1. Mineral Composition Variability
GLY 4200
Fall, 2012

2. Ionic Substitution - Size
Size: Fe2+ ↔ Mg2+ ↔ Ni2+
(0.86Å) (0.80Å) (0.77Å)

3. Ionic Substitution - Charge
Coupled substitution
Ca2+ & A13+ ↔ Na+ and Si 4+
Example: Plagioclase feldspar
NaAlSi3O8 ↔ CaAl2Si2O8
Void
Ca2+ & Void ↔ 2 Na+

4. 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
More Information:
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5. 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 %
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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6. 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

7. 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

8. Spin State High-spin versus low-spin
Source:
High-spin versus low-spin

9. Solid Solution
Source: and

10. Types of Crystalline Solution
1. Substitutional - Mg2+ for Fe2+
2. Omission Ca2+ & void for 2 Na+

11. 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, end-member 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+

12. 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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13. Beryl Cavities

14. Schottky Defect
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15. Frenkel defect
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16. HCP Stacking Defect
ABABABCABAB
H H C H H

17. CCP Stacking Defect
ABCABCABABCABC
C C H C C

18. Grain Boundary Defect
Two lattices grow together, with some displacement of ions (shown in blue)
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19. Polymorphous Minerals
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All have the formula Al2SiO5

20. Ditypous Minerals Top – sphalerite (aka zinc blende) CCP
Bottom – wurzite HCP
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21. Pseudomorphism
Pseudomorphic goethite after cubic pyrite crystals clustered on a terminated aegerine crystal
Group is 4.6cm
Eric Farquharson specimen
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22. Mineraloids Upper left –amber Lower left – obsidian
Sources: Upper left:
Lower left:
Right:
Upper left –amber
Lower left – obsidian
Right – tektite glass

23. Exsolution Augite with pigeonite exsolution lamellae
Pigeonite is a Ca-poor clinopryoxene
Left:
Right:
Exsolution in pyroxene