A. F. Rogers — Delafossite. 291 



another positive rhombohedron \hohl\, and a negative rhom- 

 bohedron \ohhl\,ja.re also present. These are probably 

 u\1014-\ and 0JO112} respectively. Fig. 2 is a plan showing 

 these doubtful forms. 



Unfortunately the crystals are not suitable for goniometrical 

 measurements. Of the various forms the basal pinacoid alone 

 gives an image when mounted on the goniometer. The unit 

 rhombohedral faces are composite and give no images at all. 

 Consequently the reflections from the faces had to be relied 

 upon, and the angles of course are only approximations. The 

 average of live measurements of the angle (0001 : 1011) on one 

 crystal gave 65°, and the average of ten measurements of the 

 same angle on another crystal gave 67°. Taking the average 

 of these two averages we have 66° as an approximate value. 

 Th_e_ value of the <?-axis for this angle is 1*94. The angle (lOll : 

 0111) was also measured ; the average of five measurements is 

 76° 45'. The calculated value, assuming the fundamental angle 

 (0001 : 1011) to be 66°, for (1011 : 0111) is 75° 26 / . Several 

 contact twins with J0001} as the twin-plane were noted. 

 There is an imperfect cleavage parallel to the prism {1010}. 



Physical Properties. — The color is black; the streak, black; 

 the luster, metallic ; and the hardness, about 5£. The mineral 

 is brittle and non-magnetic, but is easily fusible and becomes 

 magnetic when heated on charcoal. The usual pyrognostic 

 tests for copper and iron were obtained. The mineral is easily 

 soluble in hydrochloric and sulphuric acids, but is soluble with 

 difficulty, if at all, in nitric acid. 



Chemical Analysis. — A chemical analysis of the pure min- 

 eral (practically free from hematite) made by Mr. G. S. Bohart 

 of the Chemistry department of Stanford University gave the 

 following results : 



No. 1 



No. a 



Average 



Ratio 



Theory 

 CuFeO 



Cu = 41-24 



41-39 



41-32 



0-652 



4201 



Fe = 37-22 



37*30 



37-26 



0-667 



36-85 



Insol. = 0-25 



0-17 



0-21 







(Hematite) 











O = 21-29 



21-14 



21-21 



1-325 



21-14 



(by difference) 











100-00 100-00 100-00 100-00 



The ratios are very closely Cu : Fe : O = 1 : 1 : 2. The slight 

 excess of iron over copper may be explained by the fact that 

 the analyzed sample contained a small amount of hematite, a 

 part of which was soluble in sulphuric acid, the acid used to 

 get the mineral in solution. The empirical formula of the 



