Mr. F. Field on the Silicates of Copper from Chile. 363 



one of water, and one of oxide of copper, would require the fol- 

 lowing numbers * : — 



Oxide of copper . . . 41*23 



Silica 30-94 



Water '. 27'83 



100-00 



The mineral may thus be regarded as nearly approachiDg in 

 composition to CuO SiO 2 , 3110, and we have — 



Dioptase .... CuO SiO 2 , HO. 

 Chrysocolla ... CuO SiO 2 , 2HO. . 

 Blue silicate . . . CuO SiO 2 , 3HO. 



Black Silicate of Copper. — This mineral is of a dense black 

 colour, of compact structure, conchoidal fracture with a glassy 

 lustre, very much resembling obsidian, Before the blowpipe it 

 does not change colour, and only fuses round the edges with 

 difficulty. It gives off water when heated, and is easily attacked 

 by hydrochloric acid. This silicate is found in some few mines 

 in Chile, particularly those of the Higuera in the province of 

 Coquimbo, and always in very narrow veins, which are generally 

 found associated with the red oxide of copper, and the blue and 

 green silicates. An analysis by Domeyko gave the following 

 numbers : — 



Oxide of copper . . . 50*10 



Silica 28*20 



Water 19*10 



Oxide of iron .... 2*60 

 100*00 

 In the year 1858 I published a short account of a double 

 silicate of copper and manganese in the Chemical Gazette. The 

 mineral, which in an impure state is found in considerable abun- 

 dance, has a deep black colour, vitreous lustre, and is immedi- 

 ately decomposed by hydrochloric acid in the cold. A pure 

 specimen yielded on analysis, — 



Oxide of copper ...... 24*71 



Silica 18*90 



Water 15*52 



Oxide of iron *23 



Peroxide of manganese . . . 40*28 



99*64t 

 This mineral, as can be imagined, would prove highly valu- 



* Silica is taken here at 30-20, Si=142, 20=16. 

 t Chemical Gazette, vol. xvi. p. 105. 

 2B2 



