Chemical Composition of Amphibole. 51 



argument for the uniformity in structure of the acid part of 

 the molecule, enabling it to exert a controlling influence 

 throughout the series and so to determine the crystal form. 

 That fluorine and hydroxyl present are integral parts of the 

 amphibole molecule and that they are to be regarded as iso- 

 morphous with the protoxides is considered as definitely proven 

 by the results of the analyses. The presence in many amphi- 

 boles of considerable amounts of sesquioxides is explained by 

 their introduction into the metasilicate molecule in the form of 

 various basic, bivalent radicals. It is suggested that the char- 

 acter of these radicals and the degree to which they enter the 

 amphibole molecule may be due to the influence of mass effect 

 determined by the conditions under which the mineral was 

 formed. This method of interpretation of the analyses of 

 amphibole is upheld by the following facts : first, in the analy- 

 ses of the simpler varieties, tremolite and actinolite, the 1 : 1 

 ratio between RO and Si0 2 is very sharp and indicates clearly 

 that the structure of the molecule is that of a metasilicate and 

 the small, amounts of RO present cannot be introduced into the 

 formula without destroying this ratio except in the form of the 

 — R— (F,OH) 



V 



bivalent radical O ; secondly, the identity of the 



— R (F,OH) 



crystalline structure of the whole amphibole series points 

 strongly to the assumption that the acid radical is the same 

 throughout ; thirdly, in the analyses of the hornblendes, by the 

 assumption that two or more radicals, containing the trivalent 

 elements with smaller amounts of the protoxide bases, enter 

 the molecule isomorphous with RO, the residue left after the 

 subtraction of these molecules gives the metasilicate ratio of 

 RO : Si0 2 =1:1; fourthly, in all cases the calcium oxide of 

 the analyses together with the small amounts of oxides of the 

 alkalies available forms very closely 25 per cent of the various 

 radicals and bases, or in other words replaces one-fourth of the 

 hydrogen atoms of the amphibole acid. This last fact is too 

 constant throughout the analyses to be looked upon as a coin- 

 cidence, but would seem rather to be a fundamental fact con- 

 cerning the amphibole molecule, and furnishes one of the 

 strongest pieces of evidence as to the correctness of the present 

 method of interpreting the chemical constitution of the min- 

 eral. 



Mineralogical Laboratory of the 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., October, 1906. 



