some Carbonates of the Calcite Group. 353 



complicated nature, and the relative weakness of all the 

 planes having all odd indices, except planes giving curve 

 7, cannot be satisfactorily accounted for if u=3. It must 

 consequently be concluded from these data that u has a 

 value near to 0.27, certainly not appreciably greater than 

 0.27 and probably between 0.26 and 0.27. A more accu- 

 rate placing of the oxygen atoms is not possible without 

 the use of assumptions concerning the influence of spacing 

 upon intensity of reflection. 



The dimensions of the unit of structure of rhodochro- 

 site and the positions of the atoms within this unit are 

 then as follows (fig. 13) : 



Length of side of unit rhombohedron === 5.61 8 X 10~ 8 cm. 



Angle between axes = 47° 46'. 



C atoms at (000) and (a/2 a/2 a/2). 



Mn atoms at (a/4 a/4 a/4) and (3a/4 3a/4 3a/4). 



O atoms at (u u 0), (u u), (0 u u), (a/2 — u, u — a/% 



a/2), {u — a/2, a/2, a/2 — u), and {a/2, a/2 — 



■u, u — a/2), 



the length of the unit is written as a. The value of u is 

 close to 0.27. 



The Structure of Siderite (FeC0 3 ). 



Siderite and rhodochrosite have almost identical axial 

 ratios. 34 Comparison spectra against the (100) faces of 

 calcite and siderite gave the spacing of the latter as 

 2.81 X 10 8 cm., — a value which is practically the same as 

 that for rhodochrosite (2.83 X 10~ 8 cm.). A series of 

 Laue photographs through a basal section gave patterns 

 which were not distinguishable from those of rhodochro- 

 site. 35 The structures of these two compounds, then, are 

 identical within the limits of these experiments. 



The Structure of Magnesite (MgCO B ). 



Only one specimen of MgC0 3 was available for study. 

 This was a short six-sided prism. The pattern obtained 

 showed the same characteristics and reflections from the 

 same kinds of planes as did those from other members of 

 the group, but the crystal was so imperfectly constructed 

 that careful measurement was impossible. 



34 Dana, op. cit. 



35 Measurements of refractive index (by H. E. Merwin) indicated that 

 the specimens used here were about 95% FeC0 3 . 



