448 Lindgren and Hillebrand — Minerals from A 



rizona. 



Art. XLY1II. — Minerals from the Clifton-Morenci District, 

 Arizona /* by W. Lindgren and W. F. Hillebrand. 



In 1902 an examination was made of the Clifton-Morenci 

 copper district in Arizona. Study of the collections proved the 

 presence of several interesting minerals, a brief account of which 

 is here given. The copper deposits at Clifton and Morenci 

 consist in part of irregular or tabular bodies of oxidized ores 

 in Paleozoic limestones, partly of chalcocite ores connected with 

 fissure veins in a granite porphyry or in the same limestones. 



Coronadite. — On the dump of a small shaft on the west end 

 of the Coronado vein, three-fourths of a mile west of Horse- 

 shoe shaft, fairly large amounts of a dark metallic mineral were 

 found intimately intergrown with quartz and decomposing into 

 limonite. The vein at this end shows no copper minerals but is 

 stated to contain some gold and its surface ores are reported to 

 have been worked in an arrastre in the early days of the camp. 

 In color this mineral is black and its structure delicately fibrous. 

 The hardness is about 4 and the streak black with brownish, 

 tinge. 



A thin section proves it to be opaque and in reflected light 

 its fibrous and homogeneous structure is well brought out. It 

 cements angular quartz grains and its secondary nature is clearly 

 indicated. In general aspect it is not unlike psilomelane. A 

 preliminary examination showed that it contained the oxides of 

 lead and manganese ; as it did not seem to correspond to any 

 known mineral species, a separation and analysis was made. 

 The results were as follows : 



Long continued efforts to secure pure material for analysis 

 by the use of heavy solutions were not attended with success. 

 The ultimate product of specific gravity, 5*246 at 22°, yielded 

 on decomposition by hydrochloric acid a residue of from 6 to 7 

 per cent, which consisted mainly of silica, with a small amount 

 of alumina, etc. Its presence would not have mattered much 

 had it been quite indifferent to acids, but its partial solubility, 

 as shown by the varying amounts undissolved on different 

 trials and similar varying amounts of alumina and perhaps 

 other minor ingredients found in solution, renders the calcula- 

 tion of molecular ratios not altogether certain in all cases. The 

 composition as found is : 



* Published by permission of the Director of the U. S. Geol. Survey. 



