42 T. L. Walker — Crystalline Symmetry of Torbernite. 



tical axis becomes the orthodiagonal and the perfect cleavage 

 basal according to the rhombic orientation becomes clino-pina- 

 coidal. 



I give below some of the reasons which lead me to conclude 

 that torbernite as well as autunite is monoclinic and that the 

 uranite minerals form an isomorphous group. 



1. Isomorphism of torbernite and autunite as described by 

 William Phillips in 1815. — One of the first writers on this 



mineral group was William Phillips, who read a paper before 

 the Geological Society of London in 1815 " On the oxyd of 

 Uranium." His paper is accompanied by three fine plates, 

 which make it clear that in his day autunite and torbernite had 

 not been separated. His specimens were principally from Tol 

 Cam and Tin Croft in Cornwall. Speaking of the color of the 

 crystals he says : " On other specimens the crystals are trans- 

 parent and of a greenish hue, whence they pass through 

 almost every shade into deep grass green ; while in others the 

 center of the crystal is yellow and the edges only are green."* 

 He is clearly describing crystals whose centers are composed of 

 the yellowish mineral autunite while the green border mineral 

 is one of the copper uranite, most probably torbernite. At 

 that time Mitscherlich had not yet stated his Law of Isomorph- 

 ism, neither does it appear that Phillips knew the essential 

 chemical difference between the yellow and green crystals. 

 His descriptions are of great value, for in those days excellent 

 specimens were to be had in abundance. He says : " There are 

 in my possession 55 specimens of oxyd of uranium from the 

 various mines in Cornwall above cited and upwards of 200 

 detached portions each having one or more well defined crys- 

 tals, "f At present very few museums possess more than a 

 dozen specimens all told. 



If autunite and torbernite be isomorphous then what 

 Brezina has established for autunite applies also to torbernite 

 and it becomes probable that both minerals are monoclinic. 



2. Cleavages of torbernite. — All the minerals of the Uranite 

 Group have one very perfect cleavage which is parallel to the 

 basal pinacoid according to the commonly accepted orientation 

 of the crystals. In addition to this, basal cleavage fragments 

 show two other fairly perfect cleavages which appear to inter- 

 sect at right angles — in torbernite said to be parallel to the 

 prism of the second order (100) and in autunite parallel to the 

 macro- and brachy-pinacoids (100) and (010). In the latter 

 mineral these two cleavage directions are plainly unequally 

 developed, which is to be expected in pinacoidal cleavages of a 

 rhombic crystal. In torbernite however the two cleavage 



* Trans. Geol. Soc, vol. iii, p. 114. 

 f Loc. cit., p. 118. 



