X-Ray Diffraction for Soils

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. n • Planes in a crystal • Diffraction based on λ of X-rays and plane spacing http://pubs.usgs.gov/of/of01-041/htmldocs/xrpd.htm

6. The X-ray Diffractometer • Cu source, X-ray beam, interaction with specimen • Detector records diffraction pattern at varied angles http://pubs.usgs.gov/of/of01-041/htmldocs/xrpd.htm

7. 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

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 Tetrahedral

11. 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

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 Al1.67 (Mg2+,Fe2+)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 • 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/