JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. 9 MARCH 19, 1919 No. 6 



CRYSTALLOGRAPHY.— r/i^ classification of mimetic crystals. 

 Edgar T. Wherry and Eluot Q. Adams, Bureau of 

 Chemistry.^ 



As more or less fully described in all text-books on crystallog- 

 raphy, crystals which belong fundamentally to one system or 

 symmetry class may at times exhibit features characteristic of 

 other systems or classes. This may result from accidents of 

 growth, or from the approach of the angles to those of other 

 systems, either with or without twinning. It is customary to 

 group together part or all of these phenomena under the general 

 head of mimicry, mimetism, or mimetic behavior, and to add the 

 prefix pseudo- to the name of the system or class to which the 

 crystals apparently belong. In discussing certain crystallo- 

 graphic relationships, however, it may become desirable to dis- 

 tinguish the several types of mimetic phenomena on the basis 

 of their underlying causes, and to have special terms, for both 

 the original system or class and the one imitated, to apply in 

 each case. 



The principal types of mimetic behavior are presented in 

 table I, together with prefixes proposed for, and typical illustra- 

 tions of, each of them. The prefixes are derived from well- 

 known Greek roots, transliterated in accordance with accepted 

 usage. The illustrations are drawn from minerals, since the 

 features in question are most familiar in them, although the 

 greatest use for the classification may prove to be among artificial 



' Contribution from the Crystallography and Color Laboratories. 



153 



