IrtG IF. T. SchaUci' — Ludioigite from Montana. 



Art. XII. — Ludioigite from Montana I by Waldemar T. 



SCHALLER. 



Occurrence and Association. 



Thk ludwigite liere described was collected at Philipsburg, 

 Montana, by Mr. Donald F. MacDonald and sent to the 

 chemical laboratory of the Survey for determination by Mr. F, 

 C. Calkins. It is stated to occur in metamorphosed limestone 

 with large bodies of magnetite. An examination of a thin 

 section showed that a member of the olivine group was 

 sprinkled throughout the ludwigite and in addition a minute 

 amount of a carbonate and a secondary fibrous mineral ■wjas 

 present. 



The ludwigite forms small spherulites composed of radiating 

 fibers of a very dark green or nearly black color. In its 

 physical properties it resembles the original ludwigite from 

 Hungary. IJnder the microscope with high magnification the 

 fibers extinguish parallel and show a strong pleochroism, 

 parallel to the elongation a sea-erreen, normal thereto a chest- 

 nut-brown with stronger absorption in the brown than in the 

 green. The refractive index is much higher than 1"67. The 

 olivine mineral has a mean refractive index of about 1'66 and 

 cleavage sections, parallel to b {010 J, showing a and have 

 refractive indices lying between 1*65 and 1"66. The mineral 

 must therefore be very low in ferrous iron and is referred to 

 forsterite. As no calcium was noted in the analysis, the 

 carbonate is either magnesite or siderite. 



Anal^/sis. 



The sample free from magnetite but containing a small 

 quantity of forsterite and minute amounts of the colorless 

 fibrous mineral and the carbonate, gave on analysis the follow- 

 ing results, the figures in the last column showing the values 

 with the forsterite and carbonate deducted and the analysis 

 recalculated to 100 per cent. 



Tlie ferrous iron was determined by a modification of 

 Pratt's method," using finely ground material. Some experi- 

 ments conducted by Dr. Hillebrand on samples of this 

 mineral showed that the oxidizing effect of grinding on the 

 ferrous iron was slight. The first value for alumina (1'98 per 

 cent) was obtained as the difference between the gravimetric 

 determination of R^Oj and the subsequent volumetric titration 



* Hillebrand, W. F. The Analysis of Silicate and Carbonate Rocks, 

 Bull. 305, U. S. Gaol. Survey, 1907, p. 138. 



