MINERA L TRA NSFORMA TIONS. 



279 



In the alterations of minerals " atoms or groups of 

 atoms can be split off from a molecule and replaced by 

 others, and the information so gained bears directly upon 

 the question of chemical structure". 



At the present time such evidence is being actively 

 sought among the various products derived from minerals, 

 either by the processes of nature or by experiments in the 

 laboratory. 



The experimental inquiry is one from which much may 

 be expected, but too little has yet been done in this direc- 

 tion. The suggestive experiments of Lemberg, Doelter 

 and others, are of the greatest importance, but cannot be 

 adequately summarised in the present article. But the 

 natural pseudomorphs, if due care be taken, as indicated 

 above, to select only those which are beyond doubt, supply 

 examples which can never be equalled by artificial means. 



In Clarke's elaborate memoir frequent use is made of 

 this line of reasoning, and several other papers have been 

 recently published in which the transformations disclosed by 

 natural pseudomorphs are made the basis of a theory of 

 chemical constitution, on the following principle : we may 

 conceive, it is argued, that while a silicate undergoes 

 change, certain radicles constitute a comparatively stable 

 kernel which persists as such, and unites with the incoming 

 radicles. 



Among the most familiar materials of silicate pseudo- 

 morphs are Kaolin, Serpentine, and Talc : they may be 

 taken as simple and typical examples affording arguments 

 as to the structure of certain silicates from which they have 

 been derived. 



(1) Kaolin H 4 Al,Si,O g (in which the water is all con- 

 stitutional) results from the decomposition of Orthoclase 

 K Al Si 3 O s . 



Scharizer [Zeitschr. f. Krystallograpkie, xxii., p. 369, 

 1893), wno assumes that in the natural metasomatic altera- 

 tion products the liberated radicles free just sufficient 

 valencies to saturate the water or hydroxyl introduced, 

 supposes the change to be due to the entrance of four 

 hydroxyl radicles w r hich saturate four freed valencies of 



