Clay Mineralogy 101

Peter Ryan, Geology Dept, Middlebury College, June 1, 2012

Definition of “Clay”

• Grain size (e.g. < 2 mm) – could be any mineral;

• Mineral group consisting of sheets of silica tetrahedra

and Al/Mg/Fe octahedra. Known as “clay minerals”,

“phyllosilicates” and “silicate clays”.

Pedogenic smectite: ~ (Ca,Mg)0.3(Al1.2Fe0.7Mg0.15)(Si3.4Al0.6)O10(OH)2

Pedogenic halloysite: Al2Si2O5(OH)4

Silica (SiO4-4) tetrahedron

Basal plane of hexagonally-arranged silica

tetrahedra. = “tetrahedral sheet”.

An individual tetrahedron = SiO4-4, but sharing of

edges in extensive sheet like this results in Si2O5

Octahedrally-coordinated metal cation

(e.g Al+3, Mg+2, Fe+2, Fe+3)

An individual octahedron is Al(OH)6+3 but extensive

sharing of edges makes Al2(OH)6 … or Mg3(OH)6

Molecular-scale structure of clay minerals

(example here is K-mica, a “2:1 clay”)

K Al2 Si3Al O10(OH)2

Interlayer

2:1 Clays with zero layer charge

Al2Si4O10(OH)2 Mg3Si4O10(OH)2

Examples of 2:1 clays that differ based on

layer charge

LC = -1 LC = 0

LC = 0

LC = -1

K Al2 Si3Al O10(OH)2 Al2 Si4 O10(OH)2

K Mg3 Si3Al O10(OH)2 Mg3 Si4 O10(OH)2 (TALC)

Smectite (layer charge ~ -0.3)

Hydrated Na+ or Ca+2

2:1:1 clay (chlorite) 1:1 clay (e.g.kaolinite)

Mg2Al(OH)6.Mg3Si3AlO10(OH)2 Al2Si2O5(OH)4 (kaolinite)

Mg3Si2O5(OH)4 (serpentine)

~interlayer

Serpentine Group Minerals

Mg3Si2O5(OH)4

Chrysotile

Antigorite

Kaolinite [Al2Si2O5(OH)4]

Cation exchange capacity

~5 cmolc/kg ~100 cmolc/kg

(1) Layer charge, (2) Interlayer space, (3) surface area : volume

Cation exchange capacity

Unit of measure = cmolc/kg

Quartz …………… 1 cmolc/kg (low surface area, low charge)

Kaolinite clay …. 5 cmolc/kg (medium surface area, low charge)

Chlorite, mica 10-20 cmolc/kg (low surface area, high charge)

Smectite clay …. 100 cmolc/kg (high surface area, medium chg)

Vermiculite 150 cmolc/kg (~high surface area, ~high charge)

Humus……………. 150 cmolc/kg (high surface area, high chg, edges)

Iron hydroxide… (anion exchange, pH-dependent)

Nutrient retention, contaminant mitigation

Champlain Valley Soil Clay Mineralogy

Illite > chlorite > smectite (quartz is ubiquitous)

Glacial erosion of

chlorite and mica

from schists,

phyllites and other

mtamorphic rocks

in source areas of

Glacial Lake

Vermont &

Champlain Sea

Taconic slates and phyllites

Additional source

of clay minerals in

Ch V soils =

chemical

weathering of

phyllosilicates

derived from

metamorphic

rocks (in soils…?

In lake

sediments…?),

e.g. biotite →

vermiculite or

smectite

LC ~ -0.5 LC ~ -0.5

Smectite is likely pedogenic clay in Champlain Valley

g = gypsum

c = calcite Sherman, 1952

Shrink-swell clays aka expandable clays …

correlated with smectite content

Not all clays are shrink-swell

vs

Kaolinite vs smectite

< 2 mm fraction of clay soils in Addison

County contain ~10 % smectite

Significance of dehydration on aeration & vertical permeability?

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