X-Ray Diffraction for Soils
Melody Bergeron
X-Ray Diffraction
Capabilities
 Crystallography
 How it works
 Sample Preparation
 Examples

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
enstatite
beryl
albite
fluorite
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 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
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
Tetrahedral
Octahedral
Tetrahedral
 Depending
is needed
on the type of clay, further preparation
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
(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
 Smectites
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 required
 CEC - ∑Cations + ∑ Anions = 0
 CEC=
∑Cations + ∑ Anions
Calculation of Layer Charge and CEC for
Montmorillonite (M0.33Si4 Al1.67 (Mg2+,Fe2+)0.33)
Atom
Z
Si
4+
# ½ cell Total
charge
4
16+
Al(VI)
3+
1.67
5+
Mg or
Fe2+
O
2+
0.33
0.66+
2-
10
20-
OH
1-
2
Total layer charge
2-
Formula weight for ½ cell of
montmorillonite =359 g/mol
Thus CEC of montmorillonite
is 92 cmol/kg
-22
-0.33
Interlayer Charge
+0.33
(mol charge/mol
0.33 mol  mol clay 1000 g 100cmol 92 cmol
clay)
*
*
*

mol clay
359 g clay
kg
1mol
kg clay
Clay Mineral Properties
Type
Group
Formation
Occurrence
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
2:1
Pyrophyllite/ Secondary minerals
Talc
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
none
0
strong
electrostatic
aproaching ionic
Potassium
40-100
2:1
Illite
Weathered Mica, K
common intermediate
0.6-0.9,
tetrahedra and
weathered out, can be
weaterhing product in soils usually closer octahedra, all
precipitated from solution with mica
to 0.8
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 common in many soils of
mica, can be precipitated temperate regions
from solution
0.6-0.9
tetrahedra and
octahedral
med-high
10-150
electrostatic force Ca, Na, Mg other
between interlayer cations, if K then
cations
reverts to illite
600-800
2:1
Smectite
A weathering product that very common in soils of
precipitates from solution temperate regions
0.2-0.6
tetrahedra and
octahedral
high
80-150
electrostatic force Ca, Na, Mg other
between interlayer cations,lots of
cations
water
600-800
2:1:1
Chlorite
Formed in metamorphic
environments rich in Fe
and Mg.
variable
tetrahedral and
some
octahedral
low
10-40
electrostatic and
van der Wahls
forces between
2:1 layer and
hydroxide sheet
25-150
not commonly found in
soils because interlayer
easily weathered
Charge per
half cell
Source of
charge
tetrahedra
Shrink
swell
CEC cmol Force holding
layers together
kg-1
Interlayer cation
brucite and
gibbsite with
isomorphic
substution
Total
Surface
area m2
g-1
7-30
From McBride 1994
Hydrated Cations in Interlayer
From Schulze 2002
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/