318 " R. W. G. Wyckoff— Crystal Structures of 



The Structure of Calcite. 



Crystallo graphic Data: 6 — Calcite lias the symmetry of 

 the ditrigonal scalenohedral class of the hexagonal sys- 

 tem (holohedry of the rhombohedral division). 

 a (the angle between the rhombohedral axes) =101° 55'. 

 The reflection measurements 7 give the ratio of the order 

 of the reflection to the number of molecules associated 

 with the unit of structure. There are, of course, two pos- 

 sible fundamental units, a rhombohedron and an hex- 

 agonal prism. Assuming the former, if n is the order of 

 the reflection and m is the number of molecules associated 

 with the unit, and using the value of d/n = 3.04 X 10 8 

 cm., 7 the ratio n s /m becomes, 



"- = 2-08. 



Ill 



This very strongly suggests a second order of reflection 

 from a rhombohedron containing four molecules of cal- 

 cium carbonate. 



The specimens used in the present instance were from 

 a clear piece of Iceland spar. Photographs of the 

 L-series lines of tungsten, using this material as a grat- 

 ing, gave spacings that agreed with the previous measure- 

 ments to within the limit of error of the experiment. 



Laue photographs were prepared by passing the X- 

 rays through sections ground parallel to the basal plane, 

 (111), and through a cleavage piece. Exposures were 

 taken with the rays normal to these faces, (111) and 

 (100), and also inclined at small angles to the normal. 

 These plates averaged about 350 spots apiece. The in- 

 dices of the planes producing the spots were determined 

 by plotting the photographs in gnomonic projection. 

 When the crystaliographic axes (a = 101°55') are used to 

 determine the indices of the reflecting planes, there does 

 not seem to be any simple agreement with the observed 

 pattern ; such agreement, however, is obtained if axes are 

 chosen which are the diagonals of the faces of the rhombo- 

 hedron formed by the first set of axes. It is this second 

 set of trigonal axes which will be used in the computa- 

 tions. The relation between the two sets of axes is shown 

 in fig. 1, A. 



6 P. Groth, Chemische Krystallographie II, 204, 1908. 



7 W. L. Bragg, op. cit. 



