 

 Both basal and brachypinacoidal sections seemed to show the emer- 

 gence of a bisectrix with a very large axial angle, though, as stated, 

 the results were inconclusive. It coidd, however, be easily seen 

 that the optical orientation was changed. In the green crystals, 

 viewed on the flat face, h (010), the vertical axis is a direction of 

 minimum velocity, but on the lavender crystals this same direction is 

 one of maximum velocity. Similarly on basal sections, the h axis is 

 a direction of minimum velocit}^ for the green crystals, whereas on 

 the lavender ones the h axis is a direction of maximum velocity. 



The pleochroism of the lavender crystals is strong and very striking. 

 All the colors are combinations of red and blue, and therefore rather 

 hard to describe accurately. On crystals that are not too thin the 

 colors are very intense and beautiful. The brachypinacoidal sections 

 are strongly pleochroic, whereas basal sections hardly show any 

 difference in shade in different directions. The colors may be de- 

 scribed as follows: 



a axis = lavender. 



h axis = pinkish lavender. 



c axis = violet. 



The color effects given above are such as were noted on crystals 

 that bad been gradually heated up to 160° C. If the green crystals 

 be suddenly heated to a high temperature, as, for instance, by putting 

 them on a piece of platinum foil and suddenly heating them in the 

 hot flame of a Bunsen burner, the results are different. The crystals 

 are then considerably cracked and on brachypinacoidal sections the 

 optical orientation is like that of the green crystals, namely, the c axis 



