Foote and Bradley— Solid Solution in Minerals. 25 



Art. III. — On Solid Solution in Minerals with Special 

 Reference to JVephelite ; by H. W. Foote and W. M. 

 Bradley. 



It is a fact well known to mineralogists that there are 

 certain minerals to which no satisfactory chemical formulae can 

 be assigned which agree with the results of analysis. The 

 reason for this in many cases, particularly where the mineral 

 is rare and little investigated, is probably that the material is 

 impure, containing included foreign matter, or else the 

 analysis is incorrect. There appear to be cases, however, 

 where the material has been so carefully selected that foreign 

 matter could not be present except in traces, and where 

 analyses have been made with the greatest care and still the 

 formula cannot be definitely assigned. A case of this kind is 

 that of the mineral nephelite, to which the formulae NaAlSi0 4 

 and Na 8 A] e Si 9 34 besides others more complicated have been 

 given. An examination of several good analyses of this 

 mineral will show that the analytical data do not support any 

 one formula, but that there are considerable variations from 

 it which are greater than can be accounted for by the ordinary 

 errors of analysis. 



In general the composition of a mineral as obtained in 

 analysis varies from the composition of the ideal pure com- 

 pound for two reasons, aside from errors of analysis. Either 

 there is (a) isomorphous replacement of one element or radical 

 by another, or (b) there are mechanical impurities present. 

 Where there is merely isomorphous replacement, the formula 

 of the pure compound can be derived from the analysis by the 

 ordinary methods of calculation, which need not be considered 

 here. The presence of mechanical impurities can usually be 

 determined by other means, for instance, by the use of heavy 

 solutions or by microscopic examination. We wish to call 

 attention to another influence which must probably be taken 

 into account in cases like that of nephelite. It appeal's to us 

 necessary to assume that in certain cases a substance on crystal- 

 lyzing forms a solid homogeneous solution with foreign matter 

 which cannot be assumed to be isomorphous with any constit- 

 uent, and which is not to be regarded as a mechanical mixture. 

 It can be compared to the solution of salt in water, in which 

 the salt takes on the appearance aud form of the water without 

 taking any part in the formula of the water. A case of this 

 kind in minerals would not be a mechanical- admixture of the 

 foreign substance, comparable to the suspension of a solid in 

 water, but would form a homogeneous mass with the rest of 



