286 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Analysis of anthophyllite asbeslus, Kantiah 



(E. V. Shannon, analyst) 

 Constituent Per cent 



Silica (Si0 2 ) 57. 60 



Alumina (A1 2 3 ) 1. 94 



Ferric oxide (Fe 2 3 ) 1. 58 



Ferrous oxide (FeO) • 6. 62 



Lime (CaO) Trace 



Magnesia (MgO) 30. 32 



Water (H 2 0) above 110° C 2. 66 



Water (H 2 0) below 110° C . 20 



Total 100.92 



This composition is essentially that of anthophyllite. Under the 

 microscope the analyzed sample is in aggregates of exceedingly 

 fine fibers which, in the aggregates, are apparently in parallel position 

 and give a faint biaxial figure indicating the obtuse bisectrix perpendi- 

 cular to the fibrous aggregate with the optic plane parallel to the 

 length. These observations together with the fact that all of the 

 fibers show parallel extinction with positive elongation indicate that 

 the mineral is biaxial, positive with 2V large, orientation Z = c. The 

 fibers are transparent and faintly brown. The refractive indices 

 measured in the analyzed material are a = 1.605, 7 = 1.625, Bire- 

 fringence =0.020. The optical properties of anthophyllite have 

 been discussed by Bowen 3 who gives a diagram showing the increase 

 in refractive indices with the increase of iron, stated as FeSi0 3 . 

 The total of both ferrous and ferric iron shown in the above analysis, 

 calculated as FeSi0 3 gives approximately 15 per cent. The indices 

 from Bowen's curves, for an anthophyllite of this composition, are 

 a = 1.613, 7 = 1.632, appreciably higher than those found. If, how- 

 ever, only the ferrous iron be considered the indices derived from the 

 curves are a = 1.606, 7 = 1.624, almost exactly those measured. 

 This indicates that the ferric iron shown by the analysis is not es- 

 sential, especially since it would be more influential than ferrous 

 iron in increasing the indices. This supports the conclusion strongly 

 suggested by the appearance of the specimen before grinding, that 

 the ferric iron is extraneous and present as an infiltrated stain. 



A number of specimens labeled Orofino, which were sent to the 

 United States Geological Survey for examination by Dean Francis A. 

 Thompson, of the Idaho School of Mines, are probably from the same 

 area. The first of these consists of a mass of slightly curved splintery 

 brittle fibers with silky luster, abutting against a mass of granular 

 crushed white and green material. The silky fibrous material, under 

 the microscope, contains two slightly different materials which are 

 probably both anthophyllite. The first of these is coarse prismatic 

 under the microscope and is transparent and colorless with good 



N. L. Bowen. Jour. Wash. Acad. Sci., vol. 10, p. 413, 1920. 



