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Mineral Composition Variability

GLY 4200

Fall, 2015

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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

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

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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, 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+

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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)

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

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

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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.

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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

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Pseudomorphism

• Pseudomorphic goethite after cubic pyrite crystals clustered on a terminated aegerine crystal

• Group is 4.6cm• Eric Farquharson

specimen

Non-crystalline matter

• Matter may form in a non-crystalline state, or may become non-crystalline as a result of alteration

• Examples Metamict Mineraloid

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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

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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)

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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 Ca-poor clinopryoxene

• Exsolution in pyroxene