T. L. Walker — Crystalline Symmetry of Toroernite. 43 



directions are equivalent and the cleavages should be equally 

 perfect if the mineral be tetragonal. This however is not the 

 case. If we regard torbernite as rhombic or monoclinic this 

 difference of cleavage would be quite regular. 



3. Evidence from Etching Figures. — Thin cleavage plates 

 parallel to (001) were treated for fifteen seconds with hot 5 per 

 cent nitric acid. The operation is best performed in a porce- 

 lain dish using about 5 CC of the dilute acid, the action being 

 terminated by adding 250 cc of cold water when the desired 

 corrosion period is ended. After drying, the cleavage frag- 

 ments were examined under the microscope and showed mono- 

 symmetric etching figures in abundance (fig. 1). These figures 

 are symmetrical about one plane only and that plane is parallel 

 to the better of the cleavages observed in basal cleavage frag- 

 ments. This indicates that torbernite is monoclinic and that 

 the very perfect so-called basal cleavage and the less perfect of 

 the other two cleavages lie in the zone of the ortho- diagonal, 

 while the third (second best) cleavage is clinopinacoidal. It 

 must be remarked that this orientation does not agree with 

 that adopted by Brezina for autuuite, for he finds the best or 

 basal cleavage so-called to be clino-pinacoidal while the etching 

 figures indicate that in the case of torbernite this cleavage lies 

 in the orthodiagonal zone. 



010 



no 



010 



100 



Fig. 1. Basal section of torbernite showing- etching figures produced by 5 per 

 cent hot nitric acid by 15 seconds corrosion. Figures much enlarged. 



The progress of corrosion tends to obliterate the monoclinic 

 character of the etching figures, though they never lose the 

 habit completely. It may be stated as a general rule that cor- 

 rosion figures become less plain with the progress of the reac- 



tion after a certain 



stage 



has been reached, and the chief 



difficulty in preparing etching figures for microscopic examina- 

 tion is to interrupt the reaction just when that stage of best 

 definition is reached. 



