T. L. Walker — Crystalline Symmetry of Torbemite. 41 



Art. III. — The Crystalline /Symmetry of Torbemite ; by 

 T. L. Walker. 



[By permission of the Director of the Geological Survey of India.] 



The minerals of the Uranite group have the general formula 

 R(U0 2 ) 2 M 2 8 . 8H 2 . where K may be Cu, Ca or Ba and M 

 either F or As . Two of the minerals are considered to be 

 tetragonal, and the other three rhombic, though their angles 

 and axial relations are almost identical with those of the two 

 tetragonal minerals. 



Torbemite— Cu(U0 2 ) 2 P 2 M -f 8H 2 tetragonal — c = 2*9361 

 Zeunerite — Cu(UO a ) 2 As 2 B + 8H 2 " -c = 2-9125 



Autunite-Ca(U0 2 ) 2 P 2 0, + 8H 2 rhombic -9875 : 1 : 2*8517 



This similarity in chemical constitution combined with the 

 close morphological affinity indicated by the axial relation- 

 ships, leads one to ask whether the whole group may not be 

 isomorphous and belong to the same crystal system. It will be 

 generally admitted that the crystals of these minerals are sel- 

 dom suitable for accurate goniometric determinations, as is 

 evidenced by the varying results obtained by different well 

 known observers. The measurements quoted by Dana- for 

 torbemite vary in many cases from one to two degrees, very 

 considerable differences when we consider its close resemblance 

 to autunite, which is plainly not of a higher grade of symmetry 

 than rhombic. 



Brezinaf examined autunite from Johanngeorgenstadt and 

 concluded that it was monoclinic and only pseudo-rhombic. 

 He assigned the axial relationships 



d:J~:c = 3463 : 1 : 0*3525 



We may express the same proportions by the following h'gn res, 



d\b\ c = 0-9886: 2*8379: 1 



This brings out clearly the close relationship between the 

 monoclinic form calculated by Brezina for autunite and the 

 commonly accepted rhombic proportions 



d:J: c= 0*9875: 1:2-8517 



It is thus seen that Brezina orients the crystals so that the ver- 



* System of Mineralogy, 6th ed., p. 856. 

 f Zeitschr. f. Kryst, iii, p. 273. 



