274 Warren — Ilmenite Rocks near St. Urbain, Quebec/ 



cult to picture the exact character of the reaction which pro- 

 duced it. It is perhaps possible that it was formed by a 

 reaction between the alumina and silica of the plagioclase and 

 the spinel, some sodium and silica being eliminated and the 

 remaining feldspar crystallizing as a somewhat more basic 

 lime-soda feldspar. The optical properties of the feldspar asso- 

 ciated with the sapphirine do not indicate, however, that it 

 is more basic than that elsewhere in the ilmenite nor in the 

 enclosing anorthosite. Nor is it at all clear why the sapphirine 

 should not have formed in those portions of the ilmenite rock, 

 also containing the same plagioclase but free from rutile, these 

 having certainly undergone substantially the same metamor- 

 phism as the rutile-bearing portions. Taking everything into 

 consideration, the writer believes that the sapphirine in this 

 instance is a magmatic mineral. 



Quantitative Study of the Rutile-bearing Rocks. — Two 

 quantitative estimates (Rosival) of the mineral composition of 

 the rutile-bearing rock have been made on large, thin sections, 

 one cut from a sample which appeared richer than the average 

 in rutile, and the other from a rutile-poor sample. The results 

 were as follows : 



Rutile... 20-4 11-3 



Ilmenite-hematite 73-2 84*5 



Sapphirine _ 3*2 0*7 



Rest . 3*2 3*2 



100-0 100-0 



It is certain, as noted, that the sapphirine as judged by its 

 characteristic alteration products was originally more abund- 

 ant than at present, so that a part of the material listed as 

 "rest" represents sapphirine once present. The average of 

 the percentages for rutile is 15*8 per cent, which is probably 

 not far from the true average rutile content of the rutile-bear- 

 ing portion. 



As stated, it has not been deemed worth while to make at 

 present an exhaustive chemical analysis of the rock on account 

 of its considerable degree of alteration. A fairly complete 

 analysis* has, however, been made on a sample of the rutile- 

 rich type similar to that on which the Rosival estimate, given 

 in column I, above, was made. The results are as follows : 



* This analysis was made by Mr. R. S. Anderson in connection with a 

 thesis presented as one of the requirements for graduation in the course in 

 Mining Engineering at the Massachusetts Institute of Technology. The work 

 was done under the supervision of Professor William Hall of the Depart- 

 ment of Chemistry, and the writer. Each result is the average of at least 

 two, and in the case of FeO and Ti0 2 , of three determinations. 



