[ 305 



XXX. The Structure of the Spinel Group of Crystals: By 

 W. H. Bkagg, D.Sc, F.R.S., Cavendish Professor of 

 Physics in the University of Leeds *. 



THE spinel group of crystals is placed by crystallographers 

 in their Class 32, the members of which are cubic and 

 possess the highest possible number of symmetries. The 

 composition is given by the formula K"R 2 '"04, where the- 

 divalent metal R" may be Mg, Fe, Zn, or Mn, and the tri- 

 valent metal R'" may be Fe, Mn, Or, or Al. It is very 

 interesting that a somewhat complicated composition should 

 be associated with such complete crystalline symmetry. 



Magnetite, FeFe 2 4 , is a member of this group. Its 

 X-ray spectra are shown in fig. 1, the same method of 



Fiff. 1. 



(100) 







1 









(no) 



184 



I 



76 





i 





(III) 



43 ;270 



i 1! , 



8 





1 



6 

 . 

 -*-sm9 



46 

 1 



78 

 0-2 



40 



C 



20 37 

 3 



•4 

 4 0- 



0-5 



Spectra of magnetite 



representation being used as in previous cases. The heights 

 of the vertical lines represent the intensities of the various 

 orders of reflexion by the three most important planes, and 

 their positions represent the sines of the glancing angles of 

 reflexion. The X-ray used is the «-ray of rhodium for 

 which \ = 0-614 A.U. 



We first find the spacings of the three sets of planes. 

 According to the usual formula, A, = 2dsin 6. For the (100) 

 planes 6 = b° 30 . 



0'614 M^TT 



Hence 2d l00 = ^^ = 4-15 A.U. 



0*1478 



In the same way 2d no = 5*88 A.U., and 2d ul 

 We now try to connect these values with th 



dimensions of the crystal. 



* Communicated by the Author. 



Phil. Mao. S. 6. Vol. 30. No. 176. Aug. 1915. 



= 9-60 A.U. 

 molecular 



X 



