The actively venting oxyhydroxide chimneys at Franklin Seamount, as seen in situ from
a submersible, commonly have a yellowish appearance with reddish orange
patches that become more common towards their inactive bases (Fig. 2a, see also Fig. 5a,b in ref. ).
These observations have been corrected for the color changes caused by
the longer wavelengths in the visible light spectrum being filtered out
by seawater. Chimney samples removed from the water were observed to
change to dark brown and reddish hues on the deck of the ship within a
few hours of exposure to the atmosphere (Fig. 2b, see also Fig. 5d, 6a,c in ref. ).
The samples are very friable with a roughly laminated to stromatolitic to porous, clotted texture (Fig. 2b).
Amorphous iron oxyhydroxides are commonly coated by the black Mn
oxyhydroxides, birnessite and todorokite. Dark green non-tronite is
found in the interiors of some samples although distinctive bright
green patches were also observed from the submersible on the surface of
an actively forming chimney near the vent orifice.  These bright green patches are suggestive of ferrous trioctahedral smectite, a reduced precursor to non-tronite. 
In thin section, the amorphous iron oxyhydroxide exhibits a complex
mottled structure of hair-like strands or scattered circular patches
together with filaments of probable biological origin that appear to
emanate from the strands and patches (Fig. 2c,d). The filaments have a sinuous, apparently branching, morphology (Fig. 2e), or have the appearance of bunched streamers (Fig. 2f). The textures in Fig. 2 suggest that the iron-rich material forms a coating.
SEM photographs clearly show the distinctive, apparently branching, filamentous micro-texture (Fig. 3a). The filaments are 5–15 μm long and 0.8 to 2 μm wide but thicken to up to 5 μm where coated with silica (Fig. 3b). Less common are some filament types that appear to be bunched into parallel strands (Fig. 3c) or short twisted and braided stalks (Fig. 3d). Fig. 3c,d
and eof samples from an actively venting chimney also show that much of
the iron oxyhydroxide appears to consist of formless agglomerates made
up of irregularly shaped blebs which surround the filaments. In some
samples, filaments appear to have grown out of the agglomerates (Fig. 3f,g).
It is difficult to determine if samples
containing voluminous apparently non-bacterial agglomerates are just
masked filamentous textures. This is probably not the case because the
observed filaments in such samples do not appear to be especially
thickened by additional iron oxyhydroxide deposition (Fig. 3a,b,c,d,e,f,g). Filaments in some samples appear to be degraded making their distinction from the formless micro-textures uncertain (Fig. 3h). However, Fig. 3d,e,f,g show non-degraded filaments adjacent to the formless blebs. These images are different from that exhibited in Fig. 3h. No evidence was found for formless micro-textures replacing degraded filaments.
Examination of the samples at higher magnifications by TEM reveals
the sinuous pseudo-branching habit of the filaments identified by SEM
as well as several other features. Filaments have a hollow structure
with rough walls consisting of bands of amorphous iron oxyhydroxide
parallel to the long axis (Fig. 4d). The twisted stalks observed by SEM (Fig. 3d) are made up of <0.5 μm interwoven strands (Fig. 4e). Amorphous iron oxyhydroxide coats the walls of some filaments (Fig. 4f)
but most of the individual filaments contain relatively little
additional iron deposition on their walls so it is thought that such
iron is accidental.
summary, the amorphous iron oxyhydroxide in the samples displays a
variety of morphologies that can be divided into three groups: (1)
morphologies which appear to form or mold the walls of filamentous
and/or stalked structures (Fig. 3a,b,c,d, and 4c,d,e); (2) morphologies which form discrete irregularly shaped formless agglomerates (Fig. 3c,d,e,f); and (3) morphologies which appear to have accumulated on the walls of the filaments and stalks (Fig. 4f).
The first two groups are more abundant but their proportions are highly
variable from sample to sample. The second group predominates in
freshly precipitated material sampled from the actively venting part of
a chimney, whereas nearly all of the material appears to be filamentous
in samples from other parts of active chimneys and from inactive