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    GG425/625 Wk 7 L13, S2018

    Lecture 13

    Soil Formation and Chemistry

    Please read Manahan Chapter 14 AND 15 (for this week and next).


    1. Weathering the context

    2. Clay Minerals the materials

    3. Organic solids the special sauce

    4. Some soil examples

    5. Inorganic reactions/transformation in soils

    GG425/625 Wk 7 L13, S2018

    Soils Intro

    Important substrate and growth medium

    for terrestrial biosphere.

    Susceptible to many anthropogenic


    They contain many materials in a

    gradient between

    organic rich surface deposits

    deeper inorganic deposits called saprolite.

    Saprolite with original rock textures preservedhttp://www.nicholas.duke.edu/eos/geo41/



    rich topsoil


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    GG425/625 Wk 7 L13, S2018

    On a gentle slope, rock is altered in place,

    sometimes to form soil.

    On a steep slope, weathered solids are whisked

    away by wind or water and deposited elsewhere, resulting in sediment accumulation elsewhere.

    Soils Intro

    How do soils form? Initially, physical and chemical breakdown of surficial rocks (weathering) produces secondary materials.

    GG425/625 Wk 7 L13, S2018

    WeatheringThe breakdown of rock to form secondary deposits is controlled by



    and biological processes

    Chemical and biological weathering are almost always mediated by H2O.

    During weathering new solid materials are formed and the composition of the

    mediating H2O is modified.

    The rates of alteration, and thus rates of soil (or sediment) accumulation and

    maturation, are governed by climate:


    the availability of H2O

    biome factors (flora/fauna and the DOC they produce)

    The formation of a soil is also dependent upon

    the bedrock type in the area

    physical factors (such as rock porosity and texture)

    mineralogic factors (solubility)

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    GG425/625 Wk 7 L13, S2018

    Weathering Primary minerals can be weathered from the source rock intact (mineralogically) or

    dissolved (recall congruent and incongruent dissolution)

    Mineral dissolution susceptibility is related to stability at the P, T and pE conditions of

    Earths surface.

    The higher their T and P, or more reducing the pE of formation, the

    more susceptible to weathering their minerals are.

    Many crustal rocks were formed at elevated P and/or T, and lower pE, in the lower

    crust or upper mantle. They were then "moved" to their present location at the surface

    through the combined processes of tectonics and erosion.

    The Bowen's reaction series (a

    gross generalization of mineral

    stability as a function of magma

    temperature) can also be used to

    understand weathering of many

    silicate minerals, because high

    temperature minerals are the first to

    form from a crystallizing magma

    and are more susceptible to


    GG425/625 Wk 7 L13, S2018

    Sequence of events for

    weathering common rock

    forming minerals

    The most soluble chemical

    elements are transported in the

    aqueous state to a new location

    (eventually the sea)

    The least soluble elements are

    mostly left behind.

    Elements of any solubility may be

    dissolved during weathering,

    redeposited by the aqueous

    solution somewhere down its

    flow path, and then re-dissolved

    in a new, later episode of

    weathering later.

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    GG425/625 Wk 7 L13, S2018

    Inorganic constituents: Minerals stable at high temperatures and pressures are broken down into hydrous sheet silicates (clays) and oxide minerals (Fe, Al and Mn oxides)

    Organic constituents: derived from flora, and soil microorganisms.

    Org.-Inorg. Proportion: Typical composition is 95% inorganic material and 5% organic matter -highly variable though.

    Soil Composition Basics

    GG425/625 Wk 7 L13, S2018

    Three main forms:

    Very resistant Primary minerals

    Alteration minerals (incongruently formed clays/oxides)

    Precipitation minerals (mostly carbonates/hydroxides)

    Inorganic Solids in Soils

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    GG425/625 Wk 7 L13, S2018

    Alteration minerals:The structure and composition of

    these solids is important because

    they modify soil water and affect

    the availability of nutrients to


    The types of secondary minerals

    formed from the weathering and

    hydrolysis of common primary

    minerals are given below.

    The mineral names are not

    important here, except to note that

    both clays and oxides can be


    ion exchange with water is


    CECs are variable.

    GG425/625 Wk 7 L13, S2018

    Simple oxide/ hydroxide examples are goethite and gibbsite.

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    GG425/625 Wk 7 L13, S2018

    Clay minerals:

    Clays are structurally more complex.

    They are composed of layered matrices of Si, Al and Mg

    bonded to O.

    Of the 3 basic clay types (platy, fibrous and

    amorphous), the most important in soils are the

    platy "phylosilicate" clays

    The layers are of two types:

    tetrahedral and octahedral

    These occur in clay minerals in 2 layer and 3 layer


    2 Layer Clays - T-ORepeated units of 1 tetrahedral and 1 octahedral layer

    3 Layer = T-O-T)Repeated units of 2 tetrahedral and 1 octahedral layers

    GG425/625 Wk 7 L13, S2018

    tetrahedral (Si surrounded by tetrahedrally-arranged O)

    SiO4 tetrahedra share 3 basal oxygens

    with neighboring tetrahedra, forming a

    sheet structure. The Si:O ratio = 1 to 1

    lone O + 3 50% shared oxygens =

    1: (1 + 3 x 0.5) = 1:2.5 = 2:5

    octahedral (Al or Mg surrounded by octahedrally-arranged O as hydroxyl groups).

    Octahedral Mg clays are commonly formed

    only during alteration of magnesian rocks.

    Octahedral layers of pure Al and Mg occur in

    the minerals gibbsite, Al(OH)3, and brucite,


    each Al(OH)6 (or Mg(HO)6) octahedron

    shares all of its oxygens with neighboring

    octahedra. Al:O ratio of 3, as in gibbsite.

    T and O layers combine by sharing the non-

    basal O of the silica tetrahedra with the one of

    the octahedral O atoms on each Al (or Mg).

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    GG425/625 Wk 7 L13, S2018

    T-O clays:

    we can think of each Al as having effectively lost one O atom to a

    Si, and Al:O goes from 1:3 to 1:2 (octahedral O atoms are

    actually in hydroxide form).

    Kaolinite, the simplest T-O clay, has Si:O of 2:5,

    Al:Si of 1:1 (or 2:2) and Al:OH of 1:2 (or 2:4).

    This gives the formula Al2Si2O5(OH)4.

    T-O-T clays:

    Similar arguments can be made to show that T-O-T clays have

    Si:Al of 2:1

    The simplest chemical formula is Al2Si4O10(OH)2 (pyrophyllite).

    GG425/625 Wk 7 L13, S2018

    Solute-Solids interactionsStill and through flowing water interacts with solids to exchange compositional attributes:

    3 mechanisms of compositional exchange with water operate,

    as discussed last week.

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    GG425/625 Wk 7 L13, S2018

    Ion Substitution:

    Ion substitution for Si, Al and Mg gives clays exchangeable ion

    sites that can exert a compositional control on aqueous solutions

    contacting them. The degree of substitution depends on the

    environment of their formation.

    Octahedral replacement is by ions such as Fe+3, Fe+2, Cr+3,

    Zn+2, Li+.

    Tetrahedral Si replacement is less common and mostly limited

    to Al-for-Si substitution.

    Structural substitutions result in a charge imbalance on the clay

    backbone that is balanced by addition of interlayer (non-

    structural) ions and accounts for the CEC of clays (as discussed

    last week).

    GG425/625 Wk 7 L13, S2018

    Charge on clay particles:In addition to cation exchange, clays and oxides take charges in

    natural waters (discussed earlier this semester).

    The sign of the charge

    is a function of pH and

    the density of charge is

    a function of the


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    GG425/625 Wk 7 L13, S2018

    Al (OH)3

    Na, Ca containing clay Clay mineral depleted

    in Alkalis & Alkali

    Earths: Al2Si2O5(OH)4

    Increased water flow during weathering leads to increased leaching of cations

    which lowers CEC and charge on clays.

    GG425/625 Wk 7 L13, S2018

    The gain or loss of chemical constituents in saprolite records the progress of weathering/ soil formation in the absence of significant DOC.

    In practice, Al is the least soluble element during weathering followed by Ti and Fe.

    Please note that % metal oxide

    is a way of expressing bulk

    composition of a rock. Many of

    these oxides are not actually

    present in the rock as oxides.

    SOIL INORGANIC SOLIDS saprolite development


    changes that

    occur during



    changes that

    occur during


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    GG425/625 Wk 7 L13, S2018

    Elements removed during saprolite formation have high concentration in soil

    and ground waters.

    Si is removed slower than Ca and Na. Lower but still significant Si

    concentrations remain at high % Al.

    Fe and Ti continually increase with Al, suggesting that a totally weathered rock

    would be mostly Al, Fe, Ti and Si (-the