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Soil Components:

1. Mineral Components

Mineral particles are inorganic materials derived from rocks and minerals. They are extremely variable in size and composition. 

1.1.Composition

1.1.1. Types of primary and secondary minerals
1.1.2. Weathering
1.1.3. Most common elements in soils
1.1.4. Particle and bulk density

1.2 Size of soil particles 

1.2.1. Soil Texture

1.3. Soil colloids

1.3.1. Types of soil colloids
1.3.2. Specific surface area
1.3.3. Charges on soil colloids

1.1.Composition

1.1.1. Types of primary and secondary minerals

Primary minerals are formed at high temperature and pressure, under reducing conditions without free oxygen. These minerals are mainly present in soils as sand and silt particles. They are not crystallized and deposed from molten lava.

Slide Show Images - Source:© Amethyst Galleries Inc. All Rights Reserved http://mineral.galleries.com/

Secondary minerals are formed at low temperature and pressure through oxidation. They are the weathering product of primary minerals, either through alteration of their structure or through re-precipitation. Secondary minerals are usually present in soil as clay particles.

Slide Show Images - Source:© Amethyst Galleries Inc. All Rights Reserved http://mineral.galleries.com/ and © USDA http://soils.usda.gov/use/worldsoils/

 

RELATED LINKS: 

EarthZone, TERC
Visual animation of Metamorphic Rock Forming website


Department of Soil Science, NC State University
Mineral Content - This animation shows how mineral content changes over time. website 

1.1.2. Weathering

Weathering refers to the breakdown and changes in rocks and sediments at or near the Earth's surface brought about by biological, chemical, and physical agents or combinations thereof. Weathering also involves the synthesis of new (secondary) minerals that are of great importance in soil (e.g. clay minerals).  

Weathering 
Image Source: : © USDA http://soils.usda.gov/use/worldsoils/

 

RELATED LINK: 

Exploring Earth. TERC

  • Animation: Chemical weathering of feldspar to clay. website

  • Animation: Observe the effects of mechanical weathering website


Department of Soil Science, NC State University

  • Weather vs. Minerals - This animation shows how different levels of weathering affect specific minerals in soil website

  • Mineral Weathering - This animation shows how weathering affects all minerals to create soil. website

Examples of physical weathering include processes such as crystal growth, thermal expansion, moisture swelling, abrasion, etc.

Chemical weathering consists of the following processes:

1) Hydration – intact water molecules bind to a mineral transforming hematite into ferrihydrate:

2) Hydrolysis – water molecules split into their hydrogen and hydroxyl components and hydrogen replaces a cation from the mineral structure (e.g. transformation of feldspar to kaolinite):

RELATED LINK: 

Exploring Earth. TERC

  • Animation: Animation of the hydrolysis process. website

3) Dissolution (or solution) – water is capable of dissolving many minerals by hydrating the cations and anions until they become dissociated from each other and surrounded by water molecules. (e.g. dissolution of gypsum):

4) Carbonation – weathering is accelerated by the presence of acids that increase the activity of hydrogen ions in water. For example, when carbon dioxide dissolves in water (a process enhanced by microbial and root respiration) the carbonic acid (H2CO3) produced hastens the chemical dissolution of calcite into limestone (or marble):  

5) Oxidation-reduction – minerals that contain Fe, Mn, or S are especially susceptible to oxidation-reduction reactions. When for example, iron is oxidized from divalent to trivalent form, the change in valence and ionic radius causes destabilizing adjustments in the crystal structure of the mineral.

Biological weathering effects includes: 

  • the break up of rock particles by roots

  • the transfer and mixing of materials by burrowing animals and

  • the formation of organo-mineral complexes (soil biological processes produce organic acids that can solubilize Al and Si ions, which are removed from a mineral by this process).

Biological Weathering

An example of biological weathering of a 
rock under the influence of clams

Image Source: Maja Krzic

1.1.3. Most common elements in soils

The median and range of various elements present in soils from around the world are given in Table 1. The elements that are found in soils in the highest quantities are O, Si, Al, Fe, C, Ca, K, Na, and Mg. These are also major elements found in the Earth’s crust and in sediments. Oxygen is the most prevalent element in the Earth’s crust and in soils. It comprises about 47% of the Earth’s crust by weight and more than 90% by volume.

Table 1. Contents of some elements in soils, the Earth’s crust and sediments (exert from Sparks 2003)

Element

Soils (mg/kg)

Earth’s crust (mean)

Sediments (mean)

 

Median

Range

 

 

O

490,000

-

474,000

486,000

Si

330,000

250,000-410,000

277,000

245,000

Al

71,000

10,000-300,000

82,000

72,000

Fe

40,000

2,000-550,000

41,000

41,000

C (total)

20,000

7,000-500,000

480

29,400

Ca

15,000

700-500,000

41,000

66,000

Mg

5,000

400-9,000

23,000

14,000

K

14,000

80-37,000

21,000

20,000

Na

5,000

150-25,000

23,000

5,700

Mn

1,000

20-10,000

950

770

Zn

90

1-900

75

95

Mo

1.2

0.1-40

1.5

2

Ni

50

2-750

80

52

Cu

30

2-250

50

33

N

2,000

200-5,000

25

470

P

800

35-5,300

1,000

670

S (total)

700

30-1,600

260

2,200

1.1.4. Particle and bulk density

Particle density (rs) is mass of solids (Ms) per volume of solids (Vs).

 rs= Ms/Vs

In most mineral soils the mean density of the particles is about 2.6-2.7 gm/cm3 (or 2600 - 2700 kg/m3). Soils with a high content of iron oxides and various heavy minerals have a particle density of 5.2-5.3 gm/cm3, while soils with high organic matter content can have a particle density as low as 1.3 gm/cm3.

Bulk density (rb) is the mass of solids (Ms) per total soil volume (Vt).

rb = Ms / Vt 

rb= Ms / (Vs + Va + Vw)

Vs=volume of solids; Va=volume of air; Vw=volume of water

Bulk density is always smaller than rs. Since in a general case pores constitute half the volume, rb is about half ofrs, namely 1.3-1.35 g/cm3 (or 1300-1350 kg/m3).  

Volume Density: Animation is to illustrate relationships among the weight and volume of soil air, water, and solids 

Click on the image to view flash animation

 

 

1.2 Size of soil particles

The mineral particles present in soils vary enormously in size from boulders and stones down to sand grains and minute clay particles that cannot be seen by an optical microscope. An arbitrary division is made by size-grading soil into material:

a) that passes trough a sieve with 2-mm diameter holes - the fine earth (consisting of sand, silt, and clay particles),

b) that is retained on the sieve (> 2 mm) - the coarse fragments (gravel, cobbles, and stones).

Coarse fragments (diameter > 2 mm) are defined as rock fragments and do not include fragments of pads or concretions. 

1.2.1. Soil Texture

Soil texture refers to the relative proportions of sand, silt, and clay in a soil. It is often the first and most important property to be determined when describing a soil, since many conclusions can be drawn from this information (water intake or infiltration, water storage in the soil, soil aeration, soil fertility, trafficability, etc.).

A number of different classification systems have been devised to divide soil particles according to their size. The figure below shows the limits of different size classes in three systems of soil particle size classification. Although Canadian classification system is not shown, it is very similar to the USDA system, with the exception of sub-classes of silt and clay size classes.  

Particle Size Classification

Click on the image to view a larger 
Particle Size Classification

 

Image Source: Lesley Dampier

Textural names are given to soils based on the relative proportions of each of the three soil particles - sand, silt, and clay. Soils that contain predominantly clay size particles are called clay (textural class), those with high silt content are called silt (textural class), and those with a high sand content are called sand (textural class). A more complicated soil textural class that is called a loam is a mixture of sand, silt, and clay particles and it exhibits the properties of those particles in about equal proportions.  

Soil Texture Determination

Click on the image to view a larger image of the soil texture determination.

Image Source: by permission: 
Centre for Teaching, Learning and Technology, UBC

1.3. Soil colloids

1.3.1. Types of soil colloids

When suspended in water, colloids do not readily settle out due to Brownian motion (when particles are moved around by collisions with other small particles). In chemistry, colloids where particles smaller than 0.001 mm (or 1 mm), but it has been observed that soil particles smaller than 0.002 mm (or 2 mm) still behave in the same manner, hence they are called soil colloids.

Most common soil colloids are:

1)    Phyllosilicate clay minerals

2)    Oxides and hydrous oxides of iron and aluminum (e.g., gibbsite, goethite, hematite)

3)    Amorphous minerals (e.g., allophane and imogolite)

4)    Organic colloids  

Phyllosilicate clay minerals are a sub-group of silicates (an extensive group of minerals which are derived from silica SiO2). All silicates are based on the SiO44- tetrahedron with partial or complete sharing of oxygen atoms. Different silicate structures arise from the various ways in which SiO44- tetrahedra combine with one another. Two types of building blocks are present in phyllosilicates:

a)   tetrahedron (4-faced structure) where each silicon (Si) is surrounded with 4 oxygen atoms

b)   octahedron (8-faced structure) where each aluminum atom is surrounded by 6 oxygen atoms  

Phyllosilicate Clay Minerals

Click on the image to view a large image of the phyllosilicate clay minerals.

Image Source: Centre for Teaching, Learning and Technology, UBC

 

 

Two types of phyllosilicate clay minerals that are commonly present in soils are montmorillonite and kaolinite.

MONTMORILLONITE is a 2:1 phyllosilicate mineral where there is one Al-octahedral sheet sandwiched between two Si-tetrahedral sheets (one such sandwich is called a layer, while many layers make a micelle or crystal). The layers in montmorillonite crystals are loosely held together by very weak oxygen-to-oxygen and cation-to-oxygen bonds. Hence, water molecules and various ions are attracted between layers (into the interlayer space) causing expansion of the crystal. Montmorillonite is characterized by pronounced swelling when wet and shrinkage on drying. Wide cracks commonly form as montmorillonite  dominated soils (e.g. Vertisols) are dried. Typically, montmorillonite particles are thin (thickness ~1 to 3 nm  and width ~500 nm), flat disks.

Montmorillonite Clay Minerals
Image Source: Centre for Teaching, Learning and Technology, UBC

 

KAOLINITE is a 1:1 phyllosilicate mineral in which there is one Al-octahedral sheet bonded to one Si-tetrahedral sheet by shared (apical) oxygen atoms of the tetrahedral sheet. Two adjacent layers are held together by hydrogen bonding. Consequently, the structure is fixed and no expansion occurs between layers when the clay is wetted. The effective surface of kaolinite is restricted to its external surface area. The typical kaolinite particle is fairly large (thickness ~ 50 nm and width ~ 1-3 nm) in comparison with other clay particles and hexagonal in shape. Because of the strong binding forces between their structural layers, kaolinite particles are not readily broken down into extremely thin plates. Kaolinite exhibits very little shrinkage and swelling.

Kaolinite Diagram

 

Models of the 1:1-type clay kaolinite
Image Source: Clay Minerals Group

Kaolinite Photo

Photo of kaolinite crystals

Source: Clay Minerals Group

Amorphous minerals (imogolite and allophane)

Imogolite and allophane are minerals found in many soils including soils formed on volcanic ash parent material and Podzolic soil type. They are very small (about 2 nanometers in diameter) and a very reactive component of soils.

Imogolite and allophane are amorphous, which means that they do not have a repetitive structure with atoms arranged in a regular pattern over relatively long distances (at least tens of nanometers). Because their structure is somewhat ordered on a few nanometers scale, they are said to be 'paracrystalline' or to have 'short range order'.

Imogolite and allophane are both aluminosilicates, which means that they are made of variable amounts of aluminum (Al), silicon (Si), oxygen (O), and hydrogen (H) ions. They have a rather similar composition but their morphology is different. Imogolite forms tubes while allophane forms hollow spheres: 

 

1.3.2. Specific surface area

Specific surface area is defined as the surface area of particles per unit mass or unit volume of particles.  Because of their large size, sand particles have small specific surface area (e.g., 0.1 m2/g). Clay particles, on the other hand, have a very large specific surface area, giving them a tremendous capacity to adsorb water and other substances (e.g. 10-1,000 m2/g).  

1.3.3. Charges on soil colloids

The “ideal” montmorillonite composition is HSi2AlO6, which results in an “ideal” net charge of [1(+1)+2(+4)+1(+3)+6(-2)] = 0.  However, the real composition of montmorillonite involves isomorphic substitution of Mg for some Al in the octahedral sheet. With Mg2+ substituting for Al3+ there are fewer positive charges to neutralize the negative charges and a large permanent (negative) charge results (it is called permanent charge since it takes place during crystallization and it is not a subject to changes). In contrast, there are also variable or pH-dependent charges that are formed by protonation and deprotonation of functional groups like -OH.

“Ideal” kaolinite composition is H4Si2Al2O9, which results in an “ideal” net charge of [4(+1)+2(+4)+2(+3)+9(-2)] = 0. Composition of kaolinite is close to ideal, since there is very little or no isomorphic substitution, resulting in no permanent charges. Kaolinite in soil usually has some surface contamination, and net negative charge is about 8 cmolc/kg soil. In addition, kaolinite has a small amount of variable charge related to dissociable OH groups attached to Al at the edges of the crystal, where the structural regularity ends.

Faculty of Land and Food Systems
THE UNIVERSITY OF BRITISH COLUMBIA