336 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



This agreement with the calculated angles is as close as was ob- 

 tained on any of the crystals measured. That there should be a 

 discrepancy is not at all surprising in view of the character of the 

 faces which are rounded and do not give sharp signals. Usually the 

 faces give a band of signals a degree or more wide with a central more 

 distinct cross which was measured. Where no central more dis- 

 tinct cross was present it was necessary to take the mean between 

 the two ends of the band. Vicinal faces seem common on ilvaite 

 from other localities and this occurrence is no exception. In measur- 

 ing the prism zone of any of the crystals distinct signals are often 

 noted from narrow planes which give complex indices and differ from 

 crvstal to crystal. 



The best specimens of the Idaho ilvaite are the well terminated 

 prisms like those illustrated in Plate 8. The smallest of these is 

 the single large prismatic crystal about 6 cm. long and averaging 

 perhaps 1.5 cm. in diameter with some smaller attached crystals. 

 The second and larger specimen consists of an aggregate of such 

 crystals in roughly parallel position. Attached to or projecting 

 from these larger crystals were a number of small terminated prisms 

 and it was upon these that most of the measurements were made. 

 When received the specimens were covered with a thin film of brownish 

 limonite of the ocherous gossan in which they were embedded. 

 Scrubbing with a stiff brush removed all of the limonite and left the 

 crystals clean and lustrous. Some drusy coatings of calcite were also 

 easily removed leaving no scar upon the ilvaite. The ilvaite crystals 

 are fragile and liable to crumble if roughly handled owing to the 

 presence in them of minute irregular cracks which thoroughly traverse 

 the mineral somewhat after the fashion of glass which has been heated 

 and plunged into water. This fracturing of the crystals is probably 

 traceable to strains incident upon volume changes in the associated 

 and inclosing minerals during oxidation and hydration. 



Frequently a crystal seems to have been split longitudinally and 

 the two halves slightly displaced this discrepancy, in one case, reach- 

 ing five degrees. The fracture is parallel to 6(010), the most promi- 

 nent cleavage, and is not visible on the crystal, being located on a 

 deeply striated face. Its location is clearly shown on the projection 

 made from the measurements. 



The terminated crystals show the same forms in the prism zone as 

 do the crystals embedded in quartz, and the faces are not so good. 

 The crystals are frequently distorted or unsymmetrical, as shown in 

 Figure 101, where a form may be present as a broad face on one 

 side of the crystal and absent or represented only as a narrow line 

 on the opposite side. The results obtained in measuring the ter- 

 minal faces were unsatisfactory. The only measurement of the 

 angle r : r' over c, which was obtained, gave the value 67° 02' which 



