52 



The proportion of the sizes of the different forms may of 

 course be rather varying between the individual crystals, but as 

 a rule the faces are developed as seen in the figure. Of the 

 prismatic faces Л{210} may often be very narrow, and so the 

 whole crystal is compressed from the sides. i¥(llO/ is often 

 found, but always very narrow. Of the form v{340) only a 

 single instance has been found; but as it has given a very clear 

 reflex, its presence may be regarded as certain, even if a rather 

 great incongruity has taken place between the values of angles 

 really found and the calculated values. /"{O-il/ is always pre- 

 dominant over e{02l}, and is only wanting in very few instances; 

 the latter form, on the other hand, is often very inconsiderable 

 or quite wanting. t«;(80l} is always smaller than P{lOl}, and 

 may sometimes be quite wanting; only in a single piece it was 

 found in all the crystals to be considerably larger than any of 

 the other terminal faces ; but by this feature the whole appear- 

 ance is altered to so essential a degree, that these crystals 

 have been referred to a separate type II. о {ill} is in most 

 cases rather inconsiderable; it is, however, rarely quite wanting, 

 but it is also rarely found complete in number in a single 

 crystal. 



Vicinal faces. As before mentioned a very great number 

 of vicinal faces are found in the crystals of ilvaite, and they 

 impede the measurings exceedingly. Their distribution is very 

 characteristic, and may briefly be described as follows : 



All the zones passing through 6{oio}, are very abundantly 

 provided with vicinal faces that may show a deviation of up to 

 2° from the principal face; they are often larger than this latter 

 or may quite replace it. 



All other zones contain only very rarely any vicinal faces, 

 and if found they only deviate slightly from the principal face, 

 and they are always very small and only of slight importance. 



As will be mentioned hereafter, there are, however, also in 

 some of the types more rich in faces other zones with nu- 



