200 Washington — Kaersutite from Linosa and Greenland. 



FeO might solve the problem. If under such conditions the 

 cupric salt would oxidize the Ti 2 3 without acting on the FeO, 

 titration with permanganate of two portions brought into 

 solution both with and without the addition of CuS0 4 , 

 together with the determination of total iron as Fe 2 3 and of 

 total titanium as Ti0 2 by the usual methods, would furnish all 

 the data needed. The discovery of some such method capable 

 of vieldin^ accurate results under the conditions of silicate 

 analysis is now one of the most important desiderata. 



In this connection a peculiarity in the relations of the oxides 

 of iron and titanium may be pointed out. In whole numbers 

 the molecular weights are as follows : Fe 2 3 = 160, FeO = 72 

 (2FeO = 141), Ti0 2 = 80 (2Ti0 2 = 160), Ti 2 3 = 111. That 

 is, neglecting the refinement of decimals, a molecule of ferric 

 oxide is equal to two of titanium dioxide, and one of titanium 

 sesquioxide is equal to two of ferrous oxide. Therefore, to 

 oxidize either FeO to Fe 2 3 or Ti 2 3 to Ti0 2 , one atom of 

 oxygen, equivalent to one-ninth of the lower oxide, will be 

 needed ; while conversely, in the case of reduction of Fe 2 3 to 

 FeO or Ti0 2 to Ti 2 3 , one atom of oxygen, equivalent to one- 

 tenth of the weight of the higher oxide, will be subtracted. 

 Exactly the same amount of potassium permanganate, there- 

 lore, will oxidize the same weights of iron as ferrous oxide or 

 titanium as sesquioxide to the higher form, as is expressed by 

 the two equations : 



10FeSO 4 + K Mn o 8 + 8H 2 S0 4 =5Fe 2 (S0 4 ) 3 + K 2 S0 4 + 



2MnS0 4 + 8H 2 0, 



5Ti 2 (S0 4 ) 3 + K 2 Mn 2 8 + 8H 2 S0 4 = 10Ti(SO 4 ) o + K 2 S0 4 + 



2MnS0 4 + 8H 2 0. 



Fe 2 3 and 2Ti0 2 on the one hand, and 2FeO and Ti 2 3 on the 

 other, are mutually interchangeable and equivalent as regards 

 titration by permanganate or other such oxidizing aentg. 

 From this it follows that, if Ti 2 3 is present and the ferrous 

 iron is determined in the usual way, a percentage amount equal 

 to that of the Ti 2 3 -must be deducted from the apparent value 

 for FeO. while an equivalent amount must be added to the 

 apparent amount of Fe 2 3 , and deducted from that of Ti0 2 . 



In the case of minerals whose formulas are simple and well 

 established, readjustment may be made with a fair degree of 

 confidence as to probable correctness, even in the absence of 

 determinations of all four oxides. Such readjustments based 

 on the empirical formula would have still greater weight could 

 it be assumed that ferrous oxide is absent or present in only 

 negligible amounts. This consideration applies to the com- 

 position of schorlomite, which we are justified in referring to 



