X-Ray Diffraction for Soils

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X-Ray Diffraction for Soils

• Melody Bergeron

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X-Ray Diffraction

• Capabilities
• Crystallography
• How it works
• Sample Preparation
• Examples

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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!

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Crystallography

• Unit Cell
• Crystals repeating structures
• Atoms form planes in the structure

enstatite
beryl
albite
fluorite
Perkins, 1998
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• Planes in a crystal
• Diffraction based on ? of X-rays and plane spacing

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

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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
• Claysif you see broader peaks in your pattern

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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
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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

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14Å, 10Å, 7Å Clay Groups

• Smectites (shrinking-swelling clays) 14Å,
  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

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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

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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?

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Go To Software
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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

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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

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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 Total layer charge Total layer charge -0.33
Interlayer Charge (mol charge/mol clay) 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
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Clay Mineral Properties
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From McBride 1994
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Hydrated Cations in Interlayer
From Schulze 2002
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c-axis Spacing of Clay Minerals
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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
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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

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

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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
  /