:<o. 2395. LUDWIGITES FROM IDAHO AND KOREA— SHANNON. 



675 



which should otherwise fall in a uniform series between ferro- and 

 magnesioludwigite. 15 



A second point with reference to the composition of ludwigite 

 which deserves mention is the water content, which is often reported 

 in analyses and which is not released at a temperature much above 

 110° C. The results are collected below: 



Mineral. 



Ludwigite . 

 PLnakiolite 

 Ludwigite. 



Do 



Do 



Do 



Locality. 



Hungary. 

 Sweden.. 

 Montana. 

 Hungary . 



Idaho 



Korea 



Analyst. 



Whitfield . 



Flink 



Schaller... 

 do.... 



Shannon . . 

 do.... 



Water 

 below 

 110°. 



1.13 



.51 

 .03 



Water 

 above 

 110°. 



3.62 

 .47 



1.21 

 .82 



2. 28 



1.42 



Schaller, after some consideration, decided to regard the water as 

 extraneous. Whitfield, on the other hand, considered the water 

 found by him in the Hungarian material as essential and basic. On 

 the whole, the former seems the more reasonable view, especially 

 since emphasis has been placed upon the fact that adsorbed water, 

 properly so-called, may be held tenaciously up to the point of 

 fusion. 8 Those ludwigites which show the largest content of water 

 are distinctly fibrous in structure and thus possess the maximum of 

 surface area. 



Magnesia and ferrous iron seem entirely isomorphous in ludwigites 

 as known. Boric anhydride and ferric iron are apparently always 

 definite and constant in their ratio to each other although both may 

 be susceptible of replacement to a slight extent by alumina. Ex- 

 amples intermediate between pinakiolite and ludwigites proper have 

 not yet been found. 



With regard to the occurrence of the ludwigites, one fact is obvious 

 and important. Members of the group in every known occurrence 

 are contact metamorphic minerals in limestone. The boron emana- 

 tions in the contact zone seem to form ludwigite in the event that 

 the supply of silica is deficient, whereas tourmaline forms under the 

 same conditions where the supply of silica is ample. Ludwigite is 

 not known in association with free quartz. The evidence seems to 

 indicate that ludwigites are probably rather common products of 

 contact metamorphism in limestone. The minerals may have largely 

 been overlooked, as the black varieties closely resemble black tour- 

 maline, or magnetite, while light-colored varieties may closely simu- 

 late amphiboles, etc., which are frequent in lime-silicate contact 



15 Larson, Esper S., Mineral Tables, in press. 

 i«U. S. Geol. Survey, Bull. 700, pp.G4-69, 1910. 



