398
11 Manganese: Predominant Role of Nodules and Crusts
Micronodules from the C-C F.Z. occur mainly within
siliceous ooze. They consist of 10 Å manganate with
traces of quartz and sometimes phillipsite. They have
Mn/Fe ratios of 4.7 and Ni+Cu contents of 1.7%. Again,
the Mn/Fe ratios are somewhat higher than for the
associated nodules but the Ni+Cu contents somewhat
lower. The micronodules are dominantly spheroidal or
have rod-like structures. Under the SEM, the surfaces
of the micronodules consist of plates.
Micronodules from the Peru Basin occur in three
stratigraphic horizons. The upper 50-100 mm of the
core consists of a homogeneous brown calcareoussiliceous mud containing a high abundance of
micronodules. The micronodules are larger (125-500
mm) than those taken deeper in the core (40-125mm)
and have a mamillated botryoidal surface texture similar
to that of the associated macronodules. They consist
of 10 Å manganate with traces of quartz and feldspar.
They have Mn/Fe ratios of 8.7 and Ni+Cu contents of
2.1%. These sediments are underlain by a highly
bioturbated light yellowish brown mud containing
varying amounts of calcareous and siliceous organisms. These micronodules have a smooth surface
texture. The replacement of radiolaria and foraminifera
can be clearly seen under the SEM. The micronodules
are coarsely crystalline with well-defined rod-like
structures. They consist of 10 Å manganate with traces
of quartz and feldspar and have Mn/Fe ratios of 1.9
and Ni+Cu contents of 0.9%. The sharp drop in the
Mn/Fe ratios of these micronodules compared with
those from the sediment surface suggests that remobilization of Mn and associated transition elements
has taken place within this sediment horizon. At the
base lies a dark overconsolidated clay with a low
content of siliceous tests but containing volcanic glass
and fish teeth. These micronodules have Mn/Fe ratios
of 5.0 and Ni+Cu contents of 1.3%. The high Mn/Fe
ratios of these micronodules suggest a well-oxidized
sedimentary environment from which no diagenetic
remobilization has taken place. The importance of
element remobilization in the yellowish brown muds is
confirmed by the fact that micronodule abundance is
high in the surface sediments, low in the yellowish
brown muds and high again in the dark brown clays.
A plot of Ni+Cu against Mn/Fe for micro-nodules
from various areas in the Pacific is presented in Figure
11.19. It is seen that, although Mn/Fe ratios in micronodules >20 can occur, particularly in the Peru Basin,
there is no well-defined point of reversal such as
observed for Mn nodules (Fig. 11.14), although there
is a flattening of the curve at a Mn/Fe ratio of about
10. This is a consequence of the fact that the micronodules occur within the sediment column where
conditions are less oxidizing than at the sediment
surface. Pore waters are not as enriched in Mn, Ni and
Cu within the sediment as at the sediment surface and
the Mn/Fe ratios and Ni+Cu contents of micronodules
are never as high as in the associated macronodules.
Diagenetic fractionation of elements in micronodules
is therefore not as pronounced as in the associated
macronodules.
11.4.8 Mineralogy
There are three principal phases in manganese
nodules, Mn oxides which tend to incorporate
cationic transition metal species such as Ni
2+
,
Cu
2+
and Zn
2+
, Fe oxyhydroxides which tend to
incorporate anionic species such as HPO 4
2,
HAsO 4
2, HVO 4
2, MoO 4
2and WO 4
2as well as the
REE and Co
3+
and detrital aluminosilicates which
consist of elements such as SiO 2 , Al 2 O 3 , TiO 2 and
Cr 2 O 3 . In addition, carbonate fluorapatite (francolite) also occurs in older generations of Co-rich
manganese crusts and as a discrete layer between the older and younger crust generations.
Concentrations of individual elements in deepsea Mn nodules and crusts therefore tend to
covary with the relative amounts of these four
phases (cf. Koschinsky and Halbach 1995; Koschinsky and Hein, 2003).
Three principal Mn oxide minerals are found in
Mn nodules and crusts. Their principal X-ray diffraction peaks are given below together with their alternative mineral names (in parenthesis):
10 Å manganate 9.7 Å 4.8 Å 2.4 Å 1.4Å
(todorokite, buserite, 10 Å manganite)
7 Å manganate
7.3 Å 3.6 Å 2.4 Å 1.4Å
(birnessite, 7 Å manganite)
δMnO 2
2.4 Å 1.4Å
(vernadite)
Mn oxide minerals are characterized by fine grain
size in the range 100-1,000 Å. These minerals are poorly
crystalline and give diffuse X-ray diffraction patterns.
FeOOH minerals also tend to be fine grained with sizes
of the order of 100 Å and are generally X-ray
amorphous (Johnson and Glasby 1969). In chloridebearing solutions such as seawater, akagenéite (βFeOOH) has been identified as the principal iron
oxyhydroxide mineral present by Mössbauer spectroscopy (Johnston and Glasby 1978). However, there
are no independent Fe-bearing minerals in Mn nodules.
11 Manganese: Predominant Role of Nodules and Crusts
Micronodules from the C-C F.Z. occur mainly within
siliceous ooze. They consist of 10 Å manganate with
traces of quartz and sometimes phillipsite. They have
Mn/Fe ratios of 4.7 and Ni+Cu contents of 1.7%. Again,
the Mn/Fe ratios are somewhat higher than for the
associated nodules but the Ni+Cu contents somewhat
lower. The micronodules are dominantly spheroidal or
have rod-like structures. Under the SEM, the surfaces
of the micronodules consist of plates.
Micronodules from the Peru Basin occur in three
stratigraphic horizons. The upper 50-100 mm of the
core consists of a homogeneous brown calcareoussiliceous mud containing a high abundance of
micronodules. The micronodules are larger (125-500
mm) than those taken deeper in the core (40-125mm)
and have a mamillated botryoidal surface texture similar
to that of the associated macronodules. They consist
of 10 Å manganate with traces of quartz and feldspar.
They have Mn/Fe ratios of 8.7 and Ni+Cu contents of
2.1%. These sediments are underlain by a highly
bioturbated light yellowish brown mud containing
varying amounts of calcareous and siliceous organisms. These micronodules have a smooth surface
texture. The replacement of radiolaria and foraminifera
can be clearly seen under the SEM. The micronodules
are coarsely crystalline with well-defined rod-like
structures. They consist of 10 Å manganate with traces
of quartz and feldspar and have Mn/Fe ratios of 1.9
and Ni+Cu contents of 0.9%. The sharp drop in the
Mn/Fe ratios of these micronodules compared with
those from the sediment surface suggests that remobilization of Mn and associated transition elements
has taken place within this sediment horizon. At the
base lies a dark overconsolidated clay with a low
content of siliceous tests but containing volcanic glass
and fish teeth. These micronodules have Mn/Fe ratios
of 5.0 and Ni+Cu contents of 1.3%. The high Mn/Fe
ratios of these micronodules suggest a well-oxidized
sedimentary environment from which no diagenetic
remobilization has taken place. The importance of
element remobilization in the yellowish brown muds is
confirmed by the fact that micronodule abundance is
high in the surface sediments, low in the yellowish
brown muds and high again in the dark brown clays.
A plot of Ni+Cu against Mn/Fe for micro-nodules
from various areas in the Pacific is presented in Figure
11.19. It is seen that, although Mn/Fe ratios in micronodules >20 can occur, particularly in the Peru Basin,
there is no well-defined point of reversal such as
observed for Mn nodules (Fig. 11.14), although there
is a flattening of the curve at a Mn/Fe ratio of about
10. This is a consequence of the fact that the micronodules occur within the sediment column where
conditions are less oxidizing than at the sediment
surface. Pore waters are not as enriched in Mn, Ni and
Cu within the sediment as at the sediment surface and
the Mn/Fe ratios and Ni+Cu contents of micronodules
are never as high as in the associated macronodules.
Diagenetic fractionation of elements in micronodules
is therefore not as pronounced as in the associated
macronodules.
11.4.8 Mineralogy
There are three principal phases in manganese
nodules, Mn oxides which tend to incorporate
cationic transition metal species such as Ni
2+
,
Cu
2+
and Zn
2+
, Fe oxyhydroxides which tend to
incorporate anionic species such as HPO 4
2,
HAsO 4
2, HVO 4
2, MoO 4
2and WO 4
2as well as the
REE and Co
3+
and detrital aluminosilicates which
consist of elements such as SiO 2 , Al 2 O 3 , TiO 2 and
Cr 2 O 3 . In addition, carbonate fluorapatite (francolite) also occurs in older generations of Co-rich
manganese crusts and as a discrete layer between the older and younger crust generations.
Concentrations of individual elements in deepsea Mn nodules and crusts therefore tend to
covary with the relative amounts of these four
phases (cf. Koschinsky and Halbach 1995; Koschinsky and Hein, 2003).
Three principal Mn oxide minerals are found in
Mn nodules and crusts. Their principal X-ray diffraction peaks are given below together with their alternative mineral names (in parenthesis):
10 Å manganate 9.7 Å 4.8 Å 2.4 Å 1.4Å
(todorokite, buserite, 10 Å manganite)
7 Å manganate
7.3 Å 3.6 Å 2.4 Å 1.4Å
(birnessite, 7 Å manganite)
δMnO 2
2.4 Å 1.4Å
(vernadite)
Mn oxide minerals are characterized by fine grain
size in the range 100-1,000 Å. These minerals are poorly
crystalline and give diffuse X-ray diffraction patterns.
FeOOH minerals also tend to be fine grained with sizes
of the order of 100 Å and are generally X-ray
amorphous (Johnson and Glasby 1969). In chloridebearing solutions such as seawater, akagenéite (βFeOOH) has been identified as the principal iron
oxyhydroxide mineral present by Mössbauer spectroscopy (Johnston and Glasby 1978). However, there
are no independent Fe-bearing minerals in Mn nodules.