avherry: compositions of minerals 111 



MINERALOGY. — Variations in the compositions of minerals. 

 Edgar T. Wherry, U. S. National Museum. ^ Communi- 

 cated by Waldemar T. Schaller. 



"With the advance of scientific knowledge it becomes necessary 

 from time to time to revise some of our most fundamental con- 

 ceptions and definitions. In chemistry, for instance, the old 

 view is being abandoned that mixtures differ from compounds 

 in that the former can be separated into their constituents by 

 ^'mechanical means" while the latter cannot. - 



Most of our text-books of mineralogy are largely based on 

 Dana's System of 1892, and ignore the important advances which 

 have taken place since that time. In particular, the writer be- 

 lieves that the conception of mineral species which permeates 

 them is in need of revision, for, in the light of recent work, vari- 

 ability of composition must apparently be recognized as a funda- 

 mental principle of mineralogical chemistry. It is therefore pro- 

 posed that a species be defined as a natural inorganic substance 

 whose che7nical and physical properties are constant only within 

 certain limits, these limits varying widely from one case to an- 

 other. The possible ways in which variations in the chemical 

 composition of minerals can occur are through adsorption, iso- 

 morphous replacement, and solid solution. 



Adsorption. While most minerals exemplify the crj^stalloidal 

 state of matter, colloids are also not infrequently represented. 

 As has been pointed out by several writers,* all minerals appear- 

 ing wax- or gum-like, or with botryoidal, dendritic, globular, 

 mammillary, nodular, reniform, or stalactitic structures, and at 

 the same time showing an amorphous character under the mi- 

 croscope, are to be regarded as gels, or solid colloids. 



' Read at the Rochester meeting of the American Chemical Society, September, 

 1913. 



2 W. Lash Miller, Science, 34: 257 1911. 



3 F. Cornu, Centr. Min. Geol., 1909: 324; R. Marc. Fortschritte Min. Kryst. 

 Petr., 3: 11; A. Himmelbauer, ibid., 3: 32 (including a G-page bibliography), 

 1913. 



M}' excuse for treating this subject so fully at this place is that American 

 mineralogists appear to have underestimated the importance of these contribu- 

 tions: in six recent text-books 1 have been unable to find the word colloid. 



