Polymorphism and polymorphic groups

Silica polymorphs

-quartz (high T)

-quartz (low T)

6-fold (hexagonal) symmetry

3-fold (rhombohedral) symmetry

Displacive vs. Order-Disorder Polymorphism

Distortion of lattice reduces symmetry. Note that the Si4+ cations are closer together in the -quartz structure.

K-feldspar polymorphs

-quartz (high T) -quartz (low T) Sanidine (high T)

Microcline (low T) distortion again!

Quartz polymorphs

Displacive polymorphism does not involve breaking of chemical bonds, but rather only a distortion of the crystal structure. Order-disorder polymorphism occurs as a result of the change in site occupancy for a given cation, say Al3+ vs. Si4+ in the 4-fold sites in K-feldspar. NB that cation disorder usually increases w/ inc. T.

Chemical Classification of Minerals Normally group minerals based on the identity of the major (most abundant) anion or anionic group. Usually this means that the groups have similar structure, physical, and chemical properties. Note that cation contents of many mineral groups vary significantly -> this results in solid solutions!

Compositional Variation in Minerals – Solid Solutions

Most minerals have chemical compositions that may vary within restricted limits. The chemical extrema or bounds for a given mineral, say olivine or plagioclase feldspar, are called end-members.

Substitution of one chemical species, in most minerals this is a cation, is governed by two factors: 1) similarity in size, which would allow the cation to fit in the same lattice position within the structure; and 2) similarity in charge in order to maintain overall electrical neutrality. Examples include Fe2+ for Mg2+ in olivine and other ferromagnesian minerals or Na+ for K+ in alkali feldspars.

Substitutions may be more complex: for example Na+ may be exchanged completely with Ca2+ in plagioclase feldspar. Vacancy (i.e. holes) substitution may also occur in some mineral structures.

Solid Solution Mechanisms

•  Simple Substitution -  This mechanism involves substitution of similarly sized cations of the same charge on the same site within a mineral’s structure. The solid solution may be complete or partial. -  Fe2+ for Mg2+ on M1 site (octahedral) in olivine

•  Coupled Substitution -  This mechanism involves substitution of similarly sized, but differently charged cations on the same or equivalent sites within a mineral’s structure. -  Ca2+ + Al3+ = Na+ + Si4+ on M1 and T sites in plagioclase

•  Omission Substitution -  Another way to allow cations of different charge to substitute. Instead of all sites being filled, some remain vacant. -  3Fe2+ = 2Fe3+ + vacancy on M1 sites in pyrrhotite

•  Interstitial Substitution - Variation of coupled substitution in which ions are “placed” into crystallographic sites that are normally vacant. Normally observed in minerals with structures that have large openings within the crystal structure -  vacancy + Si4+ = Al3+ + (K+, Rb+, Cs+) in beryl

Solid Solutions - Examples Simple Substitution in olivine

Coupled Substitution in plagioclase

NaAlSi3O8 CaAl2Si2O8 Both cases have complete solid solution between the end-member compositions

Mineral Formula - Stoichiometry

Basic rules for writing mineral formulas: 1)  Cations are written first, followed by anions or anionic groups 2)  Charges must always balance – the sum of the cationic charge must be

equal to the sum of the anionic charge 3)  Cations that normally substitute on the same crystallographic site are

grouped together 4)  Cations in different structural sites are listed in order of decreasing

coordination number, that is cubic (8-fold), then octahedral (6-fold), then tetrahedral (4-fold)

Examples: Diopside – monoclinic pyroxene CaMgSi2O6 VIIICaVIMgIVSi2O6 VIIICaVI(Mg, Fe)IVSi2O6

Olivine – orthorhombic VI(Mg2-xFex)IVSiO4 (0 ≤ x ≤2) Mg1.56Fe0.44SiO4

Graphical Representation of Chemical Variation

Binary Diagrams: used to represent minerals with only 2 end-members, whose composition may vary. Ternary Diagrams: used to represent minerals (or assemblages of minerals) that have 3 end-members, whose composition may vary.

Fosterite-Fayalite binary olivine solid solution Ternary pyroxene solid solution

Common Silicate Structures�Complexity built through polymerization of silicon tetrahedra