55 



is partly found in the values of the angle (111): (111), which, 

 on account of their great variations, have not been entered in 

 the table. Here the values are grouped round three angles, 

 according as the faces of one or both sides belong to the vi- 

 cinal form, and only the largest one of the three angles agrees 

 fairly well with the calculated value ; the feature, however, is 

 here far from being so conspicuous. 



The planes of the prismatic zone are still less well deve- 

 loped ; here numerous vicinal faces are found at each separate 

 principal face, and as the zone moreover commonly is some- 

 what striated, it is quite impossible to determine the nature of 

 the separate faces. 



it has only still to be added that in a few especially regu- 

 larly developed crystals the whole complex of vicinal faces 

 seems to agree remarkably well with monoclinic crystallization, 

 so that the single rhombic crystal of ilvaite in these cases 

 must be interpreted as twins after the monoclinic {lOO} (== the 

 rhombic {oio}); the c-axis would be common for both systems; 

 the monoclinic è-axis would be identical with the rhombic a-axis, 

 while the monoclinic a-axis would form an angle with the c-axis 

 of about 89° 43', and be inclined towards the twin-individual. 

 As will be seen hereafter, as well the etching figures as the 

 optical behaviour indicate decidedly that the ilvaite is really 

 rhombic. Nevertheless it might very well be the fact that each 

 separate system of one set of vicinal faces formed a mono- 

 clinic whole ; but I have not been able to prove such a fact 

 with sufficient certainty by any of the crystals measured. This 

 fact, however, is one reason more for not trying to give the 

 vicinal faces any indices. 



The nature of the faces and the edges. In all the crystals 

 of ilvaite of this type the faces are remarkable for being espe- 

 cially bright and shining. The only exception from this rule 

 is made by the face e(02l}, the larger part of which is quite 

 dull, and covered with microscopic, quite close-set furrows and 



