The Soil Solid Fraction
Solids
Pores
5%
25%
Air
Water
Mineral
45%
25%
Organic
From where do mineral particles originate?
Rocks and Minerals
Rock
Limestone
Sandstone
Shale
Granite
Basalt
Dominant Mineral
Calcite
Quartz
Clays
Quartz/Feldspars
Quartz/Feldspars
Weathering
Physical: changing the size and shape of
Rocks or mineral particles without
altering their chemical composition
Chemical: altering a mineral particle’s chemical
composition.
Weathering
Physical:
Destruction and Synthesis
freezing and thawing
plants and biota
wind, rain, abrasion
Chemical: hydration
hydrolysis
oxidation/reduction
acids
dissolution
}
Reduce
Size
}
Chemical
Alteration
Granite
Minerals: quartz, feldspars, biotite
Rocks breakdown (weather) by physical means
Primary Minerals
Primary minerals (e.g. quartz, feldspars) are dominated by Si, Al, O
Primary minerals breakdown (weather) principally by chemical means
Secondary Minerals
Silicate clays
Iron oxides
Aluminum oxides
Silicate clay minerals are dominated by Si, Al, O
Primary Minerals
Quartz
Feldspar
Biotite
Albite
Muscovite
SiO2
KAlSi3O8
KMg3AlSi3O10(OH)2
NaAlSi3O8
KAl3Si3O10(OH)2
Aluminum and Silicon
Chemical Alteration and Reorganization
Primary Minerals
Quartz
SiO2
Feldspar
KAlSi3O8
Biotite
KMg3AlSi3O10(OH)2
Albite
NaAlSi3O8
Muscovite KAl3Si3O10(OH)2
Chemical
weathering
Secondary Minerals
Crystalline
Aluminosilcate
Clays
Dominated by
Aluminum, Silicon, Oxygen
Silicate Clays
Silicate Clays
Colloidal
1.Small (< 1 micron)
2.Large surface area
3.Highly reactive
Constituents of Silicate Clays
Aluminosilicates
Si4+
Al3+
O2-
OH-
These 4 ions are arranged together to form crystals
with a number of important properties related to soil reactivity.
Ions: what are they?
Silicate Clay minerals are crystalline
and formed by specific arrangement
of ionic forms of Si, Al, and O
Ions are stable forms of elements
that possess an electrical charge.
Cations are elements that have lost
electrons to become (+) charged
Anions are elements that have gained
electrons to become (-) charged.
Elements
Electrons (-)
Protons (+)
Elements in the periodic
table have equal numbers
of protons and electrons.
They are electrically neutral
Ions
Ions are stable forms of elements that acquire
an electrical charge by gaining or losing electrons
Elemental Sodium (Na)
11 protons (+), 11 electrons (-)
Sodium ion (Na+)
11 protons (+), 10 electrons (-)
By losing an electron, sodium has more protons than
electrons and becomes positively charged.
Na - 1e- = Na+
Sodium
Na (11 electrons)
e-
+
e-
e-
e-
e-
Na
e-
11 protons
e-
e-
e-
ee-
Na+
2Na + 2H20
2Na+ + 2 OH- + H2
Ions
Ions are stable forms of elements that acquire
an electrical charge by gaining or losing electrons
Elemental Chlorine (Cl)
17 protons (+), 17 electrons (-)
Chloride ion (Cl-)
17 protons (+), 18 electrons (-)
By gaining an electron, chlorine has more electrons than
protons and becomes negatively charged.
Cl + 1e- = Cl-
Chlorine
e-
_
e-
e-
e-
e-
ee-
e-
Cl
e-
17 protons
e-
e-
e-
e-
e-
e-
e-
e-
e-
Cl-
Elements that lose electrons and become
positively charged are called cations.
Na+, K+, Ca2+, Mg2+, Cu2+, Fe3+
Elements that gain electrons and become
negatively charged are called anions.
Cl-, Br-, F-, I-
CO32-, SO42-, PO4-3
oxoanions
Electrostatic Attraction
+
+
+
+
+
+
+
+
+
Na+
Cl-
-
Salts
Salts are formed by combining cations and
anions to form solids that have no charge.
Cations: K+, Na+, Mg2+, Ca2+
Anions: Cl-, CO3-2, SO4-2
K+ + Cl- = KCl
Na+ + Cl- = NaCl
KCl, NaCl, MgCl2, CaCO3, CaSO4
Conversely, if solid salts are mixed with water
they dissolve and the ions go into solution
solution
solid
KCl
NaCl
Water
Water
K+ + Cl-
Na+ + Cl-
CaCO3
Ca+2 and
CO3-2
CaSO4
Ca+2 and
SO4-2
Dissolution
ClCl
Cl Na
Cl
Cl
NaCl
Na+
Building a Crystal with Cations and Anions
Crystalline Mineral Colloids
Aluminosilicates
Cations and Anions
Si4+
Al3+
O2-
OH-
Building Blocks for Silicate Clays
Silicon Tetrahedron
oxygen
silicon
(Si4+)
Aluminum octahedron
hydroxide (OH-)
Aluminum (Al3+)
Fundamental Building Blocks
Silicon Tetrahedron
Aluminum Octahedron
Crystalline Minerals
Graphite
Oxygen (O2-)
silicon (Si4+)
Diamond
Hydroxide (OH-)
Aluminum (Al3+)
Tetrahedra and Octahedra
Sharing the Oxygens
Linkage of thousands of silica tetrahedra and
aluminum octahedra
O
{
octahedra {
Tetrahedra
Si
O, OH
Al
OH
1:1 Mineral
2:1 mineral
{
octahedra {
Tetrahedra {
Tetrahedra
1:1 minerals
2:1 minerals
Layers and Interlayers
Layer
interlayers
Electrical Properties of Soil Minerals
Charge Balance
Si4+
O2-
Al3+
OH-
Positive charge
Al3+ Si4+
=
Negative charge
OH- O2-
Uncharged silicate clay minerals
Positive Charge
Talc
Al3+ Si4+
Pyrophillite
Negative Charge
O2- OH-
AlSi2O5OH.
+3
2 x (+4) = +8
5 x (-2) = -10
Total = zero
-1
Isomorphous Substitution
Substitution of lower-charge cations for
higher charge cations during mineral formation.
Al3+ for Si4+ in tetrahedra
Mg2+ for Al3+ in octahedra
The result is a deficit of positive charge or a
surplus of negative charge in the mineral
structure.
Tetrahedral Substitution
Al3+ for Si4+
Positive Charge = Negative Charge
Octahedral Substitution
Mg2+ for Al3+
Positive Charge = Negative Charge
Tetrahedral Substitution
Al3+ for Si4+
Octahedral Substitution
Mg2+ for Al3+
Charge
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Cation Exchange
K+
K+
Na+
Na+
K+
Na+
Na+
Na+
Na+
Na+
K+
K+
Implications of Negative Charge sites.
Important Cations: H+, Ca2+, Mg2+, Zn2+, Mn2+, K+, NH4+, Cu2+
reserve
Paraquat, Diquat, Triazines
Pyridine, quinoline, arcidine