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LY. Results of Crystal Analysis. — III. 

 By L. Vegard, Dr. phil., University of Christiania *. 



[Plate XII.] 



§ 1. A S an addition to the results of Crystal Analysis 

 jljL given in two previous papers |, 1 shall give an 

 account of the determination of the structure of the following 

 crystals : 



Xenotime (YP0 4 ), 



Anatase (Ti0 2 ). 



Both crystals belong to the ditetragonal bipyramidal class 

 of the tetragonal system. 



Xenotime. 



§2. This mineral has a chemical formula analogous to that 

 of zircon (ZrSi0 4 ) ; but although the formula may have a 

 similar appearance, there are probably considerable differences 

 with regard to chemical constitution. Thus the two elements 

 associated with oxygen in the case of zircon have both a 

 valency of four, while in the case of xenotime the one 

 element (Y) has a valency of three, the other one (P) a 

 valency of five. 



Now, in spite of their chemical differences, the two 

 substances have crystals which, both as regards form and 

 physical properties, are very similar. Both belong to the 

 ditetragonal bipyramidal class, and the ratios of the axes are 

 very nearly the same. 



For Zircon a : c=l : 06391, 



„ Xenotime a : c = l : 06177. 



In a previous communication it was found that in zircon 

 the Zr and Si atoms took up a similar position in the lattice 

 as regards their relation to the oxygen adorns. The arrange- 

 ment corresponds to a formula Zr0 2 Si0 2 and not Zr(Si0 4 ). 

 In view of the crystallographic similarity, the question arises 

 whether also the atoms in xenotime might be arranged in a 

 zircon lattice, which would mean that Y and P — in spite of 

 their difference of valency — would have the same relation to 



* Communicated by the Author. 



t L. Vegard, Phil. Mag. January 1916, Paper I.; ibid. July 1916, 

 Paper II. 



Phil Mag. S. 6. Vol. 32. No. 191. Nov. 191G. 2 M 



