the nature and origin of authigenic smectites in some recent marine ...

244 T. G. Cole and H. F. Shaw
FIG 3. Scanning electron micrographs of sediments from the Bauer Deep. (a) Well-preserved
siliceous microfossils in surface sediments (x 1200); (b) Corrosion and nontronite infiIling of
siliceous microfossils in buried sediments (x 1200); (c) Nontronite infilling of siliceous
microfossils (x 3200).
that the silica source for the clay mineral is biogenic in origin. Qualitative elemental
analysis of the infilling clay material indicated that it was composed predominantly of Si
and Fe.
The iron necessary for the formation of the nontronite originates from the East Pacific
Rise as a hydrothermal precipitate. This is transported, along with other trace elements, as
X-ray amorphous Fe- (and Mn-)oxyhydroxide flocs which are ponded in the Bauer Deep
(Dymond & Veeh, 1975; Lonsdale, 1976; Heath & Dymond, 1977; 1981; Edmond et al.,
1982; Cole, 1982). The Fe-oxyhydroxide probably becomes associated with the biogenic
opal in the Bauer Deep sediments by coordination of Fe(III) to silanol groups (SiO-) at
the surfaces of the dissolving microfauna to form an Fe(III)-Si complex which is the
precursor of the nontronite. Harder (1976; 1978) has synthesized Fe-rich layer-silicates,