Penfield and Foote — Composition of llmenite. 109 



There are both crystallographic and chemical grounds for 

 accepting Mosander's formula. Hematite and artificial Ti 2 3 

 both crystallize in the rhombohedral division of the hexagonal 

 system, and the lengths of their vertical axes are respectively 

 1*359 and 1*316. llmenite, however, differs in its symmetry 

 from the foregoing, for it crystallizes in the rhombohedral- 

 tetartohedral division of the hexagonal system, and the length 

 of its vertical axis, 1*385, is not between those of hematite and 

 titanium sesquioxide, which would be expected if ilmenite were 

 an isomorphous mixture of Fe 2 3 and Ti 2 O a . Moreover, if the 

 two sesquioxides are isomorphous it would be expected that at 

 times the Ti 2 3 would be in excess of the Fe 2 3 , which has 

 never been observed, although in some cases the ratio of Fe : Ti 

 is practically 1:1. 



The presence of the protoxide magnesia in almost all of the 

 ilmenites that have been examined cannot be accounted for if 

 the mineral is assumed to be an isomorphous mixture of the 

 sesquioxides Fe 2 3 and Ti 2 3 . The quantity of magnesia is 

 usually small, less than five per cent, but Cohen* has described 

 an ilmenite from Du Toit's Pan, South Africa, occurring in 

 rounded grains which contain 12*10 per cent MgO, and Ram- 

 melsbergf a crystallized one from Layton's Farm, Warwick, 

 IS". Y., containing 13*71 per cent MgO. Groth,J in comment- 

 ing upon Rammelsberg's analysis, points out that the material 

 might have been impure. Having on hand in the Brush 

 collection some excellent crystallized specimens from this 

 locality, we thought it best to make a new analysis. The 

 material was derived from a single crystal. This was rough so 

 that accurate measurements could not be made, but the habit 

 was that of ilmenite, and_ by means of the contact goniometer 

 the forms e, 0001, r, 1011 and s, 0221 were identified. The 

 analysis (by Foote) is given below, together with that made by 

 Rammelsberg : 



I. II. Average. Ratio. Rammelsberg. Ratio. 



'727 



Si0 2 ... 



0*44 



0-31 



0*37 



Ti0 2 .. 



57*30 



57*28 



57*29 



FeO. . 



24*08 



24-23 



24-15 



MgO .. 



16-01 



15-93 



15*97 



MnO__ 



1-09 



1-12 



1*10 



Fe O - 



1*99 



1*75 



1*87 



•006 ) 

 •716 f 1ZZ 



57-71 



•721 



•335 ) 

 •399 [ -749 

 •015 ) 



26*82 



•372 



13-71 



•342 



0'90 



•013 



•012 









99*14 







4-303 





100-91 100-62 100-75 

 Specific gravity _ 4-345 



* Jahrb. Min., 1877, p. 695. f Log. cit. \ Loc. cit. 



