Results of Crystal Analysis. 



511 



calculated and observed values for the other three faces, we 

 put a =65°. 



In the following Table II. are given the calculated 

 intensities corresponding to this value of a, as also the 

 observed intensities. 



Table II. 



«=65°. 



(111). 



(110). 



(100). 



(101). 



leal. 



lobs. 



leal. 



lobs. 



leal. 



lobs. 



leal. 



lobs. 



0-4 

 

 12 

 100 



<20 



<20 



<20 



100 



100 



45 



20 



100 

 58 

 25 



100 

 74 



100 

 70 



100 



9 



1 



100 



17 









The agreement between calculated and observed values is 

 quite satisfactory. 



§ 4. Comparing the lattices found for the xenotime with 

 that of zircon, we see that the lattice at the same time 

 accounts for the crystallographic similarity and the difference 

 in chemical constitution. 



Both lattices have the symmetry properties which are 

 necessary to give a crystal of the ditetragonal bipyramidal 

 class. 



If we would imagine the oxygen atoms removed the two 

 lattices would be exactly of the same type. 



In both lattices the same relative number of oxygen atoms 

 are arranged with tetragonal symmetry in a plane perpen- 

 dicular to the tetragonal axis, which accounts for the fact 

 that the ratio c/a is almost the same for both minerals. 



The difference found for the arrangement of the oxygen 

 atoms accounts for the different chemical constitution to be 

 expected for the two minerals. In zircon both Zr and Si 

 take up essentially the same position towards the oxygen 

 atoms ; in xenotime, however, the oxygen atoms are 

 attached in a singular way to the P atoms and we get groups 

 (radicles) of the composition P0 4 . Thus the liontgen-ray 

 analysis of the space-lattice gives as the constitution formulae: 



Zr0 2 Si0 2 for Zircon and Y(P0 4 ) for Xenotime. 



