X-RAY MICROSCOPY 



luniiiar crystals showing lighter on the ra- 

 diograph. The absorption is greater because 

 the chrome grains contain MgO and AI2O3 

 as well as Cr203 and Fe203 . Fig. 19 is a good 

 example of these long cr3^stals of iron rich 

 spinel. Also, iron penetration can be seen 

 along cracks in the chrome grains and this 

 is often associated with diffusion of AI2O3 



Fig. 17. Characteristic structure formed near 

 the hot face in used chrome magnesite roof bricks. 

 A remnant grain of chrome ore is surrounded by 

 recrystallized magnesia which itself contains a fine 

 unresolved precipitate of MgO.Fe203 and is there- 

 fore lighter than that of Fig. 16. The elongated 

 iron-rich spinel crystals behind the chrome grain 

 are just differentiated. X18. Microradiograph — Ni 

 radiation. 



Fig. 18. Similar region to Fig. 17, but shuvving 

 resolution of the precipitate in the recrystallized 

 magnesia. Near the chrome grain the precipitate 

 has nucleated on the spinel grain leaving a margin 

 free from coarse precipitate. X38. Microradio- 

 graph — Ni radiation. 



Fig. 19. Chromite grain embedded in collum- 

 nar iron-rich spinel showing porosity due to loss 

 of material by diffusion. Microradiograph — Ni 

 radiation. X12.5 



Fig. 20. Iron rich magnesia sintered with 

 chrome ore showing the nucleating effect of the 

 chrome spinel and the orientation of the iron bear- 

 ing precipitate (MgO.Fe203) in the magnesia. 

 X30. Microradiograph — Ni radiation. 



away from the grain which leaves some pores 

 behind. 



The effect of high temperature sintering 

 on magnesia containing iron oxide in the 

 presence of chrome is shown in Fig. 20. The 

 precipitate in the MgO is just resolved in 

 this case but the most striking feature is 

 the nucleating effect of the spinel crystals. 



The study of porosity and iron distribution 

 in fireclay refractories has been helped by 

 radiography as is shown in Fig. 21 taken on 

 a section from a firebrick during work on 

 carbon disintegration, alkali attack and zinc 



580 



