Lecture 5 - OVERVIEW

Classification of Minerals; Crystal growth

Details

 

 

Review from last time - Paulings Rules:

 

1.  The Coordination (radius ratio) Principle Ð a coordination polyhedron of anions surrounds each cation.  The cation-anion distance is determined by the sum of the cation and anion radii and the number of anions coordinating with the cation is determined by the relative size of the cation and anion.

 

2. Electrostatic Valency Principle Ð in a stable ionic structure, the total strength of the valency bonds that reach an anion from all neighboring cations is equal to the charge of the anion.

 

3. Sharing of Polyhedral Elements I Ð the existence of edges (and particularly faces) common to coordination polyhedra decreases the stability of ionic structures

 

4. Sharing of Polyhedral Elements II Ð in a crystal containing different cations, those with large valence and small coordination number tend not to share polyhedral elements with each other.

 

5. Principle of Parsimony Ð the number of essentially different kinds of constituents in a crystal tends to be small.

 

 

 

Chemical Variation in Minerals

 

This raises an issue of terminology:

            major elements are fundamental to the mineral, control its structure and gross physical properties

            minor elements are present in small amounts (up to a few %), usually as substitutes for major elements

            trace elements are present in extremely small amounts but are often responsible for mineral color.

 

We also need to introduce the idea of mineral formulas, which is how we describe mineral compositions. 

 

            Idealized formula                            Fe2ZnO4

 

 

            Structural formula                           VIFe2IVZnO4

 

 

            General formula                              (Fe, Mn)2(Zn,Fe)O4

 

 

            Specific formula                              (Fe1.4 Mn0.6)(Zn0.8 Fe0.2)O4

 

 

Solid Solutions

The discussion above leads directly to a discussion of substitutions of one element for another within the stable mineral structure called isostructural substitutions.  This process is known as solid solution, defined in a mineral structure as specific atomic sites that are occupied in variable proportions by two or more different chemical elements.

 

Three main factors determine whether or not solid solution is possible:

            1. Comparative size of ions (atoms, molecules) that are substituting for one another

            2. The valence state (charge) of the ions involved in the substitution.

            3. The temperature at which the substitution takes place

 

Types of substitution

 

            Simple cationic/anionic: Ions of similar size and charge substitute for each other.  Examples:

 

K = Na

KCl Ð NaCl (sylvite - halite);

KAlSi3O8-NaAlSi3O8  (orthoclase Ð albite)

Mg = Fe (= Mn)

Mg2SiO4 Ð Fe2SiO4 Ð Mn2SiO4 (forsterite Ð fayalite - tephroite; olivine)

MgSiO3 Ð FeSiO3 (enstatite Ð ferrosilite; pyroxene)

Cl - Br

KCl - KBr

Fe = Zn

(Zn, Fe)S  (sphalerite)

 

 

            Coupled substitution: For electrical neutrality to be maintained, substitution of two elements requires an additional substitution.  Examples:

 

Fe2+ + Ti4+ = 2Al3+

(Al, Ti)2O3  (corundum, var. sapphire)

Ca2+Al3+ = Na+Si4+

CaAl2Si2O8-NaAlSi3O8  (plagioclase)

Mg2+ + 2Al3+ = 2Fe2+ + Ti4+

(Mg, Fe)(Al, Ti)2O4 (spinel group)

 

            Interstitial substitution: Between some ions or ionic groups there may exist  structural voids. 

 

            Vacancy solid solution: remember that close packing of anions often creates more cation sites than can be filled. 

 

            Omission solid solution: this is the opposite of filling a vacancy, that is, creating one. 

 

The result of these substitutions is a wide variety of mineral and mineral formulas!!!

 

 

Crystallization and polymorphs