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



that it contained a large proportion of magnesium sulphate. Anal- 

 yses of two different samples yielded: 



The conclusions to be drawn from these results are, first, that 

 the mineral is an isomorphous mixture of the sulphates of copper 

 and magnesium; secondly, that this double salt contains five mole- 

 cules of water of crystallization ; and thirdly, that for each molecule 

 of copper sulphate there are present (very nearly) two molecules of 

 magnesium sulphate. The shortage in the water content is doubt- 

 less owing to efflorescence, and there should be credited to the mag- 

 nesia content an amount equivalent to the percentages of the oxides 

 of iron and manganese. 



Under the name of cupromagnesite a double sulphate of copper 

 and magnesium has been described by Scacchi. It occurs in the 

 form of green crusts on lava from the Vesuvius, and is believed to 

 be isomorphous with melanterite, containing, like the latter, seven 

 molecules of water of crystallization. I have seen no reference to 

 a mineral of the same composition as that above described. 



Associated with the chalcanthite and the double sulphate of cop- 

 per and magnesium were other little masses, dirty-white in color and 

 more or less stained with ferric oxide. They were very hard and 

 consisted almost entirely of silica, containing only trifling amounts 

 of oxide of iron and magnesia. It is puzzling to explain why these 

 masses should simulate the form of the accompanying soluble 

 sulphates. 



Brochantite( ?) Containing Arsenic Acid. 

 Very small quantities only were available of an emerald green 

 mineral which was observed partly in fine acicular crystals dissemi- 



