X-Ray Diffraction for Soils

Melody Bergeron

Capabilities Crystallography How it works Sample Preparation Examples

X-Ray Diffraction

X-Ray Diffraction

Mineral Identification Element Analysis independent of crystal size, small sample,

“nondestructive,” mixtures Phases as little as 1-3% sample weight can be

identified Qualitative or Quantitative Must be crystalline!

Crystallography Unit Cell Crystals repeating

structures Atoms form

planes in the structure

fluorite

beryl

enstatite

albite

Perkins, 1998

Planes in a crystal Diffraction based

on λ of X-rays and plane spacing

n

http://pubs.usgs.gov/of/of01-041/htmldocs/xrpd.htm

The X-ray DiffractometerCu source, X-ray beam, interaction

with specimenDetector records diffraction pattern at

varied angles

http://pubs.usgs.gov/of/of01-041/htmldocs/xrpd.htm

Powder XRD Powder, crystals in

random orientations Goniometer swings

through many angles Enough crystals,

enough angles, get enough diffraction to determine mineralogy

http://pubs.usgs.gov/of/of01-041/htmldocs/xrpd.htm

XRD of Soils and Sample Prep.

XRD used for Identification of Components Silicates, Clays, Carbonates, Oxides, some

organics?, etc… Need disaggregated, powdered samples for

analysis – dry preferred Additional sample preparation is needed for

detailed clay analyses

Sample Problems Specific For Soils Methods depend on what question(s) you are

asking Dry is preferred (bake at 100 ºC for 1 hr), but I

have run wet samples for fragile clays Depending on the soil horizon - disaggregation

may be difficult, organic material may need to be removed, cements may need to be dissolved

Clays…if you see broader peaks in your pattern…

Clay Prep. and Analysis Clay fraction needs to be separated (by size) for

detailed analyses – mix sample in water, clays will be suspended, decant and centrifuge liquid to concentrate the clays

Several methods for mounting the clays – need to orient them flat

Depending on the type of clay, further preparation is needed

TetrahedralOctahedralTetrahedral

Clay Prep. and Analysis Methods include:

Solvating with ethylene glycol or glycerol (replaces water – gives a constant interlayer spacing)

Baking at various high temperatures to destroy parts of the crystal structure

Saturating with cations (Mg, K, etc.) may produce diagnostic structural changes

14Å, 10Å, 7Å Clay Groups

14Å, 10Å, 7Å Clay Groups Smectites (shrinking-swelling clays) 14+Å, greater

than 14Å if interlayer water Chlorite 14Å and 7Å peaks Kaolinite 7Å peak 10Å clays are Micas, Illite or Glauconite Vermiculite 14Å and ?Å depending on Mg, Na, Fe Sepiolite, Palygorskite, Halloysite… check for

fibrous or tubular material in microscope first

Additional Clay Problems Polytypes – many clay have several polytypes that may or

may not be distinguishable in your diffraction pattern Interlayering – different types of clays can alternate

(randomly or ordered ratios) producing a completely different diffraction pattern

How important is it that you know exactly which clay you have present?...

Determining Cations (for CEC)… Since changing cations may not alter the diffraction pattern, it is generally preferable to use EDX-SEM to determine the cations

Examples Control – 16Å peak and small peak at 10Å

EG Solvated - 16Å shifted to 17Å

Baked samples - 16Å peak collapses to 10Å peak and small 5Å peak

What clay is it?

Go To Software

Clay Mineralogy Surface charges on clays affect their absorption properties

and their “engineering” properties Ex. some clays allows water into their inner layer and by

doing so expand when wet and contract when dry Ex. other clay minerals exclude water from their inner

layer Ex. Different clays bind different cations Cation Exchange Capacity…

Cation Exchange Capacity

The amount of exchangeable cations a soil or mineral is capable of retaining on its surface.Charge balance of overall mineral is requiredCEC - ∑Cations + ∑ Anions = 0

CEC= ∑Cations + ∑ Anions

Calculation of Layer Charge and CEC for Montmorillonite (M0.33Si4 Al1.67 (Mg2+,Fe2+)0.33)

Atom Z # ½ cell Total charge

Si 4+ 4 16+

Al(VI) 3+ 1.67 5+

Mg or Fe2+

2+ 0.33 0.66+

O 2- 10 20-

OH 1- 2 2-

Total layer charge -0.33

Interlayer Charge (mol charge/mol clay)

+0.33

Formula weight for ½ cell of montmorillonite =359 g/mol

Thus CEC of montmorillonite is 92 cmol/kg

0 3 3

3 5 9

1 0 0 0 1 0 0

1

9 2.* * *

m o l

m o l c lay

m o l c lay

g c lay

g

kg

cm o l

m o l

cm o l

kg c lay

-22

Clay Mineral PropertiesType Group Formation Occurrence Charge per

half cellSource of charge

Shrink swell

CEC cmol

kg-1

Force holding layers together

Interlayer cation Total Surface

area m2

g-1

1:1 Kaolin/ Serpentine

Highly weathered soils kaolinite common in tropical env., serpentine rare, usually in coastal regions

0 none 0 Hydrogen bonds none except halloysite which has water

7-30

2:1 Pyrophyllite/Talc

Secondary minerals commonly found in metamorphic rocks

pyrophyllite rare. Talc more common but susceptable to weathering

0 none 0 van der wahls none 65-80

2:1 Mica Primary mineral formed from melts

common primary minereral in poorly weathered soils, often found in soils in large sheets,

1 tetrahedra none 0 strong electrostatic aproaching ionic

Potassium 40-100

2:1 Illite Weathered Mica, K weathered out, can be precipitated from solution

common intermediate weaterhing product in soils with mica

0.6-0.9, usually closer

to 0.8

tetrahedra and octahedra, all dioctahedra

some slight electrostaitc between K and other intrelayer cations

K, Ca, Na, etc. intermediate hydration

60-200

2:1 Vermiculite A weathering product from mica, can be precipitated from solution

common in many soils of temperate regions

0.6-0.9 tetrahedra and octahedral

med-high 10-150 electrostatic force between interlayer cations

Ca, Na, Mg other cations, if K then reverts to illite

600-800

2:1 Smectite A weathering product that precipitates from solution

very common in soils of temperate regions

0.2-0.6 tetrahedra and octahedral

high 80-150 electrostatic force between interlayer cations

Ca, Na, Mg other cations,lots of water

600-800

2:1:1 Chlorite Formed in metamorphic environments rich in Fe and Mg.

not commonly found in soils because interlayer easily weathered

variable tetrahedral and some

octahedral

low 10-40 electrostatic and van der Wahls forces between 2:1 layer and hydroxide sheet

brucite and gibbsite with isomorphic substution

25-150

From McBride 1994

From Schulze 2002

Hydrated Cations in Interlayer

c-axis Spacing of Clay Minerals

Structural Impacts on Clay Mineral Properties (1)

Isomorphic substitution creates overall negative charge on clay layers.

To balance charge cations are adsorbed in the interlayers.

From Goldberg 2000

Structural Impacts on Clay Mineral Properties (2)

Substitution originating in tetrahedral sheet leads to stronger sorption of some cations (e.g., K+) than isomorphic substitution in octahedral sheet.

Shrink-swell characteristics of clay minerals are dictated by the layer charge.

Edges of clay minerals have unsatisfied bonds and thus can form covalent bonds with sorbates

Surface Functional Groups on Clay Mineral Edges

Figure 5.3 from Sparks, 1995

Sorption to Mineral Surfaces

Heavy metals, organics, etc. can sorb to many mineral surfaces

If the mineralogy (and field conditions like pH, ppt, etc.) can be identified then the fate and transport of contaminants can be modeled

Additional Information

http://www.tulane.edu/~sanelson/eens211/x-ray.htm

X-Ray Diffraction and the Identification and Analysis of Clay Minerals – Moore and Reynolds

Minerals in general - http://mineral.galleries.com/