12-2 /•.'. I'. Shannon — Occurrence of Ilvaite in the 



ing in view of the fact that in very few cases did the 

 faces of the crystals give single distinct signals. Much 

 more frequently a face is represented by a continuous 

 band of signals, a degree or more wide. Usually there 

 is one signal in the band which stands out more clearly 

 than the others and, in such cases, this more distinct 

 signal was the one measured. In other cases it was 

 necessary to take the mean between the two ends of a 

 series of signals. Vicinal faces seem to be a constant 

 characteristic of ilvaite and in measuring a prism zone 

 of this mineral, one seldom fails to note some faint but 

 distinct signals of this kind. If one were dealing with 

 a single crystal, it might be considered justifiable to 

 record these forms and calculate indices for them, but 

 the examination of a number of crystals leads to such a 

 confusion of unsubstantiated forms with improbable 

 indices that the attempt can not but be abandoned. The 

 following vicinal forms have been recorded on ilvaite 

 from Herbornseelbach by Bauer: 6 (13.17.0), (7.11.0), 

 (4.11.0). 



The best specimens of Idaho ilvaite consist of well- 

 terminated prisms. The smallest specimen is a large 

 prismatic crystal about two and one-half inches long and 

 averaging perhaps one-half inch in diameter, with some 

 small crystals attached to the larger prism. The other 

 specimen is an aggregate of such large prismatic crystals 

 in parallel position. Attached to or projecting from the 

 larger crystals were a number of smaller terminated prisms 

 and it is on these that most of the measurements were 

 made. "When received, these specimens were covered 

 with a thin film of ocherous limonite and looked as though 

 they had been embedded in a soft gossan of oxidized 

 material. Scrubbing with a stiff brush removed all of 

 this limonite and left the crystals clean -and lustrous. 

 Some very thin drusy coatings of calcite were also easily 

 removed, leaving no scar on the ilvaite. The crystals 

 are fragile and liable to crumble away if roughly handled, 

 owing to the presence of minute, irregular cracks which 

 thoroughly traverse the mineral, somewhat after the 

 fashion of glass which has been heated and plunged into 

 cold water. This breaking of the crystals is probably 

 traceable to strains incident upon volume changes in the 

 associated and enclosing minerals during surficial oxida- 



Bauer, Jb. Min., 1, «J1, 1890. 



