GES 166/266, Soil Chemistry

Lecture Supplement 1 Soil Solids

I SOIL MINERALOGY

I-1, Mineral Stability

Minerals will both degrade (dissolution) and form (precipitation) due to meta-stable conditions. We can break the minerals into two formation categories: PRIMARY & SECONDARY. Primary minerals are formed at high temperatures and pressures through what we may term ‘geologic’ processes. Consequently, they are unstable under atmospheric conditions. Secondary minerals form under atmospheric conditions (such as those found in soils) but may be unstable if conditions change from those of their formation. Primary minerals tend to release their ionic constituents; this can lead to the "neo" formation (i.e., precipitation) of secondary minerals. Secondary minerals can also be formed from the solid-state transition of a primary mineral, e.g., mica->vermiculite.

The formation conditions of primary minerals leads to their stability, and is summarized by the Bowen Reaction Series shown below. This series starts with the minerals formed at the highest temperature with subsequent minerals formed at progressively lower temperatures. The stability of the minerals in soils increases with lower temperatures of primary mineral formed.

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Olivine

Augite

Hornblende

Phlogopite

Ca-Feldspar

Na-Feldspar

K-Feldspar

Muscovite

Quartz

LeastStable

MostStable

Bowen Reaction Series

I-2, Ion Bonding: A quick review

Ions or molecules aggregate in response to electrostatic and chemical attractions. The interaction strength depends on the type of bond, while the coordination numbers depend on the ratios of ion or molecular sizes.

Types of Bonds:

Ionic: bonds that occur between ions of opposite charge without the sharing of electrons.

- occur between elements on opposite ends of the periodic table- non-directional bonds- relatively strong bonds giving high melting points- very common for soil materials, e.g., NaCl, CaCO3, etc..

Covalent: the sharing of electron pairs between the combining atoms

- occur between atoms of similar electronegativity, e.g., H2O, CH4- strongest bond- directional- bonding within molecules is usually covalent

H-bonding: a weak electrostatic bond between H+ and highly electronegative atoms, e.g., F-, O2-.

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- although weak individually, they can be very strong when multiplied

- water forms H-bonds

van der Waals: weak attractive forces between atoms. They can be significant when present in large numbers but generally result in very weak planes of attraction

Bonding is rarely exclusively of one kind, e.g., in crystals most bonds are a composite of both covalent and ionic character; the degree of each character does vary significantly, however.

The Bond Valence () (electrostatic bond strength) is a convenient way to denote the charge balanced by each bond. It is the formal charge observed between the central cation and each coordinating atom.

= (cation valence) / cation coordination number = Zcation / CNcationexamples: Al(OH)6 : = 3 / 6 = 1/2

OH

Al

OH

OH

OH

OHOH

-1/2

-1/2 -1/2

-1/2-1/2

-1/2

SiO4: = 4/4 = 1

O

SiO

OO

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I-3, Ion Coordination

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Atoms bond to stabilize their electronic configuration or to provide charge shielding. We will call the cation the central ion, and it will be coordinated by anions that we will term ligands. The coordination of a cation by a ligand is dependent on the ratio of their sizes--it is strictly a geometric factor. The following table provides an estimate of usual coordination observed in nature for size ratios.

Table of Radius Ratios and expected coordination.

Radius Ratio Coordination Coordination(rcation / ranion) Type Number

0.15 - 0.225 trigonal 30.225-0.414 tetrahedron 40.414-0.732 octahedron 60.732-1.000 cube 81 dodecahedron 12

(closest packed)

Examples: High temp: Al-O: RAl = 0.51 , RO = 1.40 ; RAl/RO = 0.364

=>tetrahedronLow temp: Al-O: RAl = 0.60 ; RAl/RO = 0.429 => octahedron

Si-O: RSi = 0.42 ; RSi/RO = 0.3 => tetrahedronI-4, Building Blocks of Soil Minerals

Oxygen is the most abundant element in the earth's crust and is the dominant element in the solids of soils. Si is the second most abundant and Al the third; they along with O make up most of the minerals we will consider. We also need to appreciate Mg since it makes up a class of minerals we will discuss.

Basic building block on the molecular scale are:

SiO4 Al(OH)6 Mg(OH)6

OH

Mg

OH

OH

OH

OHOH

O

SiO

OO

OH

Al

OH

OH

OH

OHOH

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These units can be linking and stacked in various arrangements to give the common minerals we observe in soils. This can be thought of as either 1) the build-up of molecular linkages or 2) oxygen sheets filled with cations that are needed for electrical neutrality.

On the following page, both octahedral and tetrahedral sites can be seen by the arrangement of oxygen sheets. There are two types of 'voids' present in these oxygen sheets, which are designated by 'b' and 'c'. Due to charge constraints, only one type of void will be filled in the phyllosilicate minerals. It does not matter if it is the 'b' or 'c' type voids that fill, but only that the mineral has one type consistently (exculsively) filled. Now, based on electrical neutrality you should be able to determine the octahedral or tetrahedral cation occupancy.

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b

c

Tetradral Cavit ies,occur when an atomis placed over acavity

bc

Octahedral Cavit ies,formed when cavit iesalign over cavit ies

bc

b C

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The octahedral vacancies will be filled to maintain charge neutrality. We can determine their filling by first using the bond valence theory. For trivalent ions such as Al3+ in octahedral coordination, = 1/2. Therefore, we know that each anion will have 1/2 of a charge satisfied by each bond to the trivalent cation. If the anions (ligands) are hydroxyls, as in gibbsite [Al(OH)3], then each OH- must be bound to 2 Al3+ for charge neutrality. We can write a formula for the needed coordination of the ligand based on v:

[The number of cations coordinating each anion] = [ (anion charge) / ]So, for a divalent ions, such as Mg2+, in octahedral coordination by OH- [Mg(OH)2]:

v = 1/3, and 3 (1 / 1/3) Mg2+ must surround each OH-.In fact, these are the observed coordination environments in minerals. In the diagrams below the position of divalent and trivalent cations is shown to give the observed coordination.

= Al

= Mg

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Note that every octahedral site of the same type must be filled to obtain charge neutrality with divalent cations. For trivalent cations this is not the case; here, every third octahedral site must remain vacant in order to maintain charge balance. The vacant sites in adjacent rows are shifted so that vacancies are directly below two filled octahedral sites. Take a look at these diagrams and make sure that you can see that each hydroxyl is maintaining charge neutrality, except, of course, those at the edges. A good way to check whether the octahedral sites are filled correctly is to draw a hexagonal ring with the filled sites (see bottom diagram on the previous page). If a hexagonal ring does not result then the filling was incorrectly diagrammed.

Forming and Linking Tetrahedral and Octahedral Sheets

The main class of soil minerals are formed by linking silica tetrahedra, to from tetrahedral sheets, and either Al or Mg octahedra, to form octahedral sheets. The tetrahedral sheets are formed by a sharing of 3 O, which form hexagonal rings, and give a formula (Si2O5)2-. The octahedral. units are Me(OH)2 or Me(OH)3 (where Me is a metal cation such as Al3+, Mg2+, or Fe2+). The octahedral sites in the oxygen (hydroxyl) lattice can be completely filled by divalent cations or two out of every three sites can be filled by trivalent cations, as we have just discussed. Since 3 out of 3 sites are filled in the divalent cation case these minerals are termed trioctahedral; those with trivalent cations are dioctahedral minerals. It turns out the Al and Mg octahedra have ligand positions very closely aligned to those of the apical oxygens in the silica tetrahedra. As a consequence, the tetrahedra and octahedra can link together to form a class of minerals known as the phyllosilicates ('phyllo' being a Greek word for 'layered'). As shown on the following page, the phyllosilicates can have varying ratios of tetrahedra to octahedra. The simplest form is just a single octahedra; the next type of phyllosilicate would have a Si-tetrahedral sheet added to 1 octahedral sheet (a 1:1 mineral). Then we move on to one composed of 2 tetrahedra and 1 octahedra (known as a 2:1 mineral). The ratio of tetrahedra to octahedra is the dominant means of classifying the phyllosilicates. Additionally, this class of minerals is further identified by the cation in the octahedral position and the layer charge, which is our next topic.

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I-5, Charge Development

There are two primary processes by which minerals exert a charge: isomorphic substitution and terminal ("broken") bonds. Isomorphic substitution is the replacement of one ion with another having a different charge but with no change in the mineral structure. Examples are the substitution of Al3+ for Si4+ or Al3+ for Mg2+. This charge is termed "permanent" since it will not vary as a function of the solution composition. Terminal (“broken) bonds result from coordinately unsaturated central cations or ligands that occur at terminal ends of many minerals. Mineral that are composed of 2-dimensional or 3-dimensional growth patterns need to extend infinitely to satisfy all their charges; this of course is not possible and so that at the surfaces of these minerals a charge will emanate into the surrounding media. In solution such surfaces will react with water and its components (H+ and OH-) yielding a pH dependent charge--this is also termed a variable charged surface. Of the minerals we will discuss, the x-axis {the (100) plane) and y-axis edges {the (010) plane} of the phyllosilicates and all faces of the hydrous oxides will have variable charges.

The charges on the functional groups of variable charged surfaces can be determined based on the knowledge of the bond valences and coordination number. For example, suppose we are looking at the mineral gibbsite, Al(OH)3; Al enters octahedral coordination (CN = 6), so that 6 OH- ions must surround each Al. We also know that Al is trivalent, which leads to v = 3/6 = 1/2. From this we can also reason that each OH -

must be bound to two Al ions to satisfy its charge (each bond only satisfies 1/2 a charge and the OH- has 1 unit of charge so two bonds are needed to fully satisfy its charge). Therefore, if at the surface of gibbsite a OH- ion is only bound to 1 Al then it will have a charge of -1/2. This surface group could also react with water and become protonated yielding a +1/2 charge.

surfacemineral plane protonation positively charged surface

(occurs at low pH)

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Al

OH

OH

Al

OH

OH

OH

Al

OH

OH

Al

OH

OH

OH2

2

+ 2 H+

+1/2

+1/2-1/2

-1/2

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I-6, Crystal Planes and Faces (Miller Indices)

In many minerals, particularly the phyllosilicates, different crystal faces will have vastly different properties than others. Accordingly, we need to have a nomenclature that will allow us to identify these different mineral surfaces. To accomplish this we turn to a notation called "Miller Indices".

The Miller Index uses 3 parameters in a Cartesian coordinate system to represent a face or plane. Planes and faces are designated by: (hkl). In this system, hkl represents the reciprocal of the specific axis intercept, where h is for the x-axis, k the y-axis, and l the z-axis. Furthermore, the Miller Indices are all designated to be within 1 unit value of each axis. Why? Because for any mineral the length given by 1 unit value is that of the unit cell (the smallest repeatable unit within the crystal). Since the unit cell can be repeated to make up the crystal, moving to a second unit on an axis will simply repeat our first unit. That is to say that the crystal is exactly the same from 0 to 1 as it is from 1 to 2 units of the unit cell. Therefore, we only have to concern ourselves with faces and planes within 1 unit value of each axis.

For the phyllosilicate minerals, 3 faces are of concern for us: the (100), (010), and (001). These 3 planes are represented schematically below. Looking back a couple of pages you can see the usual orientation of the phyllosilicates. In this orientation the (100) and (010) faces will have similar properties; the (001) plane, however, will be very different. The (001) plane is composed of surface oxygen groups that are completely charge (and coordinatively) satisfied, making them chemically unreactive. In contract, the surface groups, either oxygens or hydroxyls, on the (100) and (010) faces are not charge balanced or coordinatively satisfied. They will therefore be chemically reactive and generate a charge simply from their unsatisfied coordination environment (terminal bonds).

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x

y

z

(100)

1

x

y

z

(0 1 0 )

1y

z

(0 0 1 )1

x

I-7, Phyllosilicates As already stated, the phyllosilicates are minerals composed of Si,Al-tetrahedral

and Mg- or Al-octahedra. The phyllosilicates are classified based upon:

1. The number of tetrahedra and octahedra in a layer2. The octahedral site occupancy (i.e., octahedral composition)3. The charge per formula unit for each layer.

The dominant minerals we will study are provided in the following table, based on the specified criteria above.

General Classes (layer build-up) of Phyllosilicate Minerals:

Layer Type Charge† Trioctahedral Dioctahedral

1 octahedra 0 brucite, Mg(OH)2 gibbsite, Al(OH)3

1 tet. : 1 oct. 0 serpentine, Mg3Si2O5(OH)4 kaolinite, Al2Si2O5(OH)4

2 tet. : 1 oct. 0 talc, Mg3Si4O10(OH)2 pyrophyllite, Al2Si4O10(OH)2

2 tet: 1 oct. 1 phlogopite muscoviteKMg3(AlSi3O10)(OH)2 KAl2(AlSi3O10)(OH)2

1 biotiteKFe3(AlSi3O10)(OH)2

0.6-0.8 illite (hydrous mica)K(Na,Ca) Al1.3Fe0.4Mn0.2Si3.4Al0.6O10(OH)2

0.6-0.9 vermiculite

0.25-0.6 smectite† The layer charge per formula unit

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I-8, Mineral Descriptions

Non-expandable Clay Minerals

Brucite and Gibbsite: Gibbsite is a common secondary mineral, and it is a sink for aqueous Al. Brucite is less commonly observed in soils but is found in those derived from ultramafic parent materials.

Serpentine and Kaolinite: The structural sheets composed of 1 tet. to 1 oct. are held together by H-bonds. Because of the numerous bonds these sheets are held rather tightly together and are thus only slightly expandable.

Serpentine, like brucite, is found most commonly in soils derived from ultramafic material; it exhibits a tubular morphology due to differences between the octahedral and tetrahedral layers. (Incidently, asbestos is composed of serpentine minerals and is hazardous to lung tissue because of it’s fiberous habit --a consequence of the tubular morphology).

Kaolinite is a common soil mineral and may be the most ubiquitous of all soil minerals. It has a low cation exchange capacity value (vida infra) ranging from 10 to 100 mmol / kg; the charge originates from unsatisfied bonds at the terminal ends of the sheets and is pH dependent. The surface area is also low, typically between 7 and 30 m2/g.

Talc and Pyrophyllite: These minerals are the structural basis for the 2:1 clay minerals. They, however, have no permanent charge. Adjoining layers are bonded via van der Waals forces. As a consequence, the mineral is very easily cleaved along this bonding plane and accounts for the slipperiness of these minerals; they have minimal CEC.

Micas (phlogopite, muscovite, and biotite): 1 Al substitutes for every 4th Si in the tetrahedral sheet of the micas. This produces a charge of -1 per formula unit; the additional charge is satisfied by K+ ions that resides in the interlayer cavities and is bound such that it is not exchangeable. The high layer charge results in a strong assemblage of the layers which are thus non-expandable; there is no appreciable CEC. Potassium does have a role in this collapse; it is the perfect size to fit in the cavities between layers and allows for the collapse of these layers.

The trioctahedral micas weather more rapidly than the dioctahedral ones. The K+ in the interlayer is released during weathering—a source of nutrients in soils.

Expandable Clay Minerals

The layer charge of these minerals is reduced relative to mica. This occurs from a number of processes including:

i) Fe(II) oxidation to Fe(III)

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ii) H+ incorporation into the structureiii) exchange of Si4+ for Al3+ in the tetrahedral sheetiv) cation (e.g., Al3+ for Mg2+) substitutions in the octahedral layer

Illite (hydrous mica): This mineral is derived from the weathering of the muscovite. It differs in that a divalent cation substitutes in the octahedral layer for Al. Also, the quantity of Si is increased in the tetrahedral layer. The CEC is approximately 300 mmol / Kg (30 meq/100g).

Vermiculite: Can occur in either di- or trioctahedral forms. Approximate formulas for these are:

dioctahedral: Nax(Al,Fe)2(AlxSi4-x)O10(OH)2•4H2Otrioctahedral: Nax(Mg,Fe)2(AlxSi4-x)O10(OH)2•4H2O

-The charge originates primarily from isomorphic substitution in the tetrahedral layer (Al for Si). The charge in the tetrahedral layer may be somewhat offset by Al3+

substitution for Mg2+ in the octahedral layer—a source of positive charge that diminishes (slightly) the negative charge developed in the tetrahedral sheet. The cations in the interlayer, which balance the layer charge, are exchangeable. Due to a relatively high layer charge that originates from the tetrahedral layer, the layers are not very expandable. Ions of the correct size to ‘fit’ the interlayer cavity can lead to a layer collapse of the structure; such ions include K+, NH4+, Cs+, Rb+. Once these ions collapse the structure they are no longer exchangeable. The CEC and surface area (SA) are high, generally on the order of CEC = 1200-1500 mmol/Kg, SA = 600-800 m2/g.

Smectites: The smectite minerals are the most expandable of the clay minerals and account for most of the observed shrink-swell properties in soils. This is due to their moderately-low layer charge. Additionally, they have a very high water holding capacity as a result of such swelling. Like the vermiculite minerals, smectites can be found in both dioctahedral and trioctahedral forms.

Dioctahedral Forms:

Montmorillonite: ≈ Nax(Al2-xMgx)Si4O10(OH)2 Mg2+ is substituted for Al3+ in the octahedral layer x = 0.25-0.45

Beidellite: ≈ Nax(Al2)AlxSi4-xO10(OH)2 Al3+ substituted for Si4+ intetrahedral layer x = 0.45 - 0.5

Trioctahedral Forms:

Saponite: ≈ Nax-y(Mg3-yAly)AlxSi4-xO10(OH)2 Al3+ substituted for Si4+ intetrahedral layer and for Mg2+ inoctahedral layer

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Hectorite: ≈ Nax(Mg3-xLiy)Si4O10(OH)2

The CEC ranges from 800 to 1200 mmol / Kg with a surface area of 600-800 m2/g. These minerals have very small particle sizes and are highly expandable. They are the major components of soils that limit water movement in dispersed soils. Montmorillonite is commercially available under the name "Bentonite", which is used as an non-permeable material for ponds or earthen damns.

Integrades: There is a potential for cationic metals such as Al3+, Cr3+, and Fe3+ to precipitate within the interlayers of the 2:1 minerals that have a permanent charge; these precipitates may be generally termed 'hydroxy interlayers'. Interlayer precipitates are in essence an additional octahedral layer, forming a 2:1:1 type mineral, if complete interlayer occupancy is achieved. Usually, full occupancy of the interlayer is not reached and so the resulting phase is termed a 2:1 integrate. Integrades decrease the CEC of the mineral but often increase the anion exchange capacity (AEC). The interlayer precipitates also decrease the phyllosilicate's surface area, expandability, and water-holding capacity. They occur most commonly in vermiculite but are also found in some of the smectite minerals.

Chlorite: True chlorite is a primary mineral with a 2:1:1 layer structure (also called 2:2) and has the formula (AlMg2(OH)6)x {Mg3(Si4-xAlx)O10(OH2)}. Note that chlorite is a trioctahedral mineral (both octahedra have Mg as the primary cation) with some Al substituting in the additional octahedral sheet. The surface are is 70 to 150 m2/g and the CEC 100-400 mmol/Kg. Again some positive charge is associated with the Al substituted octahedral layer giving rise to anion exchange. Pedogenic chlorites are actually end-members of the intergrades discussed above.

Tectosilicates (Frame-work silicates)

Extending the layer silicates to a three dimensional structure, so that every tetrahedra shares all of its 4 oxygens with neighbors, results in the class of minerals known as the tectosilicates. The most common and well known member of this group is quartz (SiO2). As should be expected by its abundance in the surface environment, the structural arrangement of the tectosilicates leads to a very stable configuration that resists weathering. Another well known member of this group are the Feldspars. While abundant is some soils, the tectosilicates do not significantly contribute to the chemical or electrostatic properties of soils; they are relatively inert due to their stable configuration.

Zeolites are also tectosilicates; they are unique minerals having a 3-D aluminosilicate framework that forms channels and cavities of distinct size. These channels can act like molecular sieves and account for the use of zeolites as catalysts of petroleum products. The aluminosilicate tetrahedra are connected by their vertices and are composed of 4 to 12 member rings linked together; in the center of the rings are the channels.

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Accessory Minerals: The Hydrous Oxides

In well developed, highly weathered soils the phyllosilicate clay fraction begins to diminish giving way to Al and Fe oxides, hydroxides, and oxyhydroxides (collectively referred to as hydrous oxides). Because these materials form late in the weathering sequence and at the expense of other minerals, they are also referred to as secondary minerals. Due to their high surface areas, ability to coat other particles, and reactivity, hydrous oxides can exhibit a marked influence on the soil chemical properties even when composing only a small fraction of the total solids. Most of the secondary minerals do not have permanent charges, but they do have a very active variable charge. Because of their structure, all surface planes will be coordinatively unsaturated and consequently develop a variable charge. In addition, this results in the surfaces having a strong CHEMICAL affinity for many ions, which leads to a very strong retention of ions on such surfaces.

Silica: In addition to the very stable tectosilicate forms, hydrated amorphous forms with very high surface areas are also common in soils. These materials are not rigid structures, rather they form a gel-like amorphous phases. They can contribute to the cementation of clay rich layers. The ZPC is low (at pH 2-3) so that the dominant electrostatic interaction will arise with cations.

Allophane: A generic term for amorphous hydrated aluminosilicate with a Si:Al ratio of ≈1-2. Aluminum enters octahedral coordination and Si is present in tetrahedral coordination. They are commonly observed in volcanically derived soils and sediments. The exchange capacity of allophanes is very large; values of up to 1500 mmol/Kg have been reported. As might be expected, their surface area is also large (70-300 m2/g). Allophane forms a spheroid surface topography and has short to mid-range atomic ordering.

Imogolite: (SiO2•Al2O3•2.5H2O) Similar to allophane, imogolite is also a common material found in soils derived from volcanic parent material. It has a tubular morphology, in contrast to the spherical morphology of allophane. It also has a very high CEC (1350 mmol/Kg).

Hydrous Al Oxides: The most abundant of the secondary minerals is gibbsite, -Al(OH)3. It is a very stable mineral at low temperatures and is the building block for other phyllosilicates (the dioctahedral class). In gibbsite, hexagonal sheets are bound by H-bonds. It, like the other hydrous Al oxides, has a very high ZPC that ranges from 8 to 9.5. The surface area of this mineral is moderate, 5-20 m2/g. It is found ubiquitous in well developed soils, particularly of the tropic. Another Al phase found in soils and sediments is boehmite (-AlOOH). It is the analog to lepidochrosite, an iron oxyhydroxides.

Hydrous Fe Oxides: Ferric hydrous oxides are abundant in many soils and due to their strong pigmentation they are easily recognized; the yellow and red soil colors are due to this class of minerals. Based on radius ratios, Fe(III) should enter octahedral coordination, and this is observed in nature. Accordingly, the Fe oxides are similar to the Al-oxides.

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Goethite (-FeOOH) is the most abundant of the HFOs but is closely followed by hematite (-Fe2O3). Goethite exhibits a yellowish color and is common in lower temperature, higher moisture soils. Hematite forms a bright pinkish color, and is favored in high temperature low moisture areas. Additionally, high pH favors the formation of hematite relative to goethite. The surface are of goethite is slightly higher than for hematite (10-50 m2/g for goethite and 5-20 m2/g for hematite). Ferrihydrite is another common form of Fe is soil, and although not as stable as either hematite or goethite it forms rapidly from Fe(III) precipitation. Given enough time ferrihydrite will transform to either goethite or hematite. Iron oxides have a strong affinity for many metal ions and consequently are used in wastewater treatments to partition metals out of the aqueous phase. They also form concretions and coatings in many soils.

Manganese Oxides: The manganese oxides are an important fraction of the soil because they are one of the strongest oxidants of naturally occurring material. They also have a very high affinity for cationic metals such as Fe(III), Al, Zn, Cu, etc. In marine systems, nodules composed of concentric rings alternating between Fe- and Mn-oxides are common. These interesting noodles result from redox transformation and tend to be rich in other metals such as Co as well.

The most prevalent Mn-oxide in soils is the mineral birnessite (a disordered -MnO2 phase). Manganese oxides form coatings on many particles but are also observed as discrete nodules; their strong black pigment often dominates the soil color--occasionally this is misidentified as arising from organic matter. These materials usually have a low ZPC, high surface area, and are very reactive. They are also one of the first materials lost to reductive dissolution under oxygen limiting conditions.

Titanium Dioxides: A residual from the primary minerals, the titanium dioxides are found in lesser amounts in soils. They do have both an anion and cation exchange capacity due to a moderate ZPC near 5-6. There are two polymorphs (minerals with the same chemical composition but differing structures) of TiO2, rutile and anatase. Both are found in soils, but rutile a little more commonly. Both of these minerals are strong white pigments and account for the white color of paints. In fact, anatase and rutile are extensively mined as pigments for paints.

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