MIDTERM EXAM
SOIL SCIENCE 712. Advanced Soil Chemistry and Mineralogy
1. Classify the major types of soil colloids and describe each of them according to their
properties/characteristics. Identify at least 1 mineral for each colloid. (20 pts)
Soil colloids are very small particles in soil that can hold water and nutrients. They are important
for plant growth and soil fertility. There are four main types of soil colloids: organic, non-crystalline
inorganic, crystalline inorganic, and iron and aluminum oxide clays.
Organic colloids are made from decomposed plants and animals in the soil. They can hold a lot of
nutrients and water, and they make the soil more acidic. They have a high cation exchange capacity (CEC),
which means they can attract and exchange positively charged ions such as calcium, potassium, and
magnesium. They also have a high buffering capacity, which means they can resist changes in soil pH.
Examples of organic colloids are humus, which is the dark brown or black substance that forms from the
decomposition of organic matter.
Non-crystalline inorganic colloids are formed from volcanic ash or intense weathering of igneous
rocks. They have a short-range order, which means they lack a regular crystal structure. They have a high
phosphate adsorption capacity, which means they can bind phosphate ions and prevent them from
leaching out of the soil. They also have a variable charge, which means their charge can change depending
on the soil pH. Examples of non-crystalline inorganic colloids are allophane and imogolite, which are
hydrous aluminosilicates that have a spherical or tubular shape.
Crystalline inorganic colloids are the most common and stable type of soil colloids. They are
formed from weathering of silicate rocks. They have a regular crystal structure composed of silica
tetrahedra and alumina octahedra arranged in layers. The layers can be stacked in different ways,
resulting in different types of clay minerals. They also have a fixed charge, which means their charge is
constant and depends on the substitution of some atoms by others in the crystal structure. Examples of
crystalline inorganic colloids are kaolinite, montmorillonite, illite, and vermiculite, which have different
properties depending on their layer arrangement, charge, and interlayer cations.
Iron and aluminum oxide clays are formed from weathering of iron and aluminum minerals. They
have a definite or amorphous crystal structure. They have a variable charge that depends on the soil pH
and can be positively or negatively charged. They are not very sticky or plastic as the layer silicate clays
on which they are commonly found as surface coatings. Examples of iron and aluminum oxide clays are
goethite, hematite, and gibbsite, which are oxides or hydroxides of iron and aluminum.
2. Identify the two different types of layer silicate clays which have low swelling capacities and low CEC.
What are the common and different properties of these two clays? (10 pts)
The two different types of layer silicate clays that have low swelling capacities and low cation
exchange capacities (CEC) are kaolinite and illite. These are the 1:1 and 2:1 non-expanding types of clays,
respectively.
Some of the common properties of these two clays are: (1) They have a layer-like crystalline
structure with negative charges on the surfaces and edges of the layers. (2) They are held together by
hydrogen bonding or potassium bridging, which prevents water from entering between the layers and
TUSI, ALJUN G.
MS in SOIL SCIENCE
MIDTERM EXAM
SOIL SCIENCE 712. Advanced Soil Chemistry and Mineralogy
causing swelling. (3) They have low CEC, ranging from 3 to 40 cmol/kg, because of their low surface area
and low charge density.
On the other hand, some of the different properties of these two clays are: (1) Kaolinite has one
silica tetrahedral sheet and one alumina octahedral sheet per layer, while illite has two silica tetrahedral
sheets and one alumina octahedral sheet per layer. (2) Kaolinite has a hexagonal shape and a smooth
surface, while illite has a flaky shape and a rough surface. (3) Kaolinite is more resistant to weathering
than illite, because of its lower isomorphous substitution and higher stability.
3. Discuss and differentiate the origin of charges in the colloids. Relate this to the reasons why CEC of
different soil colloids vary. (10 pts)
The origin of charges in the colloids depends on the type and structure of the colloidal particles.
There are two main sources of charges on soil colloids: isomorphous substitution and ionization of surface
groups.
Isomorphous substitution is the replacement of one atom by another of similar size in a crystal
lattice without disrupting or changing the crystal structure of the mineral. This usually results in a net
negative charge on the mineral surface. For example, in some clay minerals, such as montmorillonite and
illite, some of the silicon atoms in the tetrahedral layer are replaced by aluminum atoms, and some of the
aluminum atoms in the octahedral layer are replaced by magnesium or iron atoms. These substitutions
create more negative charges than positive charges, and thus the clay mineral becomes negatively
charged. The amount of isomorphous substitution varies among different clay minerals and affects their
cation exchange capacity (CEC).
Ionization of surface groups is the dissociation of hydrogen ions from acidic functional groups on
the edge of clay minerals or on organic matter. This can create either positive or negative charges
depending on the pH of the soil solution. For example, some of the common functional groups on soil
organic matter are carboxylic (-COOH), phenolic (-OH), and amine (-NH2). At low pH, these groups tend to
remain protonated and have no charge or a positive charge. At high pH, these groups tend to lose protons
and become negatively charged. The ionization of surface groups is more pH-dependent than
isomorphous substitution and can change with soil conditions.
The CEC of different soil colloids varies according to their charge density, surface area, and type
of charge. Generally, soil organic matter has the highest CEC, followed by 2:1 expanding type clays (such
as montmorillonite and vermiculite), 2:1 non-expanding type clays (such as illite), and 1:1 type clays (such
as kaolinite). The CEC values reflect the differences in charge origin and magnitude among different soil
colloids. Soil organic matter and 2:1 expanding type clays have more negative charges due to both
isomorphous substitution and ionization of surface groups. They also have larger surface areas than 1:1
type clays, which increases their potential to adsorb cations. Illite has less negative charge than
montmorillonite because it has less isomorphous substitution and more potassium ions bridging between
the layers. Kaolinite has the lowest CEC because it has very little isomorphous substitution and no
ionizable surface groups. Its hexagonal shape also reduces its specific surface area compared to other
clays.
TUSI, ALJUN G.
MS in SOIL SCIENCE
MIDTERM EXAM
SOIL SCIENCE 712. Advanced Soil Chemistry and Mineralogy
5. What are the different surface structures of organic colloids that can contribute to the charges of the
colloids. (10 pts)
Some cations and anions are easily displaced from the surfaces of the colloids because of the
different factors. First, the concentration and activity of the ions in the soil solution. The higher the
concentration and activity of an ion in the solution, the more likely it is to replace another ion on the
colloid surface. For example, if a soil is treated with a potassium fertilizer, the potassium ions in the
solution will increase and displace other cations such as calcium or magnesium from the colloids.
Secondly, the valence and size of the ions. The higher the valence and the smaller the size of an ion, the
more strongly it is held by the colloid surface. This is because such ions have a higher charge density and
can form more electrostatic bonds with the negative charges on the colloids. For example, aluminum ions
(Al3+) are more strongly adsorbed than sodium ions (Na+) on clay minerals and are less easily displaced by
other cations. Lastly, the type and structure of the colloids. Different colloids have different amounts and
origins of charges, which affect their cation exchange capacity (CEC) and selectivity. For example, organic
matter has a higher CEC than clay minerals, and can adsorb more cations per unit mass. Organic matter
also has more variable charges that depend on the pH of the soil solution and can adsorb both cations
and anions. Clay minerals have more fixed charges that originate from isomorphous substitution or broken
bonds at the edges of their layers and can adsorb mainly cations.
6. How does dioctahedral layer and trioctahedral layer form within the layer’s lattices of the clay structure
(5 pts)
Dioctahedral and trioctahedral layers form within the layer's lattices of the clay structure
depending on the type and number of cations in the octahedral sheet. The octahedral sheet is composed
of anions (usually hydroxyl groups) that are coordinated by cations (usually aluminum, magnesium, or
iron) in an octahedral shape. If all the six octahedral sites are occupied by divalent cations (such as Mg2+),
the layer is called trioctahedral, because there are three cations per formula unit. If only four of the six
octahedral sites are occupied by trivalent cations (such as Al3+), the layer is called dioctahedral, because
there are two cations per formula unit. The empty sites are usually arranged in a trans configuration,
meaning that they are opposite to each other across the center of the layer. The octahedral sheet is then
bonded to one or two tetrahedral sheets, which are composed of silica tetrahedra that share oxygen
atoms at their corners. The combination of tetrahedral and octahedral sheets forms the basic building
blocks of phyllosilicate minerals, also known as clay minerals.
7. Soils are formed from parent materials, which is related to Bowen's Reaction Series, which explains the
order of crystal formation from magma. (15 pts)
a) Explain discontinuous and continuous series of mineral formation.
Discontinuous and continuous series of mineral formation are two types of crystallization patterns
that occur from magma cooling. Discontinuous series refers to the sequential formation of different
minerals with distinct compositions and structures, such as olivine, pyroxene, amphibole, and biotite.
TUSI, ALJUN G.
MS in SOIL SCIENCE
MIDTERM EXAM
SOIL SCIENCE 712. Advanced Soil Chemistry and Mineralogy
Continuous series refers to the gradual change of one mineral into another with similar structures but
different compositions, such as plagioclase feldspar.
b) Why does the weathering of minerals differ? Explain this by considering the concept of Bowen's
reaction series.
The weathering of minerals differs because of their different stability and reactivity under surface
conditions. Minerals that form at high temperatures and pressures tend to be less stable and more
reactive than minerals that form at low temperatures and pressures. This is because they have higher
energy and lower entropy, and tend to release energy and increase entropy by weathering. Bowen's
reaction series explains the order of crystal formation from magma, and also the order of mineral
weathering from most to least stable. Minerals that crystallize first, such as olivine and pyroxene, are
more prone to weathering than minerals that crystallize last, such as quartz and feldspar.
c) Do you think this type of mineral formation will happen in both sedimentary and metamorphic rocks
No, this type of mineral formation will not happen in both sedimentary and metamorphic rocks.
Sedimentary rocks are formed by the accumulation and cementation of weathered materials, such as
clasts, organic matter, or chemical precipitates. Metamorphic rocks are formed by the alteration of
existing rocks due to high temperature and pressure, such as folding, faulting, or contact with magma.
Neither sedimentary nor metamorphic rocks involve the cooling and solidification of molten magma,
which is the process that produces igneous rocks and Bowen's reaction series.
8. Differentiate the structures of inorganic colloids from organic colloids and their capability to attracts
cations (20 pts)
Inorganic colloids and organic colloids are two types of soil particles that have very small size and
large surface area. They differ in their structures and their capability to attract cations.
Inorganic colloids are mainly composed of clay minerals, which have a layer-like crystalline
structure with negative charges on the surfaces and edges of the layers. These charges are due to
isomorphous substitution or broken bonds in the mineral lattice. Inorganic colloids can adsorb cations on
their external and internal surfaces, depending on the type and arrangement of the layers. For example,
2:1 expanding type clays, such as montmorillonite, can adsorb cations between their layers and swell when
wet. 1:1 type clays, such as kaolinite, can only adsorb cations on their external surfaces and do not swell.
Organic colloids are mainly composed of humus, which is a complex and amorphous substance
derived from the decomposition of organic matter. Humus has a chain-like structure with various
functional groups, such as carboxyl, phenolic, and amine groups. These groups can dissociate hydrogen
ions and create negative charges that depend on the pH of the soil solution. Organic colloids can adsorb
both cations and anions on their surfaces, depending on the type and amount of functional groups.
Organic colloids have higher cation exchange capacity (CEC) than inorganic colloids because they have
more negative charges per unit mass.
TUSI, ALJUN G.
MS in SOIL SCIENCE