1. soil minerals properties for the purpose of teaching.ppt

Teaching material

1. •Mineral is defined as:
"a naturally occurring, inorganic, crystalline solid, with a fixed
range of chemical composition and physical properties."
Rocks are composed of minerals e.g. granite is an
igneous rock composed of minerals: quartz, Feldspar
and mica

2. •Silicate minerals are the most common rock-forming
minerals
Primary silicates: appear in soil (mainly in sand and silt
and also clay) from physical disintegration of parent
material
•Olivines
•Pyroxenes
•Amphiboles
•Micas
•Feldspars, quartz

3. Name Chemical formula Mineral group
ForsteriteMg
2
SiO
4
Olivine
FayaliteFe
2
SiO
4
Olivine
EnstatiteMgSiO
3 Pyroxene
DiopsideCaMgSi
2
O
6
Pyroxene
TremoliteCa
2
Mg
5
Si
8
O
22
(OH)
2
Amphibole
HornblendeNaCa
2Mg
5Fe
2AlSi
7O
22(OH)Amphibole
MuscuviteK
2
[Si
6
Al
2
]Al
4
O
20
(OH)
4
Mica
Biotite K
2
[Si
6
Al
2
]Mg
4
Fe
2
O
20
(OH)
4
Mica
orthoclaseKAlSi
3O
8 Feldespar
Albite NaAlSi
3
O
8
Feldespar
Names and chemical formulas of primary silicates

4. •Chemically can be classified into:
–ferromagnesian (iron/magnesian) - olivine,
pyroxenes, amphiboles, biotite
–felsic (silica/aluminum) - quartz, plagioclase,
potassium feldspars
•Silicate minerals, however, are typically classified
on the basis of their silica tetrahedra
polymerization.
–Isolated (olivines), single chains (pyroxenes),
double chains (amphiboles), sheets (micas and clay
minerals), and 3D frameworks or networks (quartz
and feldspars).

5. •The fundamental building blocks of silicates is silica
tetrahedron, SiO
4
-4
•Mineral structures are built by polymerization
=> Sharing of O between Si atoms resulting linkage of tetrahedral
units
Tetrahedral unit

6. •Chemical weathering of 1
0
silicates contributes to native
soil fertility
–Major products Na
+
, Mg
++
, K
+
, Ca
++
, Mn
++
, Fe
++
in soil
solution
–Often bivalent cations coming from 1
0
silicates exceed
Na
+
=> lessen sodicity hazard
•Aluminosilicates (2
o
silicates) predominate in the
clay fractions => weathering product
–Muscovite (1
0
) weathering => Vermiculite (2
0
)
•Aluminosilicates are composed of two kinds of horizontal
sheets.
–> tetrahedral sheets – basic unit tetrahedron
–> octahedral sheets – basic unit octahedron

7. Tetrahedral sheet formed from
tetrahedral unit
Apical oxygen ion

8. Octahedral unit
or Mg
+2
or Fe
-3
co-
ordinated to O
-2
or OH
-

9. Octahedral sheets

10. •Dioctahedral: if the metal cation is trivalent (occupies two
of the three sites)
–E.g. Muscuvite
•Trioctahedral: if the metal cation is bivalent (all 3- sites are
filled)
–E.g. Biotite
•Isomorphic substitution
–Substitution of Al
+3
for Si
+4
– tetrahedral sheets
–Al
+3
(or Fe
+3
) for Mg
+2
– octahedral sheets
•Layer charge in clay minerals decreases in order
–Illite > vermiculites> smectites
•Chlorites: octahedrally coordinated cations reside in two
sheets
–One sandwiched in a 2:1 layer and one on a basal
surface of 2:1 layer

11. 1:1 Kaolinite

12. 1:1 Kaolinite

13. 2:1 clays two sheets
of silicon tetrahedra
fused to each planar
side of metal cation
octahedra
One layer

14. 2:1:1 or 2:1 with
hydroxide interlayer
e.g. Chlorite

16. •Chlorite exist as both 1
o
and 2
o
mineral. In its 1
o
form
magnesium hydroxide is in the interlayer 2
o
chlorite had
aluminum hydroxide in the interlayer sheet
Structurally disordered aluminosilicates known collectively
as allophanes and imogolites
–Common in soils formed on volcanic ash materials
–Allophanes: Si
3
Al
4
O
12
.nH
2
O amorphous hydrous
aluminum silicate. Structure not well understood.
Thought to form a hallow spherule (~5nm diameter)
–Imogolite: Si
2
Al
4
O
10
.5H
2
O exhibit tubular
morphology.

17. Oxides and hydyoxides (Nonsilicates)
•Al, Fe and Mn form the most important oxides,
oxyhydroxides and hydroxides minerals in soils
•These clays consists of modified octahedral sheets with
either Fe
3+
or Al
3+
•Have no silicon in their structure
•Goethite and gibbsite are common minerals in soils and
are mixed with silicate clays. Oxide clays may occur as
coating
•Goethite (FeOOH)
–the most thermodynamically stable iron oxide
–Geothite and ferrihydrite (Fe
2
O
3
.nH
2
O) are common in
temperate climate

18. •Gibbsite (Al(OH)
3)
–the prominent constituent of highly weathered soils
–Dioctahedral sheets bound togather by hydrogen
bonds between opposing OH groups
–Vermiculite Gibbsite Chlorite (2
o
)
•Birnessite (Na
0.7
Ca
0.3
Mn7O
14
.2H
2
O)
–The most common manganese mineral in acid soil
–Structure contains sheet of MnO
6
8-
linked in some
fashion with Mn
+3
, Mn
+2
, Ca
+2
ions
•Oxides and hydroxides represent the climax mineralogy
of weathered soils

20. •Source of charges
–1. hydroxyl and other functional charges on the
surface = variable or pH dependent charges
•e.g. >AlO
-
+ H
+
 >AlOH + H
+
 AlOH
2
+
Negative Neutral Positive
pH decreasing 
kaolinite, oxides, organic matter
–2. the charge imbalance brought about by
isomorphic substitution = Permanent or
constant charge

21. Isomorphous
substitution
•Each Mg
2+
ion that
substitutes for Al
3+
causes
a negative charge in a
dioctahedral sheet
•Each Al
3+
ion that
substitutes for Si
4+
causes
a
negative charge in a
tetrahedral sheet

22. Cation exchange capacity (CEC) of common clay minerals
and organic substances
Clay type CEC (cmol(+)/kg)% constant
Vermiculite 150 95
Smectite 100 95
Illite 30 80
Kaolinite
Gibbsite
Geothite
8
4
4
5
0
0
Chlorite
Allophane
30
30
80
10
organic 200 10

23. ©2002 Prentice Hall, Inc. Pearson Education
Upper Saddle River, New Jersey 07458
Soils: An Introduction, 5th Edition
by Michael J. Singer and Donald N. Munns
Box 2-3Fixed and Variable Charge