112 wherry: compositions of minerals 



One of the most striking features possessed by gels is their 

 power of adsorbing foreign substances. For instance, the abihty 

 of the sesquioxides, when precipitated with ammonia, to take up 

 considerable amounts of phosphoric acid, boric acid, etc., has 

 long been recognized, and analytical methods modified in the 

 presence of these substances. The amounts so adsorbed bear in 

 general no relation to the molecular weights of the substances 

 concerned — that is, the law of definite proportions is not obeyed 

 — except that ''the composition of many [natural] gels agrees 

 frequently quite exactly with the corresponding formulas of crys- 

 tallized minerals."* 



The extent to which adsorption causes variations in the com- 

 positions of colloid minerals becomes evident when the analyses 

 of bauxite, psilomelane, wad, the clays, chrysocolla, etc., are 

 inspected. It is not so certain, however, which constituents 

 should be regarded as united and which as adsorbed in these 

 minerals. Is an amorphous clay containing silica, alumina and 

 water to be regarded as an adsorption product of Si02 gel, AI2O3 

 gel, and water, or as kaolinite gel (H4Al2Si209) containing an 

 excess of one or the other constituents adsorbed? It will be nec- 

 essary to develop methods for the proximate analysis of colloidal 

 minerals — staining, fractional solution, plotting dehydration 

 curves, etc. — before such questions can be settled, a problem that 

 will keep mineralogical chemists busy for some time to come. 

 But meanwhile we should face the issue squarely, and in stating 

 the composition of such minerals admit their variable character 

 instead of attempting to assign them definite formulas, as is 

 done in text-books. 



Then there is a whole series of minerals whose superficial ap- 

 pearance is that of colloids, but whose fibrous or foliated internal 

 structure betrays their crystallinity, such as chalcedony, limon- 

 ite, malachite, serpentine, dufrenite, etc. As there appears to 

 be no single descriptive term'^ in use for these, the word meta- 

 colloid is here proposed. A meta-colloid is to be defined as a 

 substance which, though originally colloidal in character, has he- 



* Cornu, op. cit., p. 336. 



^ Himnielbauer, op. cit., p. 39, calls them "gealterte Gelen." 



