84 KELLER— NOTES ON CHILEAN COPPER MINERALS. [April 24, 



In view of the very limited amount of material available for 

 analysis, it seemed best to dispense with the water determination 

 and confine the characterization of the mineral to ascertaining the 

 specific gravity and an estimation of the base and the acids. The 

 following results were obtained : 



Specific gravity 3- 160 



L n. 



Per Cent. Per Cent. 



Sulphur trioxide 16.32 16.63 



Arsenic anhydride 2.31 2.40 



Copper oxide 68.90 68.68 



Silica 1.63 1. 18 



If we deduct the quartz from the substance taken for analysis, 

 the percentages of sulphur trioxide and oxide of copper will be 

 found to approach very nearly to those in brochantite which contains : 



Per Cent. 



Sulphur trioxide 17.7 



Cupric oxide 70.3 



Water 12.0 



and it is difficult to explain the role of the arsenic acid which is 

 equivalent to about 4.4 per cent, of sulphur trioxide. To establish 

 a definite formula for the compound it would be necessary to ascer- 

 tain by further analyses whether or not the proportion of arsenic 

 anhydride is constant, and to complete the analysis by an exact deter- 

 mination of the water. The specimens in my possession, unfor- 

 tunately, are not sufficient for this purpose. 



The specimens were collected at Copaquire, Province of Tarapaca. 



Atacamite. 

 In conclusion I desire to call attention to some magnificent speci- 

 mens of atacamite from Paposo in the Province of Antofagasta. 

 They do not show the usual slender prisms, but consist of aggre- 

 gates of fairly large crystals, closely resembling the octahedron of 

 the isometric system modified by the cube and the rhombic dodeca- 

 hedron. On closer examination, however, it would seem that they 

 are reallv combinations such as have been observed on the atacamite 



