Clay Types Study Guide

• Types of Colloids– crystalline 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 types• Isomorphous substitution• Review of clay types• Distribution• Weathering & generalized distribution in US

Basis for distinguishing crystalline silicate clays

• Based on numbers & combinations of structural units– tetrahedral and octahedral sheets–planes combined sheets combined layers crystals (fig 8.4)

• Two general categories: 1:1, 2:1–2:1 types: expanding & nonexpanding–also “2:1:1” Chlorites

• Number of cations in octahedral sheet– tri- 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 diff’s in properties of clay types

Clay minerals

1:1 clays (one tetrahedral sheet for each

octahedral sheet)

Kaolinite, nacrite, dickite, halloysite, etc.

2:1 clays (two tetrahedral sheets for each

octahedral sheet)

Montmorillonite,beidellite,

saponite, etc.

Illite, muscovite, biotite, etc.

Tri- or di-vermiculite

Cookeite, chamosite,

etc.

‘Weird’, not truly 2:1

Smectites Micas Vermiculites Chlorites

Visual comparison of common silicate clays’ structure

illitemontmorillonite“2:1:1”

more strongly heldthan in smectite

Isomorphous substitution

• The replacement of one ion for another of similar size within the crystalline structure of the clay

• Often results in change in net charge

takes eons – doesn’t change rapidly

equal shape/size

Substitution in tetrahedral sheet

Si2O4 SiAlO4

neutral

Tetrahedral sheet

+4, +3, -8 (-2*4)negative

Substitution in octahedral sheet

(OH)2Al2O2 (OH)2AlMgO2

neutral negative -2, +3, +2, -4

Octahedral sheet

1:1 Silicate Clays• Layers composed of one tetrahedral

sheet bound to one octahedral sheet• Kaolinite: one of the most widespread

clay minerals in soils; most abundant in warm moist climates

• Stable at low pH, the most weathered of the silicate clays

• Synthesized under equal concentrations of Al3+ and Si4+

Kaolinite• A 1:1 clay

• Little or no isomorphous substitution

• “nutrient poor”

• No shrink-swell (stable ‘cuz of H-bonding between adjacent layers)

• A product of acid weathering (low pH, common in soils of the SE USA

Sheets of silicon tetrahedra and aluminum octahedra linked by shared oxygen atoms.

Structure of KaoliniteNO ISOMORPHOUS SUBSTITUTION!!!

Kaolinite under low pH

Al—OH + H+ Al—OH2+

No charge positive charge

2:1 Silicate Clays• Two silica tetrahedral sheets linked to

one aluminum octahedral sheet

• Three key groups:– Smectites (e.g., montmorillonite)– Vermiculites– Micas (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 sheet

• Unstable (weathers to something else) under low pH and high moisture

• Most swelling of all clays

• “Nutrient rich”

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

Structure of Montmorillonite

Al

O

Structure of Montmorillonite

Isomorphous substitution here, in the octahedral sheet

Causes cations to move into the interlayer space, where they can be replaced by other cations

= 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 octa

• “nutrient rich!”

• Stable under moderate to low soil pH, high Mg, Fe

• Common in midwestern US

Structure of Vermiculite

Lots of charge imbalance, both sheets:

High nutrient supply capacity

= Al = Fe = Mg

Illite (2:1, a Mica)• Al3+ substitution for Si4+ on the tetrahedral

sheet

• Strong surface charge

• “fairly nutrient poor”

• Non-swelling, only moderately plastic

• Stable under moderate to low pH, common in midwestern US

Structure of Illite

Structure of Illite

1. Isomorphous substitution is in the tetrahedral sheets

2. K+ comes into the interlayer space to satisfy the charge and “locks up” the structure

K+ K+

Chlorites (2:1:1)

• Hydroxy octahedral sheet in the interlayer space

• Restricted swelling

• “Nutrient poor”

• Common in sedimentary rocks and the soils derived from them

Structure of Chlorite

Mg-Al hydroxy sheet

Mg-Al hydroxy sheet

1. Iron-rich

2. “locked” structure

3. Low nutrient supply capacity

= Al = Fe = Mg

Visual comparison of common silicate clays

illitemontmorillonite“2:1:1”

Strongly held

H-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 bonds

– Ionic are heat tolerant– Covalent generally stronger ‘cuz shared electrons

between atoms, but not heat tolerant

Weathering pattern of clay formation

Entisols, Inceptisols

Vertisols

2:1 1:1 Fe/Al Ox

Spodosols

Fig 8.16

Oxisols

Ultisols

CEC and weathering intensity

Alfisols, Vertisols,

Argiudolls*Ultisols Oxisols

*remember nomenclature structure = “argi-ud-oll”

Where to find different clays

– see Table 8.3