Soil Mineralogy

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Soil Mineralogy

• 1/25/2008
• Paulings Rules
• Short- and long-range structure in minerals
• Hematite example - CrystalMaker
• Braggs Law

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Common Minerals in Soilsfrom Handout1

• Hematite (a-Fe2O3) - today
• Goethite (g-FeOOH) and birnessite (layer-type Mn
  oxide) week 2
• Clay minerals (silicates) week 3

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Silicate Mineralsstructurearrangement of atoms
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Silicate Minerals the Si-O Tetrahedron
Structures composed of Si-O tetrahedra Corner
sharing
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Paulings Rules
Linus Paulings Rules for ionic solids (when
ionic interactions dominate, the arrangement of
atoms in a crystal structure may be treated as
packing of rigid spheres in 3-D space)
Electronegativity (EN) ability of an atom in a
molecule to attract electrons
Electronic orbitals empty
Electronic orbitals full
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Paulings Rules
Rule 1. A coordinated polyhedron of anions is
formed about each cation, the cation-anion
distance being determined by the radius sum and
the coordination number (CN) of the cation by the
radius ratio.
IRionic radius
IRcation
gt CN
IRanion
ex. Si Radius ratio0.042/0.1400.30 0.22-0.41
CN4 Si is in tetrahedral coordination to O
ex. Fe Radius ratio0.064/0.1400.46 0.41-0.73
CN6 Fe is in octahedral coordination to O
Biotite Mica K2(Mg,Fe)6Si6Al2O20(OH)4
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Paulings Rules
Rule 2. The electrostatic valence principle
states that in a stable structure, the valence of
each anion, with changed sign, is equal or nearly
equal to the sum of the strengths of the
electrostatic bonds to it from adjacent cations.
selectrostatic bond strength
O
O
-1
Si4
ex. Si4 and Al3 s(Si4) 4/4 -1 s(Al3)
3/4 -0.75 0.25 electrostatic units
unsatisfied leads structural, net negative charge
in minerals such as muscovite
O
O
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Paulings Rules
Rule 3. The presence of shared edges and
especially shared faces in a coordinated
structure decreases its stability the effect is
large for cations with large valence and small
coordination number.
Example of large Z and low CN? But we do see
face-sharing in minerals ex. Hematite
stability
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Short Range StructureHematite
Extended X-ray Absorption Fine Structure (EXAFS)
spectroscopy
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Polyhedral Linkages Local Structure
Pauling Rule 1 Fe3 IR(Fe3)/IR(O-2)0.46 CN6 o
ctahedral coordination

• 4 different Fe environments
• face-sharing Fe-Fe at 2.89 Å
• edge-sharing Fe-Fe at 2.97 Å
• corner-sharing Fe-Fe at 3.37 Å
• corner-sharing Fe-Fe at 3.70 Å
• Recall 1 Å 1x10-10 m
• Interatomic distances demonstrate
• Pauling Rule 2 face-sharing brings
• cations close together

What happens to the Fe-O octahedral symmetry
when Fe-Fe distances are short?
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Long Range StructureHematite

• 1. Locate crystallographic parameters for
  hematite in the
• American Mineralogist Crystal Structure Database
• http//www.minsocam.org/MSA/Crystal_Database.html
• Load the parameters into CrystalMaker
• http//www.crystalmaker.com/crystalmaker/download.
  html
• 3. Use the The Virtual Museum of Minerals and
  Molecules
• http//virtual-museum.soils.wisc.edu/displays.html
  
• to get a first cut at the properties and
  occurrence of hematite
• Web of Science for literature search
• http//www.lib.umn.edu/

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American Mineralogist Crystal Structure Database
Mineral name Citation (authors, journal, title) a
b c a b g space group atom x y z Fe O
Hematite Blake R L, Hessevick R E, Zoltai T,
Finger L W American Mineralogist 51 (1966)
123-129 Refinement of the hematite structure
5.038 5.038 13.772 90 90 120 R-3c atom x y z
B(1,1) B(2,2) B(3,3) B(1,2) B(1,3) B(2,3) Fe 0 0
.3553 .0080 .0080 .00029 .0040 0 0 O .3059 0 1/4
.0068 .0083 .00046 .0042 .00058 .0012
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American Mineralogist Crystal Structure Database
Space group the various ways in which motifs
(atoms in a crystal) can be arranged in a
homogeneous array There are 230 unique patterns
or space groups. Consists of information on 1.
Bravais lattice 2. Axis of symmetry 3. Mirror
and glide planes Hematite R-3c 1. rhombohedral
2. 3-fold rotation 3. axial glide plane
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CrystalMaker view of Blake et al. (1966) hematite
3.36 Å
3.71 Å
2.90 Å
2.97 Å
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Hematite
Paulings Rule 3 corner-sharing is more
stable than edge-sharing is more
stable than face-sharing octahedra
Fe-O 1.946 Å
Fe-O 2.116 Å
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http//www.eserc.stonybrook.edu/ProjectJava/Bragg/

Braggs Law nl 2d sinq n 1, 2, 3 l
wavelength of incident X-ray beam d distance
between diffracting planes of atoms q incident
X-ray beam angle