382 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



pleochroism is not marked, X pale brownish green, Y and Z bluish 

 green. The a, /3, and 7 indices are about 1.640 to 1.645. Only a small 

 amount of the chlorite could be separated for analysis and there was 

 not enough of the material to permit determination of the state of 

 oxidation of the iron. Assuming it to all be ferrous, the analysis 

 gave and following results and ratios : 



Analysis and ratios of chlorite from Sherman mine 

 [E. V. Shannon, analyst] 



Silica (Si0 2 ) 



Alumina (AI2O3).- 

 Ferrous iron (FeO) 



Lime(CaO) 



Magnesia (MgO).. 

 Water (HsO) 



Total 



This analysis leads to the formula 6(Fe,Mg)0.2Al 2 3 .3Si0 2 .5H,0, 

 or precisely that derived for the chlorite associated with the chloritoid 

 from Lemhi County, described above, and the correspondence in 

 optical properties is close enough to establish the practical identity 

 of the chlorites from these two localities. 



A similar chlorite occurs as fine granular streaks associated with 

 ferriferous calcite, quartz, and chalcopyrite in a specimen of ore from 

 the Blackbear prospect in the Avery quadrangle. The chlorite has 

 been mentioned in connection with the calcite, an analysis of which 

 is given above under that heading. 



Under the microscope this chlorite is pleochroic in X and Y deep 

 green, Z = greenish brown. It has low birefringence and gives 110 

 very definite interference figures but is apparently biaxial and positive 

 with 2V very small. The refractive indices are approximately 

 a = /3=1.630, 7 = 1.635. A very small sample was analyzed giving a 

 very inferior analysis in which the magnesia was lost through accident 

 and had to be taken by difference. The ferrous iron reported was 

 actually determined to be present as such, but total iron was not esti- 

 mated and the alumina may include a little ferric iron. The analysis 

 gave the following results and ratios : 



Analysis and ratios of chlorite, Black Bear claim 

 (E. V. Shannon, analyst) 



Per cent 



Ratios 



Silica (Si0 2 ) 



Alumina (AI2O3)--- 

 Ferrous iron (FeO). 

 Magnesia (MgO)_- 

 Water (H2O) 



Total. 



24.80 

 22.86 

 28.72 

 (13.48) 

 10.14 



100.00 



0.413 0.413 0.0S3X5 



. 225 . 225 . 075X3 



• 3^j . 737 . 082X9 



'. 563 . 563 . 080X7 



1.03X5 

 .93X3 



1.01X9 



.99X7 



