192 W. E. Ford — Optical Study of the Amphiboles. 



of the different oxides are quite irregular. The amount of 

 total iron present increases in general with the decrease in the 

 angle of extinction, yet this increase is irregular (note the low 

 percentage of total iron shown in analysis XIV) and does not 

 even begin to show the same degree of uniformity as in the 

 case of the tremolite-actinolite series. 



The fact that we can have two members of the Amphibole 

 Group differing decidedly in their percentage composition but 

 still showing the same angles of extinction proves at once that 

 the value of the extinction angle does not depend directly at 

 least upon the percentage composition. It seems probable 

 that it must depend ultimately in a large measure upon the 

 kind and amount of elements present in the mineral, but it is 

 likely that it is also influenced largely by the character of the 

 physical molecule into which they are grouped. The relation- 

 ships here are certainly complex and the available data are far 

 too few to allow us to hope that the problem can be solved as 



Various writers have previously attempted to find a correla- 

 tion between the chemical and optical properties of the amphi- 

 boles. Murgoci in a study* which was concerned chiefly with 

 the characters and classification of the glaucophane-like amphi- 

 boles makes, however, some general statements. On p. 362 he 

 says : " The chemical constituent with the most influence on the 

 physical properties seems to be Fe„0 3 (viz. Fe„Si 3 9 ) and that 

 not only in the glancophane series, but in general in the 

 amphibole family . . . Further, the size of the angle of the 

 optic axes, the position of the axial plane and even the angle 

 of extinction, up to a certain point, are in relation with the 

 amount of Fe„0 3 ." This may be true in respect to the alkali- 

 rich amphibole-like glaucophane, but the facts as given above 

 hardly bear this statement out in respect to the ordinary amphi- 

 boles. The amount of FeO shows a closer relation to the opti- 

 cal properties than the Fe 2 3 and the amount of total iron 

 present conforms more closely yet. Further, the same author 

 states on p. 373 that " A comparison of the whole group of 

 AlFe'" amphiboles has convinced me that in general the size 

 of the angle of extinction is related neither to the amount of 

 A1 2 3 nor to the amount of Fe 2 3 , but to the proportion of 

 their molecular coefficients of 'combination in the amphibole 

 constitution." A study of the analyses given above has failed 

 to confirm this theory. In fact it only needs a hasty inspec- 

 tion of the relative percentages of A1.,0 3 and Fe 2 3 in the 

 analyses quoted on p. 190 to show that the assumption does 

 not hold true with them. 



* Contribution to the Classification of the Amphiboles, Bull, of the Geol. 

 Dept. Uni. of Calif., iv, 359, 1906. 



