96 tutton: x-ray anaIvYsis and assignment of crystals 



With regard to the pyrites family of minerals, it is considered 

 that the Bragg results indicate that the space-lattice and crystal 

 molecules of pyrites, FeSo and hauerite, MnSo, in which the two 

 negative atoms are alike, possess pyritohedral (pentagonal dodeca- 

 hedral) symmetry, and those of cobaltite, CoAsS, in which the 

 two negative atoms are different, "tetartohedral" symmetry. 

 As some specimens of pyrites exhibit tetartohedral traits it is 

 further assumed that the structure of pyrites is Fe = S = S, 

 the two sulphur atoms being of different valency, tetradic and 

 dyadic. The whole group is described as belonging to the cubic 

 system, with a "pyritohedral space-lattice" (sic, particularly 

 definitely stated in the table given), and a tetartohedral structure- 

 unit. 



Barium nitrate and its strontium and lead analogues are 

 similarly assumed, from the X-ray results of Nishikawa and 

 Hudinuki, to have "pyritohedral space-lattices," with a tetarto- 

 hedral structure-unit in each case. 



Rutile, TiOo, is assumed to be of holohedral habit, but to 

 exhibit occasionally trapezoidal hemihedrism. The somewhat 

 contradictory X-ray results of Vegard and of Williams are dis- 

 cussed, and the views of Williams adopted as more reasonable. 

 The conclusion is that the system is tetragonal, the space-lattice 

 holohedral, and the structure-unit trapezoidal. 



The final conclusion in the memoir is that both the symmetry 

 of the space-lattice as a whole, and that of the crystal molecules 

 or unit cells of the space-lattice, may find expression in significant 

 physical features, and that both should be taken into account in 

 the assignment of crystals to symmetry classes, even although 

 it may be necessary at times to state two different classes for 

 the same crystal. 



With the first portion of this conclusion all can agree, provided 

 (as is not stipulated in the memoir) that it be kept clear as a 

 fundamental fact, that it is the space-lattice that determines 

 the crystal-system and the obedience to the law of rational 

 indices, while it is the structural detail (represented only by 

 a point in the space-lattice) that determines which particular 



