Download Substitution, Solid Solutions, and an Introduction to Silicate Mineral

Polymorphism and polymorphic groups
Silica polymorphs
-quartz (high T) 6-fold (hexagonal)
symmetry
-quartz (low T) 3-fold (rhombohedral)
symmetry
Displacive vs. Order-Disorder Polymorphism
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 4fold sites in K-feldspar. NB that cation disorder usually increases w/ inc. T.
Quartz polymorphs -quartz (high T) -quartz (low T) Distortion of lattice reduces symmetry.
Note that the Si4+ cations are closer
together in the -quartz structure.
K-feldspar polymorphs Sanidine (high T) Microcline (low T) distortion again! 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 endmembers.
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
Both cases have complete solid solution
between the end-member compositions
NaAlSi3O8
CaAl2Si2O8
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
VIIICaVIMgIVSi O
2 6
VIIICaVI(Mg, Fe)IVSi O
2 6
Olivine – orthorhombic
VI(Mg Fe )IVSiO
2-x
x
4 (0 ≤ x ≤2)
Mg1.56Fe0.44SiO4
Graphical Representation of Chemical Variation
Binary Diagrams: used to represent minerals with only 2 endmembers, 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