CLAY MINERALOGY
H. Chamley, Universite ´ de Lille 1, Villeneuve d’Ascq,
France
Copyright & 2001 Elsevier Ltd.
Introduction
Clay constitutes the most abundant and ubiquitous
component of the main types of marine sediments
deposited from outer shelf to deep sea environments.
The clay minerals are conventionally comprised of
the o2 mm fraction, are sheet- or fiber-shaped, and
adsorb various proportions of water. This determines
a high buoyancy and the ability for clay to be widely
dispersed by marine currents, despite its propensity
for forming aggregates and flocs. Clay minerals in
the marine environments are dominated by illite,
smectite, and kaolinite, three families whose chemical composition and crystalline status are highly
variable. The marine clay associations may include
various amounts and types of other species, namely
chlorite and random mixed layers, but also vermiculite, palygorskite, sepiolite, talc, pyrophyllite,
etc. The clay mineralogy of marine sediments is
therefore very diverse according to depositional environments, from both qualitative and quantitative
points of view.
As clay minerals are considered to be dependent
on chemically concentrated environments, and as
they commonly form in surficial conditions on land
especially through weathering and soil-forming processes, their detrital versus authigenic origin in marine sediments has been widely debated. The
transition from continental fresh to marine saline
water, marked by a rapid increase of dissolved
chemical elements, was the central point of discussion and arose from both American and European
examples. In fact the mineralogical changes recorded
at the land-to-sea transition are either important or
insignificant, are characterized in estuarine sediments
by various, sometimes opposite trends impeding
consistent geochemical explanations, and often vanish in open marine sediments. The changes observed
at the fresh-to-saline water transition in the clay
mineral composition essentially proceed from differential settling processes or from mixing between
different sources, and not from chemical exchanges
affecting the crystalline network. Such a historical
debate underlines the interest in investigating the
sensitive clay mineral associations for understanding
and reconstructing environmental conditions. This
article will consider the general distribution and
significance of clay minerals in recent sediments,
some depositional and genetic environments, and a
few examples of the use of clay assemblages to reconstruct paleoclimatic and other paleoenvironmental changes.
General Distribution and Significance
As a result of extensive reviews made by both
American and Russian research teams the general
characters of the clay mineral distribution in deep sea
sediments have been known since the late 1970s. The
maps published by various authors demonstrate the
dominant control of terrigenous sources, which
comprise either soils and paleosoils or rocks. The
impact of soils on the marine clay sedimentation is
largely dependent on weathering intensity developing
on land, and therefore on the climate. For instance,
kaolinite mostly forms under intense warm, humid
conditions characterizing the intertropical regions,
and prevails in the clay fraction of corresponding
marine sediments. By contrast chlorite and illite
chiefly derive from physical weathering of crystalline
and diagenetic sedimentary rocks outcropping
widely in cold regions, and therefore occur abundantly in high latitude oceans. The kaolinite/chlorite
ratio in marine sediments constitutes a reliable indicator of chemical hydrolysis versus physical processes in continental weathering profiles and
therefore of climatic variations occurring on the land
masses.
Other clay minerals are also able to bear a clear
climatic message, as for instance the amount of
random mixed layers and altered smectite in temperate regions, the crystalline status of illite in temperate to warm regions, and the abundance of soilforming Al-Fe smectite in subarid regions. Detailed
measurements on X-ray diffraction diagrams, electron microscope observations and geochemical analyses allow precise characterization of the different
continental climatic environments from data obtained on detrital sedimentary clays.
Some terrigenous clay minerals in recent sediments
reflect both climatic and non-climatic influences. For
instance, the distribution of illite (Figure 1), a mineral that primarily derives from the erosion of
mica-bearing rocks, shows increased percentages in
high latitude oceans due to predominant physical
346
H. Chamley, Universite ´ de Lille 1, Villeneuve d’Ascq,
France
Copyright & 2001 Elsevier Ltd.
Introduction
Clay constitutes the most abundant and ubiquitous
component of the main types of marine sediments
deposited from outer shelf to deep sea environments.
The clay minerals are conventionally comprised of
the o2 mm fraction, are sheet- or fiber-shaped, and
adsorb various proportions of water. This determines
a high buoyancy and the ability for clay to be widely
dispersed by marine currents, despite its propensity
for forming aggregates and flocs. Clay minerals in
the marine environments are dominated by illite,
smectite, and kaolinite, three families whose chemical composition and crystalline status are highly
variable. The marine clay associations may include
various amounts and types of other species, namely
chlorite and random mixed layers, but also vermiculite, palygorskite, sepiolite, talc, pyrophyllite,
etc. The clay mineralogy of marine sediments is
therefore very diverse according to depositional environments, from both qualitative and quantitative
points of view.
As clay minerals are considered to be dependent
on chemically concentrated environments, and as
they commonly form in surficial conditions on land
especially through weathering and soil-forming processes, their detrital versus authigenic origin in marine sediments has been widely debated. The
transition from continental fresh to marine saline
water, marked by a rapid increase of dissolved
chemical elements, was the central point of discussion and arose from both American and European
examples. In fact the mineralogical changes recorded
at the land-to-sea transition are either important or
insignificant, are characterized in estuarine sediments
by various, sometimes opposite trends impeding
consistent geochemical explanations, and often vanish in open marine sediments. The changes observed
at the fresh-to-saline water transition in the clay
mineral composition essentially proceed from differential settling processes or from mixing between
different sources, and not from chemical exchanges
affecting the crystalline network. Such a historical
debate underlines the interest in investigating the
sensitive clay mineral associations for understanding
and reconstructing environmental conditions. This
article will consider the general distribution and
significance of clay minerals in recent sediments,
some depositional and genetic environments, and a
few examples of the use of clay assemblages to reconstruct paleoclimatic and other paleoenvironmental changes.
General Distribution and Significance
As a result of extensive reviews made by both
American and Russian research teams the general
characters of the clay mineral distribution in deep sea
sediments have been known since the late 1970s. The
maps published by various authors demonstrate the
dominant control of terrigenous sources, which
comprise either soils and paleosoils or rocks. The
impact of soils on the marine clay sedimentation is
largely dependent on weathering intensity developing
on land, and therefore on the climate. For instance,
kaolinite mostly forms under intense warm, humid
conditions characterizing the intertropical regions,
and prevails in the clay fraction of corresponding
marine sediments. By contrast chlorite and illite
chiefly derive from physical weathering of crystalline
and diagenetic sedimentary rocks outcropping
widely in cold regions, and therefore occur abundantly in high latitude oceans. The kaolinite/chlorite
ratio in marine sediments constitutes a reliable indicator of chemical hydrolysis versus physical processes in continental weathering profiles and
therefore of climatic variations occurring on the land
masses.
Other clay minerals are also able to bear a clear
climatic message, as for instance the amount of
random mixed layers and altered smectite in temperate regions, the crystalline status of illite in temperate to warm regions, and the abundance of soilforming Al-Fe smectite in subarid regions. Detailed
measurements on X-ray diffraction diagrams, electron microscope observations and geochemical analyses allow precise characterization of the different
continental climatic environments from data obtained on detrital sedimentary clays.
Some terrigenous clay minerals in recent sediments
reflect both climatic and non-climatic influences. For
instance, the distribution of illite (Figure 1), a mineral that primarily derives from the erosion of
mica-bearing rocks, shows increased percentages in
high latitude oceans due to predominant physical
346