684 



ARBHENIUS 



[chap. 25 



matter are absent. The oxidizing conditions of deposition are further indicated 

 by the low uranium content of this type of marine francoHte. Uranium probably 

 prevails in the hexavalent state (ionic radius 0.80 A) under the normal oxidiz- 

 ing conditions in sea-water, presumably as a carbonate complex (Starik and 

 Kolyadin, 1957), and does not easily substitute for the 20% larger calcium ion in 

 the francolite structure. However, where uranium is locally reduced to the tetra- 

 valent state, its increased ionic radius (0.97 A) is close enough to that of calcium 

 (0.99 A) to substitute extensively in the crystal structure (Altschuler, Clarke 

 and Young, 1958). Consequently, the concentrations of uranium in the seamount- 

 and reef-type francolite, formed under oxidizing conditions, amount to only 

 one twelfth to one hundredth of the concentrations found in francolite when 

 it is deposited under reducing conditions in areas of high organic productivity 

 (Arrhenius and Korkisch, 1959) (Table V). 



Table V 



Substitution of Uranium in Marine Authigenic (Halmeic) Apatite and in 



Microcrystalline Fish-Bone Debris 



Those apatite deposits which crystallized under reducing conditions, indicated by the 

 presence of organic compounds, glauconite and other ferrous minerals, are high in uranium, 

 presumably substituting as U4+. Conversely, those formed under oxidizing conditions, in- 

 dicated by lack of organic matter and ferrous minerals and by the coexistence with 

 manganese oxide minerals, have a low uranium content, probably due to the difference in 

 size between Ca2+ and U6+, and to the excess charge of the latter ion. 



Similar relations appear to govern the sorption of uranium on microcrystalline bone 

 apatite in oxidizing and reducing environments as shown in the lower part of the table. 



(Data from Arrhenius and Korkisch, 1959.) 



