516 



Dr. L. Vegard on 



the lattice must be drawn out in this direction, and also in 

 this case the position of the oxygen atoms accounts for the 

 value of c/a, which in this case is greater than unity. 



To make clear the relation between the lattices of anatase 

 and rutile, we imagine that in the latter substance we 

 remove one of its two Ti lattices of the diamond type and 

 the oxygen atoms associated with the Ti atom. The lattice 

 left with the molecular axes in tetragonal arrangement in 

 planes perpendicular to the tetragonal axis does rot seem to 

 be stable. If, however, all molecular elements are turned 

 through an angle of 90°, so as to have their axes all parallel 

 with the tetragonal axis, the configuration becomes stable 

 and forms the mineral anatase. 



Photographs of models of the lattices of xenotime and 

 anatase are shown on Plate XII. 



§ 7. The absolute dimensions of the lattices are given in 

 Table Y. For the sake of comparison I have also given the 

 dimensions of the Zircon group. 







Table V. 







Substance. 



a. 



c. 



c/a. 



V. 



I. 



Zr0 2 Si0 2 



(Sn0 o ) o 



10- 8 cm. 

 9-20 



935 



9-05 



5-27 

 9-60 



10- 8 cm. 



5-87 



6-29 



5-83 



9-37 

 5-94 



0-639 



0-673 



0-644 



T777 

 0-618 



10 ~ 22 cm. 3 

 4-97 



550 



4-77 



2-60 

 549 



10- 8 cm. 

 (Zr) 2-71 

 (Hi) 1-08 



2-08 



1-99 



1-95 

 1-23 



(TiO,) 2 (Rutile) 



Ti0 2 (Anatase). 

 YP0 4 



V is the volume of the elementary lattice, I is the distance 

 from an oxygen atom to the central atom to which it belongs. 



We see that also with regard to the absolute dimensions 

 of the lattice xenotime comes very close to the Zircon group. 



The minerals of the Zircon group show a small, but regular 

 increase of dimensions with increase of atomic number of 

 the central atom of the molecular group. Comparing the 

 dimensions of rutile and anatase, we notice that in the 

 direction which is perpendicular to all molecular axes the 

 linear dimensions are nearly equal, and the value of a of 

 rutile is nearly equal to the value of c of anatase. This 

 may be due to the fact that in both cases there is the same 

 number of molecular elements inside the lattice which have 

 their molecular axis directed along the axis considered. 



We may notice that the absolute dimensions are calculated 



