SECT. 3] 



PELAGIC SEDIMENTS 



667 



minerals, dominated by goethite and disordered FeOOH. With increasing 

 cobalt concentration in the rock, more of it substitutes in the manganese oxide 

 minerals, which may achieve concentrations of well above 1% of cobalt, nickel, 

 and copper. A variable fraction (mostly ^ to |) of the soluble iron prevails in the 

 form of goethite and in some instances as nontronite (the acid-soluble fraction 

 of sample no. 2, Dwd HD 72, in Table II contains one-third nontronite). The 

 remainder of the soluble iron is located in the manganese oxide minerals 

 substituting for divalent manganese (Buser, op. cit.). Conversely, between a 

 tenth and one per cent of manganese is always found in the separation group 

 which contains the goethite, probably substituting as Mn3+ for Fe3+ in this 



*W*» 



IT 



M 



II 



10/x 



Fig. 7. Rutile needles and multiple twin, Cap. 49 BG, Quaternary, North Equatorial 

 Pacific. 



mineral. This group also tends to concentrate the major fractions of molyb- 

 denum, lead, titanium, scandium, the rare earth elements, and thorium. It is 

 as yet uncertain which of the minerals constituting the group is responsible for 

 the accumulation of the first four of these elements; it appears that thorium 

 and the rare earth elements largely prevail as phosphates. 



The elements barium and strontium in manganese nodules appear to be 

 partitioned between the manganese oxide structures and the barite-celestite 

 solid solutions (see also Section 2-B). These latter crystals account for the 

 occasional high barium content of the acid-soluble and insoluble fractions in 

 Table II. Titanium in the insoluble residue (Table II) is present as small 

 euhedral crystals of rutile (Fig. 7) and anatase, the rutile frequently twinned 

 on Oil. The relatively high niobium content of the insoluble residue is probably 



