110 P en field and Foote — Composition of Ilmenite. 



In the two analyses, the ratio of R0 2 : RO is very close to 

 1 : 1, thus indicating the existence of the molecule RO. Ti0 2 , 

 where R=Fe and Mg. 



It is thus definitely proved that in this crystallized variety of 

 ilmenite there is a molecule MgO . Ti0 2 or MgTiO s , and it 

 seems most reasonable to suppose that the iron also is present 

 as FeO . Ti0 2 , isomorphous with MgO . Ti0 2 , and not as an 

 isomorphous mixture of Fe 2 3 and Ti 2 3 . It cannot, however, 

 be told by chemical means that all of the titanium exists as a 

 tetravalent element, for on dissolving the mineral for analysis, 

 Ti 2 3 if present would oxidize to Ti0 2 at the expense of Fe 2 O s 

 and the analysis would show an equivalent of FeO . (Fe 2 3 -f- 

 Ti a O,=2TiO a + 2FeO). 



In the published analyses of ilmenite, where the ratio of 

 Ti0 2 : E.0 is very constantly 1 : 1, there is almost without excep- 

 tion an excess of Fe 2 3 , amounting in some cases to a large per 

 cent, as may be seen by examining the list of analyses in Dana's 

 System of Mineralogy, p. 218, and, as Ham berg has pointed 

 out, it is reasonable to suppose that the hematite molecule 

 Fe 2 3 or FeFe0 3 is capable of mixing with the ilmenite mole- 

 cules FeTi0 3 and MgTi0 3 , just as CaC0 3 and NaN0 3 are 

 practically isomorphous. 



Laboratory of Mineralogy and Petrography, 



Sheffield Scientific School, New Haven, Conn., April, 1897. 



