SECT. 3] 



PELAGIC SEDIMENTS 



703 



in coordination with oxygen or fluorine substituting in the tetrahedral sihcate 

 sheets. However, Harder (1959) has recently demonstrated that borate ion 

 is readily and irreversibly fixed from solution by natural iMd mica. Further, 

 measurements of the rate of passage of dissolved aluminum through the ocean 

 (Sackett and Arrhenius, op. cit.) show that only a small fraction of the total 

 aluminosilicate deposition can be accounted for in this way (of. Sillen, 1961). 



Montmorillonoid minerals of nontronite type occur in significant quantities 

 around centers of volcanic activity, like the Cape Verde and the Hawaiian 

 Islands, and in general in areas with a high proportion of pyroclastic sediments 

 (Table VII). These minerals are usually assumed to have formed in situ by 

 decomposition of the volcanic glass. The highest relative crystallinity is found 



Table VII 



Concentration and Relative Crystallinity of Montmorillonite Species in Marine 



Sediments of Varying Geological Age. 



A purified sample of the API clay mineral standard 25 a, Upton, Wyoming, was arbitrarily 

 selected to represent pure and perfectly crystalline montmorillonite. Concentrations were 

 measured against synthetic boehmite as internal standard. 



Sample 



Geological 

 age 



Montmorillonite 



A 



Concentration, Crystallinity, 



0/ 0/ 



/o /o 



Bentonite, Gotska Sandon, Sweden Ordovician*'' 

 Bentonite, Mossen, Kinnekulle, Sweden Ordovician^ 



"Red Clay", Noil Tobe, Timor Jurassic" 



McBride Chalk, Alabama Tertiary 



Pelagic Clay, S. Pacific, Dwd 61 Quaternary 



Pelagic Clay, S. Pacific, Cap 32B Quaternary 



Pelagic Clay, S. Pacific, Cap 31B Quaternary 



33 

 41 

 36 

 16 

 26 

 16 

 15 



65 

 49 

 53 

 57 

 43 

 40 

 38 



«Thorshmd (1948, 1958); Waern et al. (1948); Hagemann and Spjeldnaes (1955). 

 «'Molengraaff (1920). 



in sediments of Paleozoic and Mesozoic age (Table VII), whereas the Cenozoic- 

 Recent marine montmorillonoid is poorly crystallized and has a limited ex- 

 pansibility. Pelagic montmorillonoids and IM micas may show high disorder 

 because they form as metastable structures, which is often the case at low 

 temperature synthesis; the well ordered zeolites, on the other hand, are stable 

 phases at the composition and in the pH range of sea-water (molar ratio of 

 large cations to magnesium ion ==9, pH approx. 7.3-8.5). This suggestion is 

 supported by laboratory studies of the stability relations of the minerals in 

 question. The higher crystallinity of the ancient montmorillonites may be the 

 result of a diagenetic process active after the removal of the sediment from the 

 original alkaline environment. 



