Moses — Mineralogical Notes. 



101 



I have been unable to identify the material with any of the 

 occurrences described by Darapsky,* and it is evidently not 

 the material described from ' Sierra Gorda,' of which G. F. H. 

 Smith statesf ." that crystals of atacamite are rarely doubly 

 terminated and it is not therefore possible to determine by 

 simple measurement whether the mineral is truly holohedral," 

 whereas the Brilladora material consists almost entirely of 

 doubly terminated crystals. The material examined by Brog- 

 gerj shows greater similarity than any other both in the pre- 

 dominating faces of the crystals and in the porous ferruginous 

 gangne and associated minute crystals of gypsum. Brogger, 

 however, describes the crystals as in general " poorly formed 

 with usually deeply striated vertical zone" and rounded edges 

 and poor terminal planes, while the specimens from Brilladora 

 Mine consist of well-formed crystals, the prism faces, though 

 somewhat curved and irregular, are not striated and with few 

 exceptions the crystals are doubly terminated. 



Figure 2 shows the usual form of crystal except that the 

 modifying prism planes are represented larger than in the 

 actual crystals, in which they are little more than knife-edge 

 width. The crystals are not tabular but are usually of approxi- 

 mately equal dimensions, parallel a and b and a little longer 

 parallel c. They range in size from 4x4:X6 mm down. Under 

 magnification the faces of the prism m (110) are seen to be 





e 





m 



b 



mi 



m 



/ 



e 



\ 



slightly curved and they yield only fair reflections. The face 

 b (010), parallel to the cleavage, is in some crystals rough and 

 bright, in other crystals smooth but dull and in others is cen- 

 trally smooth and dull but with a narrow periphery (fig. 3) of 

 bright material, as if the adjoining faces of m and e were cov- 

 ered with a thin veneer. This face b sometimes shows rec- 

 tangular pittings similar to etch figures as also shown in fig. 3. 



*Neues Jahrb. f. Min., ii, 1-18, 1889. 

 JZeitschr. f. Kryst , iii, 488, 1879. 



f Min. Mag., xii, 15, 1898. 



