188 TT. E. Ford — Optical Study of the Amphiboles. 



Analysis 16 is of the basaltic hornblende from Bilin, ana- 

 lyzed by Stanley, the optical properties of which follow: 

 Mean index = 1*692. C/\C = 1° 12' in the acute angle ft. 

 Pleochroism, a = light yellow-green, fi = orange, c = dark 

 reddish brown. Absorption, c>b>ct. The percentage of 

 silica present is about what would be expected from a horn- 

 blende with a mean index of refraction of 1*69. The sesqui- 

 oxides are low, the amount of iron protoxide is very deficient 

 and the magnesia is about double what would be expected. 

 This hornblende contains 1*68 per cent of TiO„, which undoubt- 

 edly has a marked influence upon its optical properties, although 

 it would hardly seem sufficient to account for all its peculiari- 

 ties. It is characterized optically by its unusual extinction 

 angle and by the deep reddish brown color of the light vibrat- 

 ing parallel to c. This hornblende shows little correspondence 

 with the series given above. 



Analysis 17 is of the variety of amphibole known as kaer- 

 sutite, from Kaersut, Greenland, as it is given by Washington. 

 Its optical properties follow : a = 1'676, /3 = 1-694, y = 1*708. 

 Mean index - 1-692. y-a= *032. 2Y = 82° 6'. This is an 

 unusual amphibole in that it contains over 10 per cent of TiO„. 

 As might be expected, the amount of iron present is small for 

 an amphibole having this index of refraction. It obviously 

 cannot be classed as a normal amphibole. 



Analysis 18 is of the similar mineral from Linosa, also given 

 by Washington. Its optical properties are given as follows : 

 a = 1*692, ^3 = 1*730, 7 = 1*760. Mean index = 1*726. y-a 

 = *068. 2Y = 79° 54'. This, like the Kaersut mineral, has a 

 large percentage of Ti0 2 (8*47 per cent). It differs in having 

 higher percentages of the iron oxides and lower percentages of 

 the alkalies. Optically it is distinguished by having the high- 

 est mean index of refraction and strongest birefringence of any 

 of the amphiboles described in this paper. 



From the foregoing study the following tentative conclu- 

 sions may be drawn. Given a normal and typical member of 

 the Amphibole Group, its composition can be predicted within 

 reasonable limits from knowing its mean index of refraction. 

 Of the various constituent oxides, the silica, lime and magnesia 

 follow most closely in their variations the change in the index 

 of refraction. The alumina varies more widely but does not 

 seem to have a great effect upon the index of refraction. The 

 iron oxides in a less degree also frequently show a variation 

 from what might be expected, but on the other hand the total 

 iron present shows a close approximation to the theory. The 

 introduction of notable amounts of titanium, alkalies and 

 fluorine produces marked changes in the optical character. 

 The presence of titanium serves apparently to raise the index 

 of refraction while the alkalies and fluorine tend to lower it. 



