Washington — Constitution of Some Salic Silicates. 559 



tion ; this functional substitution (isomorphous replacement) 

 involving only slight (morphotropic) change in the crystal 

 form, and the isomorphous compounds (mutually either wholly 

 or partially miscible) forming homogeneous mixtures whose 

 properties (chemical and physical) are continuous functions of 

 their composition. 



The replacement* may be complete, as is illustrated by barite 

 and celestite, or apatite and mimetite, and an element can be re- 

 placed by an atomic group (radical) of like valency, as K or Na is 

 replaced by NH„ or F by OH. Isomorphously replaceable ele- 

 ments or radicals may partially replace each other, their total 

 molecular amounts being equal to that needed to satisfy the for- 

 mula. This is of very rare occurrence or quite unknown among 

 organic compounds, but is so frequent among minerals that 

 examples need not be given. Finally there is a third type of 

 isomorphous replacement, namely, that between atomic groups 

 of the same total valency. Thus a bivalent atom or radical 

 replaces or is replaced by two univalent ones, two bivalent 

 atoms by a univalent and a trivalent one, or two trivalent 

 atoms by three bivalent ones, and vice versa. This is illus- 

 trated by the mutual isomorphous replacement of Ca and Na 2 

 in many silicates, of Pb and Cu„ in sulpho-salts, of CaMg and 

 NaFe'" in the pyroxenes, and so on. 



These last mentioned atomic groups differ from radicals in 

 the ordinary sense. In radicals the atoms are linked together, 

 and the valency of the group is that of the free bond or bonds, 

 and may therefore be spoken of as residual. In the last case 

 the atoms are not linked together between themselves, but are 

 in stoichiometric ratios and together replace a number of other 

 atoms the sum of whose valencies is equal to theirs. While 

 apparently not joined together inter se, they act together, and 

 the valency of such a group may be spoken of as total. Our 

 knowledge of this sort of replacement is comparatively recent 

 and is still not well understood, but so many examples are 

 known that it is now generally accepted. f As this type of 

 atomic group is of importance in mineralogy and the study of 

 isomorphism, it will be well to distinguish it from the ordinary 

 radical by a special name. For this the word "integral" 

 seems to be appropriate,^; and by this term such atomic groups 

 will be known in this paper. 



To sum up, the fundamental characters of isomorphism or 

 isomorphous replacement are adherence to the same type 

 of chemical formula and agreement in crystal form, and 

 structure, subject to only very slight morphotropic changes, 



* Of course actual replacement of a previously present element is not meant. 



+ Cf. Groth, Introd. Chem. Oryst., New York, 1909, p. 85. 



\ Cent. Diet., "A whole formed of parts spatially distinct or of numerical 

 parts." Standard Diet., adj. " constituting a completed whole " ; noun, " a 

 function of variables that remains constant." 



