Transcript Introduction to Environmental Geochemistry

Mineral Composition Variability
GLY 4200
Fall, 2012
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Ionic Substitution - Size
• Size: Fe2+ ↔ Mg2+ ↔ Ni2+

(0.86Å)
(0.80Å)
(0.77Å)
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Ionic Substitution - Charge
• Coupled substitution
 Ca2+ & A13+ ↔ Na+ and Si 4+
 Example: Plagioclase feldspar
 NaAlSi3O8 ↔ CaAl2Si2O8
• Void
 Ca2+ & Void ↔ 2 Na+
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Victor M. Goldschmidt
• Swiss-born Norwegian
mineralogist and
petrologist who laid the
foundation of inorganic
crystal chemistry and
founded modern
geochemistry
• Born 1888, died 1947
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Goldschmidt’s Rules - Size
• Atomic substitution is controlled by size (i.e.,
radii) of the ions
• Free substitution can occur if size difference is less than
~15%
• Limited substitution can occur if size difference is 15 30%
• Little to no substitution can occur if size difference is
greater than 30%
• If there is a small difference of ionic radius the
smaller ion enters the crystal preferentially
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Goldschmidt’s Rules - Charge
• Atomic substitution is controlled by charge
of the ions --> cannot differ by more than 1
• For ions of similar radius but different
charges, the ion with the higher charge
enters the crystal preferentially
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Other Factors Affecting Solid
Solution
• Temperature
 Minerals expand at higher T
 Minerals contract at lower T
 Greater tolerance for ionic substitution at higher T
 Pressure
 Increasing pressure causes compression
 Less tolerance for ionic substitution at higher P
 Availability of ions – ions must be readily
available for substitution to occur
Spin State
• High-spin versus low-spin
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Solid Solution
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Types of Crystalline Solution
• 1. Substitutional - Mg2+ for Fe2+
• 2. Omission
- Ca2+ & void for 2 Na+
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Crystalline Substitution 2
• 3. Vacancy - normally vacant sites can be filled as part
of a coupled substitution. An important example is in
the mineral group amphibole. An abundant, endmember component of this group of minerals is
tremolite which ideally has the formula:
 []Ca3Mg5Si8O22(OH)2
 where [] represents a vacant crystallographic site.
Minerals can utilize this vacant site in coupled
substitutions such as:
• [] + Si4+ = Na+ + Al3+
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Crystalline Substitution 3
• 4. Interstitial - Atom or ion occupies space
in between the normal sites
 Often this is H+, a very small cation
 In some crystal structures these voids are
channel-like cavities. A good example is the
mineral beryl (Be3Al2Si6O18)
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Beryl Cavities
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Schottky Defect
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Frenkel defect
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HCP Stacking Defect
• ABABABCABAB
• H H C H H
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CCP Stacking Defect
• ABCABCABABCABC
• C C H C C
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Grain Boundary Defect
• Two lattices grow
together, with some
displacement of
ions (shown in
blue)
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Polymorphous Minerals
• All have the formula Al2SiO5
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Ditypous
Minerals
• Top – sphalerite
(aka zinc blende)
CCP
• Bottom – wurzite
HCP
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Pseudomorphism
• Pseudomorphic
goethite after cubic
pyrite crystals
clustered on a
terminated aegerine
crystal
• Group is 4.6cm
• Eric Farquharson
specimen
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Mineraloids
• Upper left –amber
• Lower left – obsidian
• Right – tektite glass
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Exsolution
• Augite with
pigeonite exsolution
lamellae
• Pigeonite is a Capoor clinopryoxene
• Exsolution in pyroxene
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