THE MINERALS OF IDAHO 441 



the fibers and green perpendicular to them. The absorption is 

 greatest in the direction of elongation and the extinction is parallel. 

 The birefringence is moderate while the indices of refraction are 

 high. The ludwigite yields a small amount of adsorbed water in 

 the closed tube and is slowly soluble in nitric, hydrochloric, and 

 sulphuric acids. When moistened with sulphuric acid it gives the 

 green flame of boron. A selected sample of the purest material upon 

 analysis gave the following results. 



Analysis of ludwigite from Lemhi County, Idaho 



(E. V. Shannon, analyst) 



Per cent 



Silica (Si0 2 ) 0. 90 



Ferric iron (Fe 2 3 ) 35. 90 



Alumina (A1 2 3 ) 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) above 105°C 2. 28 



Water (H 2 0) below 105°C . 03 



Sulphur (S) . 22 



Total 100.42 



The sulphur, copper, silica, and water are probably present as 

 extraneous materials, the sulphur and a part of the copper as bornite, 

 the silica as diopside, and the water adsorbed. After deducting 

 these the remaining constituents give the following ratios: 



MgO . 903) 



Fe 2 3 .J .225 .225 1.00X1 



Blot:::::::;::::;::::::::::::::::::: :Z)™> ■•«' 



These ratios give the general formula: 



4 (Mg,Fe)O.Fe 2 3 .B 2 3 , 



or an isomorphous mixture of ferroludwigite and magnesioludwigite 

 in the ratio of 3 to 7. The essential constituents shown by the analy- 

 sis are reclaculated to 100 per cent and below compared with the 

 theoretical composition of a mixture of 7 parts of magnesioludwigite 

 and 3 parts of ferroludwigite. 

 54347— 26t 29 



