80 



Dr. L. Vegard on 



Zr and Si atoms relative to the axis of the crystal and 

 relative to each other. 



With regard to the direction of the lines, let us first 

 determine their position with regard to the tetragonal axis, 

 and consider the following three cases : — 



(1) All lines are parallel to the tetragonal axis. This 

 orientation is excluded because for zircon it would make the 

 second order spectrum of face (111) vanish. 



(2) The lines belonging to one sort of atoms (Zr say) 

 might be parallel to the tetragonal axis, the lines of the 

 other sort perpendicular to this axis. The latter lines are 

 divided into two groups in such a way that an individual 

 of one group is perpendicular to one of the other. Such an 

 arrangement would not explain the distribution of the 

 intensities of the face (111) for zircon and the disappearance 

 of the first order spectrum of the (111) face of rutile and 

 kassiterite. 



(3) All lines are perpendicular to the tetragonal axis. 

 Let us consider the lines through the Zr atoms. The lines 

 through the atoms belonging to one of the face-centred Zr 

 lattices must be perpendicular to the lines through the atoms 

 of the other face-centred lattice. In order to preserve the 

 right spacing for the faces (100) and (111), the lines must 

 be drawn so as to halve the angle between the sides (a) of 

 the square base of the lattice. 



The lines through the Si atoms must be arranged in the- 

 same way. 



Fie:. 4. 



-#r-V — 



<U — kL_J>: — A 



H>* 



V—^ 



i ^ A • 1 



• Zr atoms 

 OS/ -„— 



With regard to the relative position of the lines through 

 the Zr atoms and those through the Si atoms, there are two 

 different possibilities, which will be apparent from fig. 4, 



