7
Kaolinite is a regularly structured di-octahedral
two-layer mineral and preferentially develops
under warm and humid conditions, by chemical
weathering of feldspars in tropical soil. Good
drainage is essential to assure the removal of cations released during hydrolysis. Abundance and
distribution of kaolinite reflects soil-forming processes in the area of its origin which is optimal in
lateritic weathering in the tropics. Owing to the
fact that the occurrence of this mineral in ocean
sediments is distinctly latitude-dependent, it is often referred to as the “mineral of low latitudes”.
Illite is a three-layered mineral of the mica
group and not really a specified mineral, instead
the term illite refers to a group of mica-like minerals in the clay fraction; as such, it belongs to the
most frequently encountered type of clay minerals. Illites are formed as detrital clay minerals by
fragmentation in physical weathering. Chemical
weathering (soil formation) in which potassium is
released from muscovite also leads to the
formation of illites. For this reason, illite is often
referred to as incomplete mica or hydromica. The
distribution of illites clearly reflects its terrestrial
and detrital origin which is also corroborated by
K/Ar-age determinations made on illites obtained
from recent sediments. There are as yet no indications as to in-situ formations of illites in marine
environments.
Similar to the illites are the cation-rich, expandable, three-layered minerals of the smectite
group. The smectites, a product of weathering and
pedogenic formation in temperate and sub-arid
zones, hold an intermediate position with regard
to their global distribution. However, smectites are
often considered as indicative of volcanic environments, in fact smectite formation due to lowtemperature chemical alteration of volcanic rocks
is even a quite typical finding. Similarly, finely
dispersed particles of volcanic glass may transform into smectite after a sufficiently long
exposure to seawater. Smectites may also arise
from muscovite after the release of potassium and
its substitution by other cations. There is consequently no distinct pattern of smectite distribution
discernible in the oceans.
The generally higher occurrence of smectite
concentrations in the southern hemisphere can be
explained with the relatively higher input of volcanic detritus. This is especially the case in the
Southern Pacific.
Chlorite predominately displays a trioctahedral
structure and is composed of a series of three
layers resembling mica with an interlayered sheet
of brucite (hydroxide interlayer). Chlorite is mainly
released from altered magmatic rocks and from
metamorphic rocks of the green schist facies as a
result of physical weathering. It therefore charac1.2
Sources and Components of Marine Sediments
7 A
Kaolinite
10 A
Smectite
Illite
14 A
Chlorite
Exchangeable cations
nH 2 O
Oxygens
Hydroxyls
Aluminium, iron, magnesium
and Silicon, occasionaly aluminium
tetrahedra
octahedra
hydroxyoctahedra
tetrahedra
tetrahedra
tetrahedra
tetrahedra
octahedra
octahedra
Fig. 1.4 Schematic diagram of clay mineral types: Left: According to the combination of tetrahedral- and octahedralcoordinated sheets; Right: Diagrammatic sketch of the structure of smectite (after Hillier 1995 and Grim 1968).
Kaolinite is a regularly structured di-octahedral
two-layer mineral and preferentially develops
under warm and humid conditions, by chemical
weathering of feldspars in tropical soil. Good
drainage is essential to assure the removal of cations released during hydrolysis. Abundance and
distribution of kaolinite reflects soil-forming processes in the area of its origin which is optimal in
lateritic weathering in the tropics. Owing to the
fact that the occurrence of this mineral in ocean
sediments is distinctly latitude-dependent, it is often referred to as the “mineral of low latitudes”.
Illite is a three-layered mineral of the mica
group and not really a specified mineral, instead
the term illite refers to a group of mica-like minerals in the clay fraction; as such, it belongs to the
most frequently encountered type of clay minerals. Illites are formed as detrital clay minerals by
fragmentation in physical weathering. Chemical
weathering (soil formation) in which potassium is
released from muscovite also leads to the
formation of illites. For this reason, illite is often
referred to as incomplete mica or hydromica. The
distribution of illites clearly reflects its terrestrial
and detrital origin which is also corroborated by
K/Ar-age determinations made on illites obtained
from recent sediments. There are as yet no indications as to in-situ formations of illites in marine
environments.
Similar to the illites are the cation-rich, expandable, three-layered minerals of the smectite
group. The smectites, a product of weathering and
pedogenic formation in temperate and sub-arid
zones, hold an intermediate position with regard
to their global distribution. However, smectites are
often considered as indicative of volcanic environments, in fact smectite formation due to lowtemperature chemical alteration of volcanic rocks
is even a quite typical finding. Similarly, finely
dispersed particles of volcanic glass may transform into smectite after a sufficiently long
exposure to seawater. Smectites may also arise
from muscovite after the release of potassium and
its substitution by other cations. There is consequently no distinct pattern of smectite distribution
discernible in the oceans.
The generally higher occurrence of smectite
concentrations in the southern hemisphere can be
explained with the relatively higher input of volcanic detritus. This is especially the case in the
Southern Pacific.
Chlorite predominately displays a trioctahedral
structure and is composed of a series of three
layers resembling mica with an interlayered sheet
of brucite (hydroxide interlayer). Chlorite is mainly
released from altered magmatic rocks and from
metamorphic rocks of the green schist facies as a
result of physical weathering. It therefore charac1.2
Sources and Components of Marine Sediments
7 A
Kaolinite
10 A
Smectite
Illite
14 A
Chlorite
Exchangeable cations
nH 2 O
Oxygens
Hydroxyls
Aluminium, iron, magnesium
and Silicon, occasionaly aluminium
tetrahedra
octahedra
hydroxyoctahedra
tetrahedra
tetrahedra
tetrahedra
tetrahedra
octahedra
octahedra
Fig. 1.4 Schematic diagram of clay mineral types: Left: According to the combination of tetrahedral- and octahedralcoordinated sheets; Right: Diagrammatic sketch of the structure of smectite (after Hillier 1995 and Grim 1968).