some Carbonates of the Calcite Group. 



347 



IV), though favorably situated for reflection, do not show 

 upon the plates. Fig. 14 represents the curve obtained by 

 plotting the relative amplitudes for the planes of part 

 A (Table IV) that are to be expected if u has a value be- 

 tween zero and one-half of the length of a side of the 

 unit when the arrangement is that of (g). Clearly since 

 all of these planes are found to give reflections, they must 

 agree in giving large amplitudes at the particular value of 

 u corresponding to the position of the oxygen atoms. 

 Fig. 15 is a similar curve for those planes which do not 

 appear (part B). It is to be expected that the ampli- 

 tude of all these planes should be slight at the value of u 

 for oxygen. 



An inspection of these two curves shows that the oxy- 

 gen atoms must lie close to positions defined by u = V±. 

 Furthermore, from the second curve it can be concluded 

 that the value of u for calcite is somewhere between u = 

 0.24 and u = 0.26. This seems to be as accurate a plac- 

 ing of the oxygen atoms as the photographs permit with- 

 out making rather uncertain assumptions concerning the 

 influence of mass and spacing of like planes upon reflect- 

 ing power. 



Table IV. 







A (Planes 



appearing) 







hkl 



Eelative spacing 



Curve 



No. 



hkl 



Eelative spacing 



Curve No, 



021 



.436 



1 





720 





.174 



3 



241 



.254 



2 





461 





.173 



4 



052 



.244 



2 





601 





.173 



5 



342 



.202 



3 





245 





.170 



5 



610 



.196 



3 





294 





.156 



4 



896 



.175 



1 





702 

 281 





.139 

 .137 



7 

 6 



B (Planes not appearing) 



hkl 



Eelative spacing 



Curve No 



131 



,325 



3 



540 



.224 



5 



351 



.206 



1 



261 



.179 



2 



551 



.179 



8 



443 



.154 



6 



085 



.149 



7 



751 



.149 



4 



