112 Mr. D. Forbes on the Chemical Composition 



Protoxide of copper. 44* 5 6 containing Oxygen 8" 88 "1 



Lime 2-58 „ „ 072 \ 9-931 



Magnesia . . . 0-86 „ „ 0-33 1 21-78 



a 



i> 



072 



}) 



)) 



0-33 



}) 



>) 



8-14 



)i 



)) 



371 



a 



a 



11-63 



}„. 



85 



Alumina .... 17*44 

 Sesquioxide of iron. 12'17 

 Silica 22-39 



100-00 



The equivalents most approximating to the above composition 

 are 9CuO, lFe 2 3 , 2Al 2 3 + 3Si0 3 , in which it is considered 

 that the lime and magnesia present replace a portion of the prot- 

 oxide of copper ; these would give the following percentage com- 

 position for the mineral : — 



Protoxide of copper. 49-28 containing Oxygen 9'91 

 Sesquioxideofiron. 1273 „ „ 3 ' 90 \ii ka, f 21-45 



Alumina . . .16-36 „ „ 7-64J 1L 04 J 



Silica . . . . 21-63 „ „ 11-24 



10000 



As above stated, it will be seen that the amount of oxygen 

 contained in the bases is double that of the silica, also that the 

 oxygen contained in the protoxides is about equal to that of the 

 sesquioxides ; consequently we obtain the formula 



6CuO, Si0 3 + 2(Al 2 3 Fe 2 3 ), SiO 3 , 



in which the sesquioxide of iron replaces a part of the alumina, 

 and the lime and magnesia a part of the protoxide of copper. 

 This may probably represent the constitution of the mineral ; but 

 it is much to be desired that a new analysis be made of the 

 mineral, and for this purpose that the purest portions obtainable 

 be employed. 



Hydrous Silico -phosphate of Alumina and Copper. 



Under the names of cupreous phosphate of alumina and 

 turquoise, Domeyko* has described a mineral whose chemical 

 composition does not very correctly entitle it to either of these 

 denominations. The mineral has a pale turquoise-blue colour, 

 has a compact homogeneous earthy fracture, and is scratched by 

 the nail. It is found on the estate of San Lorenzo de la Ligua, 

 in the province of Aconeagua, where it forms blue veins in a 

 white decomposed felspathic rock. When heated, it gives off 

 water and changes colour, becoming of a yellowish-grey tint : it 

 is decomposed by acids. Its chemical composition is stated as 

 follows : — 



* Elementos de Mineralogia, 2nd edit. p. 425. 



