156 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tioned as follows : — NaaFejSiiOia, 0.582 ; R^Sii O12 ; 0.834 ; SiOj left, 

 0.058. This may also be expressed by the ratio, Si02 : R2O8 : 

 (RO + R2O + H2O + F) = 8 : 0.90 : 4.76. The excess of silica is 

 considerable. What the cause is, is not clear. It may be due to im- 

 purity of the material analyzed. The potash seems rather high and 

 may be in error. The presence of even a little feldspar or quartz in the 

 sample would easily produce the apparent excess of silica as calculated. 

 In the cases of several of the other analyses it is necessary, in order to 

 make them correspond to the metasilicate formulae, to assume the 

 presence of a molecule, Il"Fe2Si40i2 on account of the excess of Fe203. 

 In any case, the ratios lack sharpness and are far from satisfactory. It 

 seems highly probable, however, that if the fluorine and water were 

 correctly determined, and the precise role which they play known 

 (whether they are a part of the chemical molecules or are merely held 

 in solid solution or both), also probably some minor changes made in the 

 figures for some of the other constituents, the molecular ratios of these 

 amphiboles would show satisfactory agreement and the formulae 

 would correspond quite closely to, the simple metasilicate molecules. 

 Table III shows the molecular ratios apportioned among the molecules 

 mentioned above ; also the percentages of the molecule Na2FeaSi40i2 

 for each of the minerals whose analyses have been given. 



TABLE III. 



12345678 9 10 



Na2Fe:Si40i2 0.582 0.888 0.900 0.558 0.738 0.630 0.588 0.720 0.606 0.618 



RFe2Si40i2 .. 0.174 0.252 0.234 .. 0.126 0.048 



(R. R2)4Si40i2 0.834 0.294 0.252 0.668 0.838 0.785 0.612 0.85S 0.812 0.984 



Rem'nderSiOz 0.058 0.022 0.072 0.134 0.087 .. 0.014 0.044 0.025 0.086 

 Per cent of 



Na2Fe2Si40i2 42 68 69 43 57 44 45 55 47 47 



The Quincy mineral (1), that from Colorado (4), from Cape Ann 

 (5), from Red Hill (6), and one of the crocidolites (8) show perhaps the 

 greatest similarity, although there is considerable divergence among 

 even these. In every one but No. 6 there is an excess of Si02. The 

 amount of the Na2Fe2Si40i2 radical varies from 69 per cent for the Soco- 

 tra mineral to 42 per cent for the Quincy variety. Six numbers, 1, 4, 6, 

 7, 8, 9, fall below the average which is 51.7. Statements in the literature 

 regarding the composition of riebeckite and crocidolite are to the effect 

 that they consist essentially of the radical Na2Fe2Si40i2 with varying 

 amounts of R4Si40i3 and analogous molecules. While the former mole- 

 cule is certainly essential, the above data indicate that we are not 

 justified in considering it the predominant molecule. 



