wherry: classes of crystals 485 



space-lattices and crystal molecules (in so far as the structure 

 of the latter affects X-rays) of the first two to have pyritohedral 

 symmetry, while the last is tetartohedral. It is to be inferred 

 that all members of the group in which the two negative atoms 

 are ahke would show the first type of symmetry, those in which 

 these atoms are unlike the second. The fact that many speci- 

 mens of pyrite actually show some tetartohedral features indi- 

 cates that the two sulfur atoms in these instances at least must 

 be unhke, not sufficiently to affect the X-ray spectra, yet dis- 

 tinctly enough to influence the habit and electrical properties. 

 This could occur if one of the sulfur atoms were tetravalent, the 

 other divalent, the structural formula of the compound (whenever 

 such a formula could apply) being Fe = S = S. On the other 

 hand the tetartohedral properties may be limited to pyrites in 

 which enough of the sulfur atoms, scattered through the mass, 

 are replaced by arsenic or some other element to give the struc- 

 ture the symmetry characteristic of the CoSAs class of com- 

 pounds. Further work will be necessary to decide between these 

 two possibihties, but it is evident that the crystallization of at 

 least some specimens of pyi'ite may be described as: 



System, cubic; structure, pyritohedral; structure-unit, tetarto- 

 hedral. 



Barium nitrate and the isomorphous strontium and lead salts 

 have been studied by X-rays by Nishikawa and Hudinuki,^ and 

 their space-lattices found to possess pyritohedral symmetry. 

 The habit and etch-figm-es are often tetartohedral, however, 

 although neither electric polarity nor optical rotatory power have 

 been observed in the crystals. The structure-units may there- 

 fore possess the diminished symmetry, while the structure as a 

 whole is pyritohedral: 



System, cubic; structure, pyritohedral; structure-unit, tetar- 

 tohedral. 



The crystal habit of rutile is usually holohedral, but hemi- 

 hedrism of an apparent trapezohedral type occasionally ap- 

 pears.^ The two series of observations which have been made 



' Proc. Tokyo Math. Phys. See. II, 9: 197. 1917. 

 8 ScHRAUF, Zeitschr. Kryst. Min. 9: 433. 1884. 



