674 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.50. 



The above type of formula was adopted by Tschermak and con- 

 tinued by Flink and Schaller. It is supported by the ratios of the 

 three end members thus far found. The ratio of B 2 3 to Fe,0 3 

 seems invariably constant. It is possible to interchange the ratios 

 of the bivalent bases, however, and the minerals can almost equally 

 well be expressed by any of the following formula types: 3RO.B 2 0... 

 RO.Fe 2 3 ; 2RO.B 2 3 . 2RO.Fe 2 3 ; RO.B 2 3 . 3RO.Fe 2 3 - 



The general formula for the group is thus 4(Mg,Fe,Mn)0. 

 (B,A1)A- (Fe,Mn,Al) 2 3 . 



The formula as written by Tschermak for the original ludwigitt- 

 (ferroludwigite) assumes 1 molecule of trimagnesium borate, 3MgO, 

 BX) 3 , plus 1 molecule of ferrous ferrate, FeO.Fe 2 3 ; while Schaller 

 deduces for magnesioludwigite 1 molecule of trimagnesium borate 

 plus 1 of magnesian ferrate, MgO.Fe 2 3 . Pinakiolite, which is the 

 best evidence in support of the above type of formula, is written 

 1 molecule of trimagnesium borate plus 1 of manganous manganate. 

 Vonsenite, as written by Eakle, obviously does not agree with this 

 series. An interesting hypothesis is developed from the observation 

 that the ludwigite from Korea, described above, is decidely magnetic. 

 It is to be remembered that the molecule ferrous ferrate is the 

 magnetite molecule, and it is quite conceivable that this compound 

 might preserve its magnetic properties even when united chemically 

 with the borate molecule in a ludwigite. Assuming, for the sake of 

 discussion, that in ludwigites which are markedly magnetic the ferrous 

 iron is present as ferrous ferrate, while in those which are not notably 

 magnetic the ferrous iron is present as borate, the following formulas 

 may be written for the several occurrences : 



Hungarian ludwigite; not sensibly magnetic: 3(Mg,Fe)O.B 2 3 . 

 MgO.Fe 2 3 . 



Idaho ludwigite; not sensibly magnetic: 3(Mg,Fe)O.B,0 3 . 



MgO.FeA- 



Vonsenite; not sensibly magnetic. Very close to 3FeO.B 2 3 . 



MgO.Fe 2 0,. 



Montana ludwigite; moderately magnetic: 3MgO. B 2 3 . (Mg,Fe) 



O.FeA- 



Korean ludwigite; decidedly magnetic: 3MgO. B 2 3 . (Fe,Mg) 



O.FeA- 



Of course, the term magnetic as here used is relative only, as all 

 ludwigites are attracted to a sufficiently powerful electromagnet, 

 as are all iron-bearing materials. The magnetism as stated above 

 is in terms of an ordinary horseshoe magnet. While not put forward 

 as a definite conclusion, this evidence is recommended as worthy of 

 consideration. These interpretations may, in part, explain the 

 anomalous variation in optical properties observed in ludwigites 



