398 



[August 18, 



The analysis a shows an excess of neavlj^ 5 per cent, of alumina, which 

 is remarkable, as the separations in the analysis were most perfect. This 

 gahnite does not come from a corundum locality, and it is therefore im- 

 probable that any has been inclosed in it. 



In ^Ir. Keller's analysis, 2.10 per cent, of alumina are wanting to form 

 spinel, RR,Oj. 



(\ Alterations of the Gahnite from Cotopaxi. 



Even the best and purest specimens from this locality, which appear to 

 be quite fresh, show innumerable cracks, breaking them up into small 

 angular fragments. 



a. In most instances these are coated with a white earthy mineral, 

 which dissolves in strong boiling hydrochloric acid. A qualitative analy- 

 sis shows this coating to be a hydrous silicate of alumina and magnesia, 

 and it is probably the same substance which in thicker coatings, has a 

 finely fibrous structure, a white or greenish-white color and silky lustre. 

 The thickest were not over 3°"" in thickness, and were very much 

 mixed with ferric oxide, and other impurities, some of them carbonates, as 

 dilute hydrochloric acid liberates carbon dioxide. Does not exfoliate on 

 ignition. The ignited mineral is readily decomposed by sulphuric acid. 



The best material which I could obtain for analysis, although still very 

 impure, was sufficiently pure to determine the position in the system 

 where the mineral belongs. It was decomposed by sulphuric acid after 

 ignition, then the silica extracted by sodium hydrate, and separated from 

 this solution. About 6 per cent. , insoluble in sulphuric acid and sodium 

 hydrate, mostly gahnite, were deducted, and the following results 

 obtained : 



Ignition = 13.83 



SiO., = 28.08 



AlA = 18.20 



Fe^O-j =n 4.32 



Cub = 0.82 



PbO - = 1.80 



ZnO r= 1.7o 



MgO = 29.85 



98.64 

 Lead and zinc are probably present as carbonates, the ferric oxide as 

 such, if I therefore deduct these as impurities, the following composition, 

 which places this mineral near ripidolite, will probably not be far from the 

 truth. 



100.00 



