GEOLOGICAL, MINERALOGICAL, CERAMIC APPLICATIONS 



the radiograph than optically. So, also, does 

 the fine structure of concentric layers charac- 

 teristic of the ooliths. The latter are often 

 built around a quartz fragment and contain 

 layers of varying iron content, most prob- 

 ably in the form of a hydrous iron-alumino- 

 silicate. 



The presence of films of minerals with 

 strong optical absorption or of staining 

 within grains can give a false impression of 

 the relative amount of opaque and trans- 

 parent material in a section. If in the case of 

 iron ores, say, the iron content is judged by 

 the amount of opaque material then clearly 

 the estimate will always be high. Figs. 3 and 

 4 show a photomicrograph and a radiograph 

 of an iron ore which demonstrates this 

 effect. Some of the ooliths are transparent 

 to both light and x-rays and must contain 

 little iron. Others which are opaque to light 

 do in fact contain some iron but, as can be 

 clearly seen in the radiograph, the iron is 

 very thinly distributed and the total iron is 

 still very small compared to the siderite 

 matrix. 



Figs. 5 and 6 show a similar ore of marine 

 origin which consists of ooliths of iron silicate 

 clay minerals etc. along with fossil fragments 

 embedded in a fine grained matrix of crys- 



FiG. 4. Microradiograph which reveals the iron 

 content of the ooliths to be low and generally dis- 

 persed. The matrix of siderite strongly absorbs the 

 Cu radiation and hence shows white. X45 



Fig. 3. Iron ore showing opaqne and trans- 

 parent ooliths embedded in siderite matrix. 

 Photomicrograph — ordinary light. X45 



Fig. 5. A fossiliferons oolitic iron ore. The 

 matrix is iron carbonate and the ooliths mainly 

 iron alumino-silicate. The bivalve shell in the 

 lower part is completely replaced by iron car- 

 bonate. Photomicrograph — ordinary light. X17 



talline iron carbonate. The lower half of the 

 section contains the fossil of a bivalve shell 

 and shows the internal loop-like structure 

 very well. The whole of the shell has been 

 replaced by iron carbonate and a part of 

 the internal cavity has become filled with 

 siderite mud, some crystalline siderite and 



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