Clay Types Study GuideTypes of Colloidscrystalline silicate clays (covered by this guide)non-crystalline silicate clays (p 314)Fe & Al oxides (p 315, 322ff)Organic (p 315, 325)Basis for distinguishing silicate clay typesIsomorphous substitutionReview of clay typesDistributionWeathering & generalized distribution in US

Basis for distinguishing crystalline silicate claysBased on numbers & combinations of structural unitstetrahedral and octahedral sheetsplanes combined sheets combined layers crystals (fig 8.4)

Two general categories: 1:1, 2:12:1 types: expanding & nonexpandingalso 2:1:1 Chlorites

Number of cations in octahedral sheettri- vs. di-octahedra (fig 8.5)

Size and location of layer charge (see also lecture 16 slides)

Type of bonding between layers (see also lecture 16 slides):Strong: ionic > H-bonding > van der Waals :Weak

Absence or presence of a cation interlayerfine-grained micas

See lecture 16 slides: review of diffs in properties of clay types

Clay minerals2:1 clays (two tetrahedral sheets for each octahedral sheet)Montmorillonite,beidellite, saponite, etc.Illite, muscovite, biotite, etc.Tri- or di-vermiculiteCookeite, chamosite, etc.Weird, not truly 2:1SmectitesMicasVermiculitesChlorites

Visual comparison of common silicate clays structureillitemontmorillonite2:1:1more strongly heldthan in smectite

Isomorphous substitution

The replacement of one ion for another of similar size within the crystalline structure of the clayOften results in change in net charge

takes eons doesnt change rapidlyequalshape/size

Substitution in tetrahedral sheetTetrahedral sheet+4, +3, -8 (-2*4)negative

Substitution in octahedral sheetnegative -2, +3, +2, -4Octahedral sheet

1:1 Silicate ClaysLayers composed of one tetrahedral sheet bound to one octahedral sheetKaolinite: one of the most widespread clay minerals in soils; most abundant in warm moist climatesStable at low pH, the most weathered of the silicate clays Synthesized under equal concentrations of Al3+ and Si4+

KaoliniteA 1:1 clayLittle or no isomorphous substitutionnutrient poorNo shrink-swell (stable cuz of H-bonding between adjacent layers)A product of acid weathering (low pH, common in soils of the SE USA

Structure of KaoliniteSheets of silicon tetrahedra and aluminum octahedra linked by shared oxygen atoms. NO ISOMORPHOUS SUBSTITUTION!!!

Kaolinite under low pHAlOH + H+ AlOH2+No charge positive charge

2:1 Silicate ClaysTwo silica tetrahedral sheets linked to one aluminum octahedral sheetThree key groups:Smectites (e.g., montmorillonite)VermiculitesMicas (e.g., illite)And one 2-1-1 (chlorites)

Montmorillonite (2:1, a Smectite)Layer charge originates from the substitution of Mg2+ for Al3+ in the octahedral sheetUnstable (weathers to something else) under low pH and high moistureMost swelling of all claysNutrient rich

Structure of MontmorilloniteStructure of montmorillonite (a smectite): it is built of two sheets of silicon tetrahedra and one sheet of aluminum octahedra, linked by shared oxygen atoms. AlO

Structure of MontmorilloniteIsomorphous substitution here, in the octahedral sheet= Mg

Vermiculites (2:1)Alteration product of micas (rock form)Formed from loss of K+Interlayer K+ of mica replaced with Mg2+Limited shrink-swell

Vermiculites (cont.)High layer charges: BOTH tetra and octanutrient rich!Stable under moderate to low soil pH, high Mg, FeCommon in midwestern US

Structure of VermiculiteLots of charge imbalance, both sheets:High nutrient supply capacity= Al= Fe= Mg

Illite (2:1, a Mica)Al3+ substitution for Si4+ on the tetrahedral sheetStrong surface chargefairly nutrient poorNon-swelling, only moderately plasticStable under moderate to low pH, common in midwestern US

Structure of Illite

Structure of Illite

Chlorites (2:1:1)Hydroxy octahedral sheet in the interlayer spaceRestricted swellingNutrient poorCommon in sedimentary rocks and the soils derived from them

Structure of ChloriteMg-Al hydroxy sheetMg-Al hydroxy sheet

Iron-richlocked structureLow nutrient supply capacity= Al = Fe= Mg

Visual comparison of common silicate claysillitemontmorillonite2:1:1Strongly heldH-H

Factors affecting mineral stability Number and type of base cations in the structure (base cations are soluble)Number of silica tetrahedra that are linked (more sharing of oxygens = more stable)Al3+ proxy for Si4+ (more proxy = less stable)Presence of Fe (more Fe = less stable)Kinds of bondsIonic are heat tolerantCovalent generally stronger cuz shared electrons between atoms, but not heat tolerant

Weathering pattern of clay formationEntisols, InceptisolsVertisols2:11:1Fe/Al OxSpodosolsFig 8.16OxisolsUltisols

CEC and weathering intensityAlfisols, Vertisols, Argiudolls*

Ultisols

Oxisols

*remember nomenclature structure = argi-ud-oll

Where to find different clays see Table 8.3

(only 2:1 with no subsitution)(only 2:1 with no subsitution)v(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)(only 2:1 with no subsitution)See the following webstie:http://www.gly.uga.edu/schroeder/geol6550/CM06.htmlhttp://www.gly.uga.edu/schroeder/geol6550/CM08.html-addt box off 2:1 box Pyrophyllites: only 2:1 with no substitution