Boiven — Birefringence of Torbernite. 195 



Art. XIII. — Abnormal Birefringence of Torbenite; by 



N. L. BowEisr. 



In checking over the properties of an unknown mineral 

 it was found that they agreed in general with those given 

 for torbernite, except in the case of the optical proper- 

 ties. Torbernite, or copper uranite, is described as uni- 

 axial and negative with indices 1-592 and 1-582. It 

 should, therefore, have a reasonably high birefringence. 

 The unknown mineral had a mean index of about 1-62 

 and very weak positive birefringence, at least it appeared 

 positive in white light. At first, it was thought the 

 mineral could not be torbernite but examination of two 

 labeled torbernites in the University collection, one from 

 Cornwall and one from Spain, revealed the fact that they 

 corresponded exactly with the unknown mineral. It was 

 then found that Rinne had studied the dehydration of 

 torbernite or copper uranite and had described the series 

 of so-called " Metakupf eruranite " so obtained. One of 

 these, representing the first step in the dehydration, is 

 described as uniaxial and positive with very weak bire- 

 fringence. 1 Later Buchholz determined the nature of 

 the change represented by this first step and found it 

 corresponded with a loss of 4 molecules of water, a 

 change from 12 H 2 to 8 H 2 0. 2 He ascribed the fact 

 that most analyses of torbernite have yielded only 8 

 molecules of water to the keeping of the material in a 

 desiccator before analysis. Though Rinne gives no 

 details of the properties of the optically positive variety 

 of torbernite it would appear that the present torbernites 

 correspond entirely with his and, therefore, that the first 

 step in the loss of water may take place spontaneously on 

 exposure. The specimens here described were not dried 

 in any way, whether in a desiccator or by heating to 

 mount them in Canada balsam. It is probably to this 

 spontaneous loss that the common finding of only 8 mole- 

 cules of water is to be attributed. 



The torbernites were studied merely by crushing them 

 and examining the powder in immersion liquids under 

 the microscope. On account of the highly eminent basal 

 cleavage most of the grains so obtained represent basal 



1 Centralblatt Min., 1901, p. 623. 



2 Centralblatt Min., 1903, p. 364. 



