668 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.59. 



specimen appears to the unaided eye to be made up of pure ludwigite 

 containing irregular masses of bornite. Under the microscope the 

 ludwigite is seen to contain disseminated crystals and ragged grains 

 of diopside. In the replacement of the calcite-diopside rock by 

 ludwigite and sulphides the calcite has been selectively replaced 

 before the diopside was attacked, and residual diopside crystals 

 showing sharp idiomorphic boundaries occur embedded in a mass 

 of ludwigite and bornite. In an advanced stage of the replacement 

 the diopside has been attacked and the crystal outlines destroyed. 



Physical properties. — To the unaided eye the Idaho lugwigite is 

 black in color, with a very fine felted structure, in which the fibrous 

 structure is too fine to be detected by the unaided eye, but is mani- 

 fested by a faint silky luster, which might easily be overlooked, and 

 the material might readily be mistaken for magnetite. The hardness 

 is above 5, as the mineral scratches apatite with ease, but is itself 

 scratched by orthoclase. The streak is moderately dark greenish- 

 brown. The material is not notably attracted by an ordinary magnet 

 of the horseshoe type. 



Optical properties. — The ludwigite is transparent in very thin frag- 

 ments and is intensely pleochroic in tones of brown parallel to the 

 elongation and grass green perpendicular to it. The birefringence is 

 moderate, while the indices of refraction are high. The extinction 

 is parallel to the elongation of the prisms. 



Pyrognostics and chemical properties. — This ludwigite yields a 

 small amount of water in the closed tube, all of which is probably 

 extraneous. It is soluble slowly but completely in sulphuric, nitric, 

 and hydrochloric acids. When moistened with sulphuric acid it 

 gives the green flame of boron. 



Composition. — The material for analysis was hand selected and was 

 contaminated by small grains of included impurities. The purest 

 pieces of the felted aggregate were selected, ground in an agate 

 mortar, and a small amount of magnetite extracted with a magnet. 

 The remaining material upon analysis yielded the following results: 



Analysis of Ludwigite from Lemhi County, Idaho. 



Silica (Si0 2 ) 0.90 



Ferric oxide (Fe 2 3 ) 35. 90 



Alumina (A1 2 0,) 2. 08 



Ferrous oxide (FeO) ; 5. 14 



Magnesia (MgO) .- 36.42 



Lime(CaO) Trace. 



Manganous oxide (MnO) Trace. 



Copper oxide (CuO) 2. 87 



Boric anhydride (B 2 3 ) 14-59 



Water (H 2 0) -105° C 03 



Water (H 2 0) +105° C 2. 28 



Sulphur (S) 22 



Total ' 100.43 



