Unit 7 - Clay Mineralogy

Clays and Colloids are sometimes

used interchangeably

Clays -

mineral or organic particles (2μm)

Colloids -

between 0.1μm and 1nm and can be composed of mineral or organic macromolecules

Physiochemical properties of clays

Extremely small size

• Very large specific surface area

• Very reactive

• Stay suspended for a long time which enables clays to translocate

Usually negatively charged

• Certain clays can take on a positive charge under low pH (extremely low, acidic soil)

The ionic charges that are important in clay minerals are classified into:

Cations - positively charged ions

Anions - negatively charged ions

The most important ions related to clays are

Hydrogen (H+), Potassium (K+), Sodium (Na+), Ammonium (NH4+), Calcium (Ca2+), Magnesium (Mg2+), Iron (Fe3+), Silicon (Si4+), Manganese (Mn2+, Mn4+), Aluminum (Al3+), Hydroxide (OH-), Oxygen (O2-)

When clays interact with water they form

a micelle

A negatively charged clay or colloid will attract

hundreds of thousands of loosely held cations

The swarm of cations and their water envelope is

called a micelle

Cations exchange between the

inter-micelle solution and the micelle

A large portion of soil clays (especially in temperate environments) are

phyllosilicates

“Phyllo” in Greek means

“leaf”; these are leaf-like minerals

The building blocks of these clays are

polyhedra (plural, singular in polyhedon)

There are two types that relate to phyllosilicates

tetrahedron

octahedron

Tetrahedron has __ anions and __ sided

4, 4

In tetrahedrons, anions are usually

O2, OH-

In tetrahedrons, cations are usually

Si4+, Al3+

Cations are tetrahedronally coordinated, are

center of polyhedron

Polyhedron is formed by connecting

the lines between adjacent anions

Octahedron has __ anions and __ sides

6, 8

Octahedron anions are usually

O2-, OH-

Octahedron cations are usually

Al3+, Mg2+, Fe2+, and Fe3+

Cations have a relatively small ionic radius and anions

have a large ionic radius

The number of anions that can be conducted depends on

the ratio of the ionic radii of the anion and cation

Individual polyhedra link together to form

large sheets as metal cations share their anions with neighboring cations

Tetrahedron sheet:

composed of tetrahedrons

Dioctahedral sheet: 

2/3 are filled

Trioctahedral sheet:

3/3 are filled

The tetrahedral and octahedral sheets link together

forming the phyllosilicates

Unit cell - where

we cut off the mineral to write down its formula

Any cations found in this space are called

"interlayer cations"

The clays are described by the number of tetrahedra to octahedral sheets they have and

the number of cations in the octahedral sheet

a 2:1 clay mineral

2:1 Clays -

2 tetrahedral sheets : 1 octahedral sheet (Sometimes called T-O-T minerals)

Pyrophylite - Si8 Al4 O20 (OH)4

Tetrahedral cation: Si8

Octahedral cation: Al4

Balancing anions: O20(OH)4

Charge balance of Pyrophylite

0 net charge

Muscovite - K2 (Si6,Al2) Al4 O20 (OH)4

Interlayer cation: K2

Tetrahedral cations: (Si6,Al2)

Octahedral cation: Al4

In muscovite, the K+ in the interlayer

does not exchange

Charge balance of Muscovite

0 net charge

Muscovite: Isomorphous substitution in the tetrahedral layer

2 of the Si have been replaced by Al lowering the layer charge

Biotite - K2 (Si6Al2) (Mg,Fe2+)6 O20 (OH)4

Isomorphous substitution in the tetrahedral and octahedral sheets

6/6 octahedral cations : trioctahedral mineral

Weathers early as Fe2+ > Fe3+ (oxidizes)

Biotite net charge is

0 net charge

Illite - Kx (Si8-x,Alx) Al4 O20 (OH)4

Where x ~ 1.3 (1 out of 6)

Needs less K to balance the charge

• Dioctahedral, isomorphous substitution in the tetrahedral layer

• NH4+ can replace K+ in the interlayer space and become non-available to plants (“fixed”)

Montmorillonite

Dioctahedral (4 octahedral cations)

Also known as “bentonite” which is its commercial name

Mined in Wyoming and often known as “Wyoming bentonite”

This is a mineral in a class of minerals called smectites (expanding clays) and include nontronite, saponite, beidellite (rare), and montmorillonite (extremely common)

These minerals can occur in playa lakes and tend to dominate soils high in clay

Expands with water and cracks when dry

Polar Organic matter can get into interlayer as well as polar and cationic herbicides

Montmorillonite - Mx Si8 (Al4-x,Mgx) O20 (OH)4

Tetrahedral cations: Si8

Octahedral cations: (Al4-x,Mg)

Balancing anions: O20(OH)4

• Where X ~ 0.65

• M = cation

charge of montmorillonite

Has a charge of 0.65 layer charge

Balanced by interlayer cation

Vermiculite

Occurs commonly in eastern US soils which are strongly leached

Forms from biotite when K is lost from the interlayer, creating interstratified clays

Mg and water moves into the interlayer expanding the biotite

Used extensively as a medium for potting soil and as packing material

Interstraitified:

layers of different minerals within the same clay

1:1 Clays

1 tetrahedral sheet : 1 tetrahedral sheet (sometimes called T-O minerals)

Kaolinite - Si4 Al4 O10 (OH)8

Tetrahedral cations: Si4

Octahedral cations: Al4

Balancing anions: O10(OH)8

Kaolinite has a net charge of

0 net charge

Kaolinite is the most 

common type of clay in tropical regions of the world

Used for paints, filler, paper industry (slick coating on paper)

Mined in Georgia

Non expanding, fairly mature clay

H-bonds keeps the sheets together

Halloysite

Same chemical composition at Kaolinite

Not plate shaped but tube-shaped known as “tubular morphology”

Develops from volcanic ash

2:2 Clays

Also known as 2:1:1 clay mineral - sometimes T-O-T-O minerals)

Chlorite has

a Mg, Fe trioctahedral sheet in the interlayer space

Non-expanding

Acicular Clays

Sepiolite and Palygorskite

Acicular - needle like

Occurs in caliche deposits

• Especially Bkkm horizons

Can be inherited from lacustrine environments

• Lake depositional environments

Oxides, Oxyhydroxides, Hydroxides

Hematite

• Fe2O3

Geothite

• FeO(OH)

Gibbsite

• Al(OH)3

Not silicate clays

Negatively charged above a pH of 5 (due to surface charge)

• Can be positively charged below a pH of 4.5

Amorphous Clays

Allophane

• Amorphous

  • Develops from volcanic ash

• Forms several years after ash deposits

• Low bulk density and high porosity

• Ash transforms to allophane which weathers to halloysite weathering further into kaolinite or montmorillonite

• Described as having a smeary feel