182 IF. E. Ford — Optical Study of the Amphiboles. 



1*64 ; that of typical hornblende between 1*66 and I'll. 

 The birefringence of the whole series is about the same, the 

 extremes observed being *0206 in the hornblende from Edenville 

 and *0325 in the tremolite from Lee, the average of the series 

 being *0259. The axial angles as far as they could be calculated 

 range from about 77 ° to 86° 30'. The extinction between c and 

 6 measured on b (010) ranges from 16° to 20° in tremolite, from 

 13° 30' to 15° in actinolite and from 9° to over 33° in the typical 

 hornblendes. Strong pleochroism was found only in the horn- 

 blendes, the actinolite members showing it but faintly, if at all. 

 The absorption was always c > b > a, although frequently little 

 difference could be distinguished in the color of the light 

 vibrating parallel to o and 6. The plane of the optical axes in 

 these minerals was always in the symmetry plane and the 

 bisectrix c lay in the obtuse angle /3. The optical character, 

 where it could be proven, was found to be uniformly negative. 



With our present meager knowledge concerning the relations 

 existing between chemical composition and optical characters, 

 such general statements, as given above, are perhaps all that 

 should be attempted. The amphiboles, because of the wide 

 variation in their composition and the large number of variable 

 radicals which they contain, present extraordinary difficulties 

 to any exact con-elation between the chemical and optical 

 properties. Such a correlation it is quite possible can never 

 be made. Yet it is thought, that within somewhat wide 

 limits and with the recognition of numerous exceptions, 

 a tentative correlation based on the work with this series of 

 amphiboles can be given. 



It was found on studying the above optical determinations in 

 respect to the corresponding analyses that the variation in the 

 mean index of refraction came nearer to showing a correlation 

 with the variation in composition than any of the other optical 

 characters. Consequently the analyses have been chiefly studied 

 in the endeavor to discover, if possible, how they varied 

 with increase in the value of the mean index of refraction. 

 The analyses, as given on page 181, are arranged in order of 

 the rise in the value of their mean indices of refraction. 

 With the purpose of making these relationships clearer, figs. 

 1 to 9, which show them in graphical form, are given. In 

 these figures the mean indices of refraction are shown as 

 the abscissas while the percentages of the different radicals 

 present in the minerals form the ordinates. The numbers 

 given to the different points located on the figures correspond 

 to those of the analyses on page 181. In each figure a 

 straight line has been drawn which falls as closely as possible 

 to the different points. Very likely a curve would have better 

 expressed the relations between the indices of refraction and 



