THE 



AMERICAN JOURNAL OF SCIENCE 



[FIFTH SERIES.] 



Aet. XVII. — The Crystal Structure of Alahandite 

 (MnS) ; by Ealph W. G. Wyckoff. 



Method of Investigation. — The determination of the 

 crystal structure of this mineral furnishes an illustration 

 of the manner of combining, as a source of the data upon 

 which to base the study of the structure of a crystal, (1) 

 the reflection measurements from a single face of a 

 crystal, which give the absolute dimensions of the unit 

 cell together with the number of chemical molecules to be 

 pssociated therewith, and (2) reflections from the pow- 

 dered substance. The general method of obtaining the 

 structure of the crystal from these data is the same 

 which has previously been used^ and is based primarily 

 upon the analytical results of the theory of space 

 groups.^ 



The Crystallography of Alahandite, — The specimen 

 used for this investigation was a cube nearly a centi- 

 meter upon a side.^ Alahandite is usually assigned to 

 the tetrahedral class (hemimorphic hemihedry) of the 

 cubic system on the basis of its showing tetrahedral faces 

 and faces of the form (211). From the existing crystal- 

 lographic evidence, however, it seems equally capable of 

 assignment to the tetartohedral class of symmetry. 



The Number of Chemical Molecules within the Unit 

 Cell. — A comparison reflection photograph of the L 

 series lines of tungsten using as gratings the (100) face 



^ Ealph W. G. Wyckoff, this Journal, 1, 138, 1921 ; etc. 



^ Ealph W. G. Wyckoff, this Journal, 1, 127, 1921, and other work as yet 

 unpublished; P. Niggii, Geometrische Krystallographie des Discontinuums, 

 Leipzig (1919). 



^ This material was loaned by the United States National Museum through 

 the courtesy of W. T. Foshag. Locality: Puebla, Mexico. Museum No. 

 19565. Though its exact composition is unknown, it seems to be quite pure 

 manganese sulphide. 



Am. Jour. Sci.— Fifth Series, Vol. II, No. 11.— November, 1921. 

 17 



