1. X-Ray Diffraction for Soils
Melody Bergeron

2. X-Ray Diffraction Capabilities Crystallography How it works
Sample Preparation
Examples

3. 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!

4. Crystallography Unit Cell Crystals repeating structures
Atoms form planes in the structure
enstatite
beryl
albite
fluorite
Perkins, 1998

5. Diffraction based on λ of X-rays and plane spacing
Planes in a crystal
Diffraction based on λ of X-rays and plane spacing n

6. The X-ray Diffractometer
Cu source, X-ray beam, interaction with specimen
Detector records diffraction pattern at varied angles

7. Powder XRD Powder, crystals in random orientations
Goniometer swings through many angles
Enough crystals, enough angles, get enough diffraction to determine mineralogy

8. 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

9. 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…

10. 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
Tetrahedral
Octahedral

11. Clay Prep. and Analysis Methods include: 14Å, 10Å, 7Å Clay Groups
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

12. 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

13. 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

14. 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?

15. Go To Software

16. 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…

17. 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 required
CEC - ∑Cations + ∑ Anions = 0
CEC= ∑Cations + ∑ Anions

18. Calculation of Layer Charge and CEC for Montmorillonite (M0. 33Si4 Al1
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
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
-22

19. Clay Mineral Properties

20. From McBride 1994

21. Hydrated Cations in Interlayer
From Schulze 2002

22. c-axis Spacing of Clay Minerals

23. 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

24. 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

25. Surface Functional Groups on Clay Mineral Edges
Figure 5.3 from Sparks, 1995

26. 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

27. Additional Information
X-Ray Diffraction and the Identification and Analysis of Clay Minerals – Moore and Reynolds
Minerals in general -