SECT. 3] PELAGIC SEDIMENTS 663 



In the map showing the distribution of pelagic sediments (Fig. 1), this broad 

 genetic classification is attem23ted, based in some instances on assumptions 

 which, admittedly, are as yet unproven. However, the accomit given below of 

 the composition of such sediments is based solely on the observed properties 

 of the constituent minerals, whose possible sources of origin are discussed 

 subsequently. In this way the basic observations have been separated from the 

 interpretations . 



A. Elements and Oxides 



Iron and manganese oxide mineral aggregates constitute one of the major 

 types of rock encountered on the ocean floor; according to Menard (unpublished) 

 about 10% of the pelagic area of the Pacific is covered by such nodules. 

 Measurements from the northeast Pacific by Skornyakova (1960) and Mero 

 (1960a) give similar averages and indicate a considerable local variability in 

 concentration (Fig. 5). 



The nodules consist of intimately intergrown crystallites of different minerals ; 

 among those identified, besides detrital minerals and organic matter, are opal, 

 goethite, rutile, anatase, barite, nontronite, and at least three manganese 

 oxide minerals of major importance, described by Buser and Griitter (1956), 

 Griitter and Buser (1957, 1959). One of these minerals, identical with 

 a synthetic phase described in the chemical literature as 8-Mn02, forms 

 aggregates of randomly oriented sheet units as small as 50-100 A. The other 

 two minerals possess a double layer structure similar to that of lithiophorite 

 and to synthetic phases, in chemical nomenclature called manganites.i These 

 consist of ordered sheets of Mn02, alternating with disordered layers of 

 metal ions, co-ordinated with water, hydroxyl and probably also other anions. 

 The most prevalent cations in the disordered layer are Mn2+ and Fe3+. It is 

 assumed that Na, Ca, Sr, Cu, Cd, Co, Ni, and Mo also substitute in this part of the 

 structure. Two species of this general type have been observed, one with a basal 

 spacing of 10 A, the other with 7 A. Buser has interpreted the 10 A spacing as 

 due to the existence of two discrete layers, OH and H2O, whereas in the 

 collapsed structure these groups are assumed to form a single layer, but owing 

 to the disorder, the structure has not yet been precisely described. Syn- 

 thesis experiments demonstrate that the three manganese oxide minerals 

 represent increasing degrees of oxidation at formation in the order 10 A 

 "manganite," 7 A "manganite", and 8-Mn02. Buser has pointed out the 

 potential use of this information in interpreting the conditions of formation of 

 pelagic sediments. 



Recent determinations of the bulk composition of manganese nodules have 

 been made by Goldberg (1954), by Riley and Sinhaseni (1958), and by Mero 

 (1960, and unpublished), and specific elements have been reported on by a 

 number of authors, Mero's data, compiled in Table I, demonstrate the wide 



1 Not identical with the mineral manganite, y-MnOOH, which is monoclinic-pseudo- 

 rhombic and isotypical with the diaspore group (Strunz, 1957). 



