Examples of common clay minerals in soils

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Examples of common clay minerals in soils

1)      1:1 Clay minerals: Most of these minerals have no layer charge or have very small layer charge because the tetrahedral cation sites are all occupied by Si⁴⁺ and the octahedral sites are occupied by Al³⁺ or Mg²⁺. Where there is substitution in one sheet of 1:1 layer silicate, neutrality is maintained by substitution in the other sheet.  The di-octahedral and tri-octahedral varieties of 1:1 layer silicates differ from one another based on the manner in which the layers are stacked. Examples of this clay minerals are kaolin  and serpentine groups

a)   Kaolin group: All are di-octahedral clay minerals with Al³⁺ as the main octahedral cation. The minerals have little or no isomorphic substitution and hence have a negligible layer charge. Representative minerals are kaolinite, dickite, nacrite and halloysite. The first three have a general formula Al₂Si₂O₅(OH)₄ and differ from one another by stacking arrangement of the neighbouring 1:1 layers. The dickite and nacrite form in the hydrothermal environment and are less common in soils. Kaolinite is very common in soils. It has a low shrink-swell capacity and a low cation exchange capacity (1-15 meq/100g). It is a soft, earthy, and usually it is white in colour. It forms by alteration of other aluminosilicates (like feldspars) in slightly acidic, well drained and highly weathered soils. Also it can be of hydrothermal or pedogenic origin. Halloysite (Al₂Si₂O₅(OH)₄.2H₂O) has two interlayer water molecules per unit cell. The mineral form by weathering in acid soils or in volcanic sediments. In general, the minerals under this kaolin group are held together by hydrogen bonds between the OH of the octahedral sheet and basal O of the tetrahedral sheet.

b)   Serpentine group: All are tri-octahedral with Mg²⁺ as their main cation in the octahedral sheet. Mg²⁺ in octahedral sheet can be substituted with Co, Cr, Ni and Al. The minerals are found in soils derived from ultramafic rocks or hydrothermal alteration of olivine, amphiboles, pyroxenes and peridotite. The minerals under this group weather easily and thus, they are less common in clay fraction except in young, unweathered soils. Weathering of serpentines form pedogenic chlorite or iron rich smectite. Representative mineral is chrysotile (Mg₃Si₂O₅(OH)₄). The mineral is fibrous and the H-bond keeps the layer together. Other minerals under this group are antigorite and lizardite. Soils derived from materials weathered from serpentine –rich rocks tend to be highly infertile. Weathering of serpentine releases high amounts of Mg and other metals. Its residuum and soils have very low Ca : Mg ratios as well as high levels of Ni, Co and Cr. In addition, the soils tend to have deficiencies of macronutrients. In most horizons, >80% of the exchangeable sites are occupied by Mg. The high Mg contents block the plant’s ability to take in nutrients, especially Ca. The soils tend to be bare with no or little vegetation.  For a plant to grow on this soil it must have ability to absorb calcium which offsets the negative effect of Mg (Schaetzl and Anderson, 2005). The soils tend to have high pH and high Fe contents.

2)      2:1 clay minerals:

a)   Talc-pyrophyllite group: The minerals have no isomorphic substitution in either the tetrahedral or octahedral sheet and hence, they do not have charge. Pyrophyllite (Al₂Si₄O₁₀(OH)₂) has an Al di-octahedral sheet  and is rare in soils.

Talc (Mg₃Si₄O₁₀(OH)₂) is a secondary mineral that forms by the alteration of magnesium silicates, such as olivine, pyroxenes, and amphiboles. Talc is tri-octahedral 2:1 layer silicate mineral with Mg²⁺ as octahedral cation. The mineral lack isomorphic replacement of ions and thus, has no layer charge.

b)      Mica group: There are three leading varieties of micas, namely muscovite, biotite and phlogopite. All micas have a net negative charge of 1mole per unit cell. This negative charge is balanced by K⁺ that is held tightly between the adjacent layers by a strong electrostatic force. This makes interlayer K⁺ less exchangeable and thus, low cation exchange capacity for the micas. The interlayer K⁺ in micas can be partly replaced by Na, Ca, Ba, Rb, and Cs whereas the hydroxyl group can be replaced by F. In octahedral sites for muscovite Al can be replaced by Mg, Fe²⁺, Fe³⁺, Mn, Li Cr, Ti and V, whilst for biotite and phlogopite octahedral Mg and Fe²⁺ can be replaced by Mn, Ti, Fe³⁺ and Li (Deer et al., 1992). These replacements lead to colour variation in micas.

Biotite & phlogopite – K(Mg, Fe)₃(AlSi₃)O₁₀(OH)₂ : Both are tri-octahedral with Fe²⁺ and Mg²⁺ as the dominant octahedral cations. However, phlogopite has less Fe²⁺ and more Mg²⁺ in octahedral sheet than biotite. Biotite is usually dark because of high Fe²⁺ content. In comparison with muscovite, biotite and phlogopite weather faster than muscovite and thus, they less common in highly weathered soils. Their weathering involves loss of interlayer K⁺ and oxidation of Fe²⁺ to Fe³⁺ (Fanning et al., 1989)^. Part of Fe³⁺ ions are released from their crystal structure on weathering.

Muscovite - KAl₂(AlSi₃)O₁₀(OH)₂: Muscovite is colourless and di-octahedral with Al³⁺ as the main octahedral cation (Fig.2). In the tetrahedral sheet Al³⁺ substitute for ¼ of Si⁴⁺.

c)      Vermiculite (Mg, Ca)_0.6-0.9(Mg, Fe³⁺, Al)_6.0[(Si, Al)₈O₂₀](OH)₄.nH₂O (Deer et al., 1992). The mineral has a net negative layer charge falling between -0.6 and -0.9 per for formula unit. This layer charge is less than that of micas but higher than that of smectites. It can either be tri-octahedral or di-octahedral. The tri-octahedral vermiculite forms by hydrothermal alteration or normal weathering of mafic minerals particularly biotite, phlogopite and chlorite. The di-octahedral varieties are common in soils and originate from the alteration of di-octahedral illite. Vermiculites have interlayer water that facilitates the exchange and migration of cations in the interlayer space. The minerals have a high cation exchange capacity (CEC) ranging between 50 and 150 cmol₍₊₎/kg. The high CEC of vermiculite is attributed to a number of factors. Among these factors is the interlayer ion exchange associated with the isomorphic replacement of Al³⁺ for Si⁴⁺ in its tetrahedral sheets and Al³⁺ and / or Fe³⁺ for Mg²⁺ in the octahedral layers. These isomorphic substitutions create a net deficiency of positive charges on the surface of vermiculite which is compensated by exchangeable cations.

Vermiculite when exfoliated by heating expands and becomes light and porous. The good liquid absorption and cation exchange capacities as well as light weight, particularly when exfoliated, make vermiculite an excellent product in agriculture, horticulture, and other sectors. These properties make it a suitable material for use as an additive to mulch, potting soils, and growing mixes and also for seed germination and transplanting trees and other plants. When applied to the soil it has a high ability to retain moisture and nutrients as well as to release them gradually to plants. Its high liquid absorption property also makes it suitable for use as a carrier medium in transportation of fertilizers and pesticides as a free-flowing solid. It can also be used as an adsorbent in removal of heavy metals from the soil and treatment of metal-contaminated wastewater which is dispersed to the land.

d)     Smectite group: Covers minerals with 0.2-0.6 net negative layer charge. The minerals can be tri-or di-octahedral. Di-octahedral smectites are more stable and abundant in soils than tri-octahedral (Borchardt, 1989). The minerals form in soils rich in silica, Mg²⁺ and Ca^2+. The soils must be poorly drained and low leaching with intense and long dry seasons (Folkoff and Meentemeyer, 1985). Most smectites are pedogenic: form by either partial weathering of other 2:1 clay minerals such as mica, vermiculite and chlorite or by dissolution of other minerals followed by precipitation or re-crystallization of dissolved ions. Smectites have weak electrostatic forces between the 2:1 layers. This weak forces permit water to enter the interlayer region and force the layers apart. The phenomenon leading to swelling of the minerals. The degree of swelling depends on the layer charge and whether the charge is in the tetrahedral or octahedral sheet.

Common dioctahedral smectites are:

i)                    Montmorillonite: Octahedral Al is partly substituted by Mg²⁺ and other divalent cations. The mineral has high ability to swell and shrink on wetting and drying, a phenomenon that facilitates soil genesis by mixing of the soil through the cracks that develop on drying. Also facilitates soil creep and landslides.

ii)                  Beidellite: In the tetrahedral sheet Si⁴⁺ is partly substituted by Al³⁺.

iii)                Nontronite: It is Fe-rich smectite with main substitution occurring in tetrahedral sheet.

Tri-octahedral smectites: Saponite and hectorite. The minerals are less common in soils

e)      Chlorites: These are tri-octahedral clay minerals with a layer charge similar to that of mica (1mole per unit cell). The octahedral sheet is dominated by Mg²⁺ or Fe²⁺. The chlorites have positively charged octahedral sheet. The interlayer hydroxide is continuous and blocks the ions from entering the interlayer region. The minerals originate from mafic igneous and metamorphic rocks. They are general unstable and alter to other clay minerals easily. Some are primary minerals and others are pedogenic, form from alteration of vermiculite or smectites.

f)       Sepiolite

Sepiolite is a hydrated magnesium silicate mineral. It is also known as meerschaum. The mineral is found in association with arid to semi-arid soils, Paleosols and alkaline lake sediments. It is tri-octahedral 2:1 layer silicates. It may contain Fe, Mn, Al and Ni in octahedral positions. In Tanzania, the mineral is found at Lake Amboseli, in Arusha Region at the border with Kenya. It is whitish in colour and light in specific gravity. The mineral occurs as spongy masses or fibrous in morphology (Fig.3). In agriculture it can be used as a liming material to correct soil acidity (Singh and Uriyo, 1976). In addition, sepiolite has considerable absorbent and adsorbent properties, which makes it an ideal material to remediate soils polluted with metals (Ẩlvarez-Ayuso and Garcia-Sànchez, 2003).

     

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