Chemical Composition of Amphibole. 37 



the analyses. Thus, for example, the constituents A1 2 3 and 

 Fe 2 3 may be regarded as combined with F and OH to form 

 the following- radicals : 



_A1— F — Al— OH — Fe— F — Fe— OH 



>0 >0 >0 >0 



_A1— F — Al— OH — Fe— F — Fe— OH 



The foregoing radicals are bivalent and, as may be seen, the 

 addition of F 2 to the protoxide ratios means the introduction 

 of Al 2 (or Fe 2 ) into the molecule, and when F 2 fails or is not 

 sufficient to satisfy all of the sesquioxides, (OH) 2 , derived from 

 H 2 0, accomplishes the same result. If, therefore, the ideas as 

 set forth on page 28 are correct, namely that amphibole is a 

 salt of a complex metasilicic acid, the assumption is now made 

 that some of the hydrogen atoms of the acid are replaced by 

 bivalent radicals, containing aluminium and ferric iron in com- 

 bination with fluorine and hydroxyl as given above. These 

 bivalent radicals therefore are regarded as isomorphous with 

 Fe and Mg. It may be asked why the radicals [R 2 /// OF 2 ]' / and 

 [R 2 '"0(OH) 2 ]" have been selected instead of others? for exam- 

 ple, [R'"F]" or [R^FJ', and their respective hydroxyl equiva- 

 lents, and the answer is simply because only the former satisfy 

 the 1 : .1 ratio of silica to protoxide bases shown by the analyses. 



Since, however, the amounts of A1 2 3 and Fe 2 3 in tremolite 

 and some varieties of actinolite are small, the assumption of 

 either of the radicals [R'"(F, OH)]" or [R'"(F, OH),] 7 would 

 not materially affect the ratios in some cases. For the sake of 

 comparison the possible fluor-hydroxyl radicals of trivalent ele- 

 ments in combination with metasilicic acid may be expressed 

 graphically as follows : 



Q Q j£ rp QTJ\ Q T>/ (^' / 



o=si/ \r-(f,oh) o=Si/ " >o o=$\/ V \ ( £'oh! 



NV \ -K-(F,OH X °- R <^[f;0H) 



According to the first assumption, bringing R"' into the 

 molecule as a bivalent radical [R — (F, OH)]", the protoxides 

 as given on page 36 would be increased ; according to the last 

 [R„(F, OH) 4 ]" the protoxides would be diminished, the result- 

 ing ratios being as follows : 





Si0 2 



: E"0 + [R'"(F, OH)]"*} 



Si0 2 : 



R"0 



+ [R ,// (F,OH) 2 ] / 2 



I 



1 



: 1-02 





1 







0-98 



II 



] 



: 1-01 





1 : 







0-97 



III 



1 



: 1-02 





1 : 







0-98 



IV 



1 



: 1-04 





1 







0-95 



V 



1 



: 1-06 





1 







0-93 



