398 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.59. 



Hercules mine in the same district. This mineral, which was in 

 amount too small for analysis, had in general the properties of antho- 

 phyllite. Determination of refractive indices made by the writer 

 on the material of Ransome's original specimen which is preserved 

 in the United States National Museum (Cat. No. 77071-91) indicate 

 that this also is the iron amphibole entirely similar to that from the 

 Tamarack-Custer mine. 



DESCRIPTION. 



The material as received consists of splintery-fibrous masses of a 

 pale grayish-green color when dry and olive green when moist. In 

 all, about 250 grams of the mineral were received. The individual 

 fibers reach a maximum length of 6 centimeters. The material 

 separates readily into very fine silky fibers, which are quite strong 

 and flexible, being comparable to a poor quality of chrysotile, which 

 the material greatly resembles. The true hardness of the mineral 

 could not be determined as its fine fibrous structure renders it too 

 weak to scratch any mineral harder than calcite. The specific 

 gravity as determined on the Kraus modified Joly balance on pieces, 

 probably not entirely free from included air, is 3.24 (mean of 5 deter- 

 minations). The only associated mineral is fine granular galena, 

 which occurs as minute veinlets cutting across the fibers, as thin 

 fillings between the fibers, and as small nodules around which the 



fibers are curved. 



OPTICAL PROPERTIES. 



Under the microscope the ferroanthophyllite is seen to consist 

 of aggregates of exceedingly fine fibers, which singly appear color- 

 less and transparent. Aggregates of a number of fibers show the 

 green color and pleochroism of the mineral. The extinction is in 

 all cases exactly parallel to the edges of the fibers. No regular ter- 

 minations could be seen nor could the shape of cross section of the 

 fibers be determined owing to their extreme thinness. The elonga- 

 tion is positive, as in anthophyllite, while the birefringence is slightly 

 lower than that of anthophyllite. The indices of refraction and the 

 birefringence as determined by immersion are as follows: 



o=l. 668 ±.003 



7=1. 685 ±.003 



y-a= .017±.003 



The pleochroism is distinct in the aggregated fibers, as follows: 



a=pale brownish green. 

 7= deep brownish green. 



Since the fibers are exceedingly minute and adjacent fibers are 

 not in parallel position, the optic axial angle and the intermediate 

 value for refractive index (0) could not be accurately determined 

 although the axial angle (2 V) is apparently moderately large. 



