LUDWIGITES FROM IDAHO AND KOREA. 



By Earl V. Shannon, 



Assistant Curator, Department of Geology, United States National Museum. 



INTRODUCTION. 



The ferric magnesian borate, ludwigite, which was described by 

 Tschermak in 1874 as a new species from Hungary, was regarded as 

 a rare mineral for 40 years. Within the past few years, however, 

 ludwigite and closely related members of the ludwigite group have 

 been described from six localities in the United States. Recently 

 ludwigites from two additional localities have been received at the 

 United States National Museum. These have been analyzed in the 

 Museum laboratory and are described in some detail below. 



LUDWIGITE FROM LEMHI COUNTY, IDAHO. 



The specimen of ludwigite from Lemhi County, Idaho (catalogue 

 number 94,145), is labeled "Copper ore, Bruce Estate." No further 

 information accompanies the specimen. Umpleby l mentions that 

 a group of claims known as the Bruce Estate extends along the moun- 

 tain slope near its summit for 2 miles south from Dry Gulch in the 

 Texas district. Lead-silver ore is reported from several claims, but 

 the most interesting feature is a big low-grade copper deposit found 

 in association with large quantities of magnetite. The deposit occurs 

 on the side of a big dike, which is called "syenite" by the miners, 

 but which is probably quartz-diorite, as an abundance of the latter 

 and none of the former was noted in the bowlders in the gulches 

 below. 



Description and associated minerals. — The hand specimen is about 

 half light gray and half black in color. The light gray portion is 

 seen in thin section under the microscope to consist of a fabric of 

 idiomorphic crystals of colorless diopside, with interstitial areas of 

 calcite partly replaced by ludwigite, chalcopyrite, and bornite. The 

 ludwigite forms irregular masses and prismatic needles, which are, 

 for the most part, opaque, but which are transparent when very thin. 

 The borate and the ore minerals are intergrown in a manner suggest- 

 ing contemporaneous deposition. The darker colored portion of the 



i Uinpleby, J. B., U. S. Geol. Survey Bull. 528, p. 89, 1913. 



Proceedings U. S. National Museum. Vol. 59— No. 2395. 



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