84 



(101) 



(021), 



(111) 



(по\ 



Д021) 



(111) 



(i2o; 



(301) 



Fig. 



31. Natural etching 

 figures on {101}. 



P(lOl} is also often pro- 

 vided with distinct and regular 

 figures. 



(Fig. 31) The form some- 

 what resembles that of those 

 found on the same face by 

 means of acids ; but they are 

 generally more lengthened, and 

 are inverted with the point 

 turned downward. 



Physical Properties. 



With regard to cleavage, hardness etc. the reader is referred 

 to the common literature. 



The specific weight, determined by the pycnometer, is 4,0065. 



The colour is shining black; some of the faces sometimes 

 show a peculiar iridescent gleam in certain directions, but not, 

 however, by far so conspicuous as in the crystals from Elba. 



Pleochroism in reflected light. In the ilvaite we find a 



phenomenon, which is surely unique among the minerals; this 



phenomenon is surface-pleochroism. By examining a face of 



a crystal through a Nicol prism , this phenomenon is seen 



rather indistinctly; a far better view is got by a prism placed 



in the objective ; when the table is turned, different colours are 



seen quite distinctly according to the following table: 



ф: the a and è-axis brownish vellow 



I 

 ф: the c-axis greenish. 



This distribution of the colours is rather extraordinary, as 

 it almost corresponds with the different colours obtained by 

 transmitted light, which are to be mentioned hereafter; before- 

 hand we should rather expect that the colours most absorbed 



