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



Figure 1 shows the decrease in percentages of Si0 2 with the 

 rise in value of the mean index of refraction. The relations 

 here are reasonably well expressed by the straight line given 

 in the figure. Of the first nine analyses, the one which departs 

 farthest from this theoretical line is No. 4 and this is only 

 about 2*5 per cent out of the way. In figure 2 the increase 

 in the percentages of A1. 2 3 are shown corresponding to the 

 increase in value of the mean refractive index. The points 

 determined in this case show more general variation from the 

 straight line than in the case of the silica, yet here the greatest 

 variation among the first nine analyses is only a little over 2 

 per cent. Figure 3, which shows the variation in the percent- 

 ages of Fe„0 3 , is much the same, the greatest variation being 

 about 2*7 per cent in the case of analysis 7. Figure 4 shows 

 the variation in the sum of A1„0 3 and Fe 2 3 . As might 

 be expected, the position of the points in this figure in general 

 approximate much closer to the straight line than in the two 

 previous figures, analyses 4, 9 and 10 being the only ones that 

 depart from it in any notable dgree. Figure 5 shows the 

 variation in amount of FeO with the increase in value of the 

 index of refraction. Eight of the analyses give points that lie 

 reasonably close to the line ; analyses 8 and 9 however depart 

 considerably from it, 9 being nearly 6 per cent out of the way. 

 Strikingly different is the agreement shown in figure 6, in 

 which the amounts of total iron are given. Here analysis 

 number 9 is the only one that shows any notable discrepancy 

 and the amount of variation in this case is only about 2 

 per cent. Figure 7 shows that the decrease in percentage of 

 MgO follows closely the rise of the index of refraction, the 

 greatest variation from the line being 2 per cent in the case of 

 analysis number 9. Figure 8 gives the percentages of CaO in 

 relation to the indices. There is a slight decrease in the 

 amount of CaO as the index rises but the extreme variation is 

 not much over 3 per cent. Consequently the influence of the 

 lime upon the index of refraction must be fairly uniform 

 throughout the series. Figure 9 gives the percentages of 

 alkalies present, K 2 + Na 2 0. They show a slight increase in 

 amount with the rise of the indices, but as the largest amount 

 of the alkalic oxides is under 5 per cent they cannot have 

 much influence upon the optical characters. 



From the above it can be seen that with this series of 

 amphiboles it is the silica, the total iron and the magnesia that 

 follow in their varying percentages most closely the variation 

 in the mean index of refraction. It must be to them that this 

 variation is chiefly due and in all probability the percentage of 

 total iron present is the factor possessing the greatest influence. 

 With full realization of the incompleteness upon which these 



