Department of Petroleum Geology & Sedimentology,
Faculty of Earth Sciences,
King Abdulaziz University,
Jeddah, Saudi Arabia
EPS 421
CLASTIC SEDIMENTARY ROCKS
Lecture 13: Mineral constituents of mudrocks,
diagenesis of clay minerals and mudrocks
Prof. Dr. Mahmoud A. M. Aref
Mineral constituents of mudrocks
1)
2)
3)
Clay minerals
Quartz
Other constituents
EPS 321 Lecture 13
Mineral constituents of mudrocks
1)
2)
3)
Clay minerals
Quartz
Other constituents
EPS 321 Lecture 13
1. Clay minerals
„
Clay minerals are hydrous aluminosilicates with a sheet or layered
structure; they are phyllosilicates, like the micas.
„
The sheets of a clay mineral are of two basic types:
1)
One is a layer of silicon-oxygen tetrahedra with three of the oxygen
atoms in each tetrahedron shared with adjacent tetrahedra and
linked together to form a hexagonal network. The basic unit is Si2O5
but within these silica layers aluminum may replace up to half the
silicon atoms.
EPS 321 Lecture 13
Single silica tetrahedron (shaded) and the sheet structure of silica
tetrahedrons arranged in a hexagonal network .
EPS 321 Lecture 13
1. Clay minerals, cont.
„
The second type of layer consists of aluminium in octahedral coordination
with O2- and OH- ions so that in effect the Al2+ ions are located between two
sheets of O/OH ions. In this type of layer, not all the Al (octahedral)
positions may be occupied, or Mg2+, Fe and other ions may substitute for the
Al3+.
„
Layers of Al-O/OH in a clay mineral are referred to as gibbsite layers
because the mineral gibbsite (Al(OH)3) consists entirely of such layers.
„
Similarly, layers of Mg-O/OH are referred to as brucite layers after the
mineral brucite (Mg(OH)2) composed solely of this structural unit.
EPS 321 Lecture 13
Single octahedron (shaded) and the sheet structure of octahedral units .
EPS 321 Lecture 13
1. Types of clay minerals
Five common clay minerals
Kaolinite
1)
Illite
2)
Montmorillonite
3)
chlorite
4)
mixed-layer clays:
5)
A. Illite-montmorillonite
B. chlorite-montmorillonite
EPS 321 Lecture 13
1. Clay minerals
kaolinite
illite
montmorillonite
™
Clay minerals consist of sheets of silica tetrahedra and aluminium or
magnesium octahedra linked together by oxygen atoms common to
both. The stacking arrangement of the sheets determines the claymineral type, as does the replacement of Si and Al ions by other
elements.
™
Structurally, the two basic groups of clay minerals are the kaolinite
group and smectite group.
EPS 321 Lecture 13
1. 1. kaolinite group
Members of the kaolinite group have
a two-layered structure consisting of
a silica tetrahedral sheet linked to an
alumina octahedral (gibbsite) sheet
by common O/OH ions.
Replacement of Al and Si does not
occur so that the structural formula
is (OH)4Al2Si2O5.
Kaolinite in SEM
„ Forms booklets
„ Greatly reduces porosity but spaces
between layers allow flow, so it does
not reduce the permeability
„
EPS 321 Lecture 13
Kaolinite
EPS 321 Lecture 13
1. 2. Smectite group
„
Members of the smectite group have a three-layered
structure in which an alumina octahedral layer is
sandwiched between two layers of silica tetrahedra.
„
Smectites have the ability to adsorb water molecules,
so they are often called 'expandable clays', is utilized
in their X-ray identification.
„
The common smectite is montmorillonite; it
approximates to Al4(Si4O10)2(OH)4.H2O but substitution
of the Al3+ by Fe2+, Mg2+ and Zn2+ can take place. A net
negative charge resulting from such substitutions is
balanced by other cations, especially Ca2+ and Na+,
which are contained in interlayer positions.
„
Smectite in SEM (montmorillonite)
„
Forms honeycombe structures
„
Some reduction of both porosity and
permeability
„
Smooth fusing of edges distinguishes from chlorite
EPS 321 Lecture 13
Smectite
EPS 321 Lecture 13
1. 3. Illite
„
Illite, the most common of the clay minerals in
sediments, is related to the mica muscovite.
„
It has a three-layered structure, like the
smectites, but Al3+ substitution for Si4+ in the
tetrahedral layer results in a deficit of charge,
which is balanced by potassium ions in
interlayer positions. Some hydroxyl OH-, Fe2+
and Mg2+ ions also occur in illite.
„
Illite in SEM
„
„
Forms hair-like threads
Low overall volume: porosity not reduced
much, permeability is reduced
EPS 321 Lecture 13
Illite
Illite - fibrous
Illite - platy
Illite - laths
EPS 321 Lecture 13
1. 4. Chlorite, glauconite, sepiolite and
palygorskite
„
Chlorite, like the smectites and illite, has a
three-layered structure, but with a brucite
(Mg-OH) layer in-between. Substitution by
Fe2+ occurs in chlorite (imparting the green
colour).
„
Chlorite in SEM
„
Many forms: platy, honeycombe, rosette…
„
Some reduction of both porosity and
permeability
„
Glauconite is related to illite and the micas,
but contains Fe3+ substituting for Al3+.
„
Sepiolite and palygorskite are magnesiumrich aluminosilicates.
EPS 321 Lecture 13
Brucite sheet
Brucite sheet
Fe and Al rich chlorite
Fe-Al rich, vermiform Chlorite
Fe-rich chlorite
EPS 321 Lecture 13
1. 4. Mixed-layer clays
„
In addition to the four common clay minerals — illite,
kaolinite, montmorillonite and chlorite — mixed-layer
clays also are common.
„
These consist of an interleaving of sheets of the
common clays, in particular illite-montmorillonite and
chlorite-montmorillonite.
EPS 321 Lecture 13
The identification of clay minerals in a mudrock
™
Through X-ray diffraction for sediment samples less than 2 um fraction.
™ The basal spacing of the clay minerals are deduced from the X-ray
reflections for:
1) The untreated sample,
2) The treated sample with glycol, which causes expansion of the
lattice of any smectites present
3) The treated sample with heat, which reduces the lattice spacing of
smectite.
™ The clay minerals in mudrocks can rarely be identified with the petrological
microscope because of their fine crystal size, only where authigenic in
sandstones.
™ A clearer picture of clay-mineral crystal shape can be obtained from the
scanning electron microscope, which is now increasingly used for clay
mineral identifications and mudrock textural studies, especially in the backscattered mode.
EPS 321 Lecture 13
Mineral constituents of mudrocks
1)
2)
3)
Clay minerals
Quartz
Other constituents
EPS 321 Lecture 13
2. Quartz
„
Quartz in mudrocks is chiefly of silt-grade, although coarser,
sand-size grains do occur, especially where the mudrocks
grade laterally or vertically into sandstones.
„
Quartz silt is invariably angular in comparison with typically
more rounded quartz sand.
„
Silt-grade quartz is derived from grain collisions in aqueous and
aeolian media and from glacial grinding.
„
Some quartz in mudrocks is diagenetic rather than detrital.
EPS 321 Lecture 13
Mineral constituents of mudrocks
1)
2)
3)
Clay minerals
Quartz
Other constituents
EPS 321 Lecture 13
3. Other constituents
„
„
„
„
„
„
„
Feldspars are generally present in low concentrations in mudrocks in view of
their lower mechanical and chemical stability relative to quartz.
However, because mudrocks are less permeable than sandstones, feldspars
and other less stable grains may be preserved preferentially in muddy
sediments, so that the non-clay fraction can be used for provenance
information.
Muscovite is common but biotite much less so.
Calcite (or aragonite) may occur in the form of skeletal debris. Diagenetic
calcite, as well as dolomite and siderite, occur as microscopic crystals,
evenly disseminated or concentrated into nodules. Mudrocks may grade into
calcareous mudrocks (marls).
Pyrite occurs as cubes, framboids and nodules in dark, organic-rich
mudrocks.
Other minerals present locally are glauconite, hematite, gypsum-anhydrite
and halite.
Organic matter is common in mudrocks, particularly black shales.
EPS 321 Lecture 13
The formation and distribution of clay minerals in
modern sediments
Clay minerals in a sediment or sedimentary rock have three
origins:
1)
Inheritance: (the clays are detrital and have been formed in another area,
perhaps at a much earlier time, but they are stable in their present
location). Inherited clays will give information on the provenance of the
deposit and probably the climate
2)
Neoformation: the clays have formed in situ, and they have either been
precipitated from solution or formed from amorphous silicate material.
3)
transformation: With transformation, inherited clays are modified by ion
exchange or cation rearrangement.
EPS 321 Lecture 13
Fractionation of clay minerals
Kaolinite may flocculate first as clays reach the salty sea because of its low
charge and low cation exchange capacity (CEC), or it may settle first because of
its equant shape.
Illite has a higher CEC and a platy morphology. It is wafted farther out onto the
shelf.
Smectite has the highest CEC and a flaky morphology that resists deposition. It
tends to accumulate in the pelagic environment, the deep sea .
Turbidites bring sediment from the shelf to the deep sea, enriching pelagic
sediment in "shelf" components, such as illite and kaolinite .
Interpretation of paleoclimate from clay composition must be done with caution.
The best way to do this is to use clays in place ,in situ ,such as in paleosols .
EPS 321 Lecture 13
Diagenesis of clay minerals and mudrocks
„
Clay minerals can be modified and altered during early and late
diagenesis, and into metamorphism.
„
The main physical post-depositional process affecting the
mudrocks as a whole is compaction.
„
Compaction in mudrocks expels water and reduces the
thickness of the deposited sediment by a factor of up to 10.
„
When muds are deposited they contain in the region of 70-90%
water by volume. Compaction through overburden pressure
soon removes much of the water so that at depths of 1000m or
so, the mudrocks contain around 30% water. Much of this is not
free pore water but is contained in the lattice of the clay
minerals and adsorbed onto the clays.
EPS 321 Lecture 13
Diagenesis of clay minerals and mudrocks
„
Further compaction through water loss requires temperatures
approaching 100°C and these are attained through burial at
depths in the region of 2 - 4km. Dehydration of clays then takes
place, accompanied by some changes in the clay mineralogy.
Final compaction to give a mudrock with only a few per cent
water requires a much longer period of over-burden pressure
with elevated temperatures.
„
Evidence of compaction in mudrocks is provided by the fracture
of shells, flattening of burrows, and the bending of laminae
around shells and early diagenetic nodules.
Diagenesis of clay minerals and mudrocks
„
Changes in clay mineralogy during diagenesis take place
principally through the rise in temperature accompanying
increased depth of burial.
„
The main vertical change with depth is:
1) An alteration of smectites to illite via mixed-layer clays of
smectite-illite. This alteration involves the incorporation of K+
ions into the smectite structure and loss of interlayer water.
„
The process is largely temperature dependent and the
temperature at which smectite begins to disappear is of the
order of 70-95°C; that is, at depths of 2-3 km.
2) At slightly higher temperatures and greater depths, kaolinite is
replaced by illite and chlorite.
Diagenesis of clay
minerals and mudrocks
™ Changes of clay minerals with
increasing depth of burial and into
metamorphism.
™ Illite: no conversion but becomes
more crystalline with burial
™ Subvarieties of kaolinite convert to
illite & chlorite with burial
™ Phyllosilicates replace clay minerals
during metamorphism:
™ Illite & chlorite can survive