156 WHERRY AND ADAMS: CLASSIFICATION OF MIMETIC CRYSTALS 



multiple twinning. The symmetry is usually increased by such 

 twinning, hence good descriptive prefixes are ana-, meaning "up- 

 wards" and syn-, "together." The common twinned crystals 

 of aragonitc, for instance, would then be termed: "ana-rhombic 

 but syn-hexagonal;" phillipsite, "ana-monoclinic but syn- 

 tetragonal;" boracite, "ana-rhombic but syn-cubic," and so on. 

 Less frequently the symmetry is decreased by twinning, where- 

 upon the original class may be designated by kata-, meaning 

 "downwards" while para- "beside" referring to the individual 

 parts of the twins, may be used for the imitated one. Thus 

 spinel and other cul^ic minerals, when twinned on the octa- 

 hedron, often become apparently trigonal; they may be de- 

 scribed as "kata-cubic but para-trigonal." 



There is still another situation in which the crystals of one 

 system may imitate those of another, consisting in the mere ap- 

 proach in angular values without twinning or distortion. It 

 should be noted here that the prefix hypo- has been applied 

 rather extensively to this type of relationship, though it is not 

 limited to mimetic crystals, but is used broadly for approach 

 of angular values to within 15° of those of the cubic, tetragonal, 

 or hexagonal systems. A new term is therefore needed for the 

 purpose of the present classification. The prefixes suggested to 

 describe this type of relationship are Icpto-, which means slight, 

 and peri-, which means around or about. For instance, the 

 triclinic plagioclase feldspars, such as albite, approach the mono- 

 clinic orthoclase very closely in angles and habit. The deviation 

 of their interaxial angles a and 7 from 90°, the value character- 

 istic of the monoclinic system, is but slight, and this can be well 

 expressed by calling them "lepto-triclinic but pcri-monoclinic," 

 that is, "weakly triclinic, and approaching monoclinic angular 

 relationships." Other well-known examples are chalcocite, which 

 is lepto-rhombic and peri-hexagonal; muscovite, lepto-mono- 

 clinic and peri-hexagonal; and chondrodite, lepto-monoclinic 

 and peri-rhombic. 



In conclusion it may be noted that the purpose of this paper 

 is not primarily the development of terms for these types of 

 mimetic phenomena, but rather the pointing out that it may at 



