1. Mineral Composition Variability
GLY 4200
Fall, 2017
One ion may substitute for another, if the size and charge can be accommodated.

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+
If the charge is different a double (or coupled) substitution often occurs –
Ca2+ may substitute for Na+ with only half the positions filled by Na + being filled by Ca 2+

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:
Source:
In 1937, Victor Goldschmidt formulated some observations about atomic substitution, which have come to be known as Goldschmidt’s Rules.
He said substitution was controlled by size and charge.

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
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HS refers to a high-spin state.
Some metals have two possible electronic configurations - High Spin and Low Spin. Low spin involves pairing of electrons in orbitals.
The two states have different magnetic, optical and structural properties. One of the differences is the difference in ionic radius.
High-spin versus low-spin

9. Solid Solution
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Some minerals form a complete “solid solution” series between two “end-members”.
Ex. Olivine Mg2SiO4 forsterite } end-members
Fe2SiO4 fayalite }
Any mixture of Mg and Fe is possible.
Ni2SiO4 is also possible but it is very rare to find much Ni in olivine type structures.
Example 2: Plagioclase Feldspar
Albite NaAlSi3O8
Anorthite CaAl2Si2O8
Frye prefers the term crystalline solution to solid solution. It is actually a better term but solid solution is very common and people are not likely to change.

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)
Image:

13. Beryl Cavities Defects in crystal structures
In discussing crystal structures it is often assumed that the structure is perfect or ideal. In fact, no real crystal is ever perfect, all have some defects.
1. Chemical defect – substitution of one ion for another is one possible defect.
2. Physical defects – many types

14. Schottky Defect
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Schottky - When an atom is massing from its normal lattice site, a lattice vacancy is created. It is called Schottky defect.

15. Frenkel defect
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Frenkel - When an ion is missing from its normal crystal site and occupies interstitial position the defect is another point defect called Frankel defect.

16. HCP Stacking Defect ABABABCABAB H H C H H
Stacking - An incorrect layer occurs in a closest packed structure

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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Grain boundary – two lattices meet.
Many others are possible.

19. Polymorphous Minerals
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When a single compound exists in more than one structural form it is said to be dimorphous (2 forms) or polymorphous, more than two forms.
Example Kyanite, andalusite, and sillimanite are three minerals with the formula Al2SiO5, but different crystal structures.
Graphite and diamond are dimorphous forms of carbon. Graphite is said to be stable. It is the lowest energy form at room temperature and pressure.
Diamond is metastable – it is not really stable being a higher energy state but is converted so slowly to graphite that for all practical purposes it is stable (kinetic barrier).
All have the formula Al2SiO5

20. Ditypous Minerals Top – sphalerite (aka zinc blende) CCP
Bottom – wurzite HCP
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Polytypism – one dimensional polymorphism – identical sheets stacked in different ways – example sphalerite, ZnS, is CCP whereas wurtzite, ZnS, is HCP

21. Order-disorder
If one type of ion substituting for another prefers a certain type of site over another the structure is ordered.
Example: There are three polymorphs of potassium feldspar (KAlSi3O8)
Sanidine
Orthoclase
Microcline

22. Sanidine-Microcline Transition
Sanidine has a high degree of structural symmetry and a relatively random distribution of Si and Al (both of these elements can fit into tetrahedral sites, surrounded by four oxygens)
When cooled, contraction occurs
This produces a tendency for Al to go into some of the smaller sites, and Si to go into some of the larger ones, which means the distribution of aluminum and silicon is more ordered
The low-temperature polymorph formed from sanidine by disorder polymorphism is microcline.

23. Effect of Ordering
The ordering of elements in the sanidine-microcline transition reduces the structural symmetry
Sanidine has a 2-fold rotation axis and a mirror plane not found in microcline

24. Pseudomorphism
Pseudomorphic goethite after cubic pyrite crystals clustered on a terminated aegerine crystal
Group is 4.6cm
Eric Farquharson specimen
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Pseudomorphism – when one mineral replaces another it may retain the external appearance of the other crystal – such replacements are said to be pseudomorphs of one mineral after another.
Examples: Native copper after cuprite (Cu2O)
Gypsum [CaSO4 . 2H2O] after anlychrite [CaSO4]
Goethite (FeO(OH)) after pyrite (FeS2)

25. Non-crystalline matter
Matter may form in a non-crystalline state, or may become non-crystalline as a result of alteration
Examples
Metamict
Mineraloid

26. Metamict
Certain minerals occasionally contain interstitial impurities of radioactive compounds, or are composed of radioactive elements
Alpha radiation emitted from the radioactive elements is responsible for degrading a mineral's crystal structure through internal bombardment.
If the structure is destroyed completely (or nearly) then it is said to be metamict

27. Effects Effects of metamictization are extensive
The process lowers a mineral's refractive index, hardness, and specific gravity
An example of a mineral containing a radioactive element is thorite (ThSiO4)
A frequent host of radioactive impurities is zircon, (ZrSiO4)

28. Mineraloids Upper left –amber Lower left – obsidian
Sources: Upper left:
Lower left:
Right:
Mineraloids – A mineraloid is a mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. They are
amorphous natural solids which form at low temperature. Another name is gel mineral or just gel.
Upper left –amber
Lower left – obsidian
Right – tektite glass

29. Exsolution Augite with pigeonite exsolution lamellae
Pigeonite is a Ca-poor clinopryoxene
Left:
Right:
Solutions of many ions, when cooled quickly, may crystallize into a badly crystallized material. This material will have some ions of the wrong size and charge crystallized another structure type.
Solution of halides and alkali gives one example. Large alkalis, such as Cs, should be eight coordinated. If coiled quickly, the Cs might be forced into VI coordination with Na. Slowly the Cs tads to migrate and finally may recrystallize in VIII coordination – this is known as ex-solution.
Another example – Apollo 12 samples yielded pigeonite (CaxMg2-xSi2O6)
Ex – solution lamellae (bonds) of augite (between CaFeSi2O6 & CaMgSi2O6)
Visible when examined by electron microscope, because the lamellae were about 200Å wide
Exsolution in pyroxene