506 



Dr. L. Vegard on 



the oxygen atoms. Thus the determination of the structure 

 of xenotime is of considerable importance for the question 

 of the part played by the valency in the constitution of 

 solids. 



The experimental determination of the X-ray spectra of 

 the different faces is somewhat difficult on account of the 

 fact, that most crystals are more or less transformed into 

 amorphous mass, are partly " metamict/' and as a conse- 

 quence, it was impossible to get reflexion from a number of 

 specimens tested. 



At last I got from the Geological Laboratory some fresh 

 and pure samples which gave fairly good reflexion for some 

 of: the faces. 



The spectra of the faces (100) and (110) were obtained 

 from a sample from the Langesundsfjord, Norway; those of 

 the faces (111) and (101) from a crystal from Raade, Norway. 



Several attempts to get reflexion from the base (001) 

 were not successful. 



The glancing-angles (0 n ) and intensities (I n ) of the 

 observed spectra for the four faces are given in Table I. and 

 in fig. 1. 



Table I. 

 YP0 4 . 



n. 



(100). 



(110). 



(101). 



(111). 



Bn, 



In. 



9 n . 



In. 



n . 



In. 



On- 



In- 





 

 

 1-5 



1 



2 



3 



4 



7° 16' 

 14 39 



1-4 



1-0 



5° 9' 

 10 21 

 15 37 



6-0 

 35 

 1-5 



6° 58' 

 14 2 



3-5 

 0-6 

 



15° 42' 



In the elementary cell ^ioo 2 ^ooi = ^ 



100 



we find from 



observations a number of molecules equal to 0'127 or J, 

 which is the same as was found for zircon. 



In a similar way, as in the case of zircon, we are led to 

 a similar space-lattice for the Y and P atoms, as found for 

 the Zr and Si atoms in zircon. If so, both the P and 

 Y atoms are arranged in a lattice of the diamond type. 



The lattice of P can be made to cover that of Y by a trans- 

 lation along the tetragonal axis a distance c/2. 



