ERTH 2001: Classification

1
ERTH 2001 Classification

• Classification of Minerals by Chemical
  Composition
• Dana's System of Mineralogy (and its derivatives)
• ca. 15 major groups of minerals with a few
  important subgroups
• - we will see about 10 of these in the lab
• named according to dominant anion or anionic
  group
• basis for organisation of your textbook (Ch.
  12-20) and field guide
• by far the most important group is the
  silicates
• - 6 subgroups based on crystal structure
• before tackling this system, you need to know
  the basics of
• Crystal Chemistry (Nesse, Ch. 3)
• Crystal Structure (Nesse, Ch. 4)
• crystallography (Nesse, Ch. 2) will be
  dealt with a little later

2
ERTH 2001 Crystal Chemistry
Sizes of Atoms and Ions atomic radius
(measured in ?) reflects of
electron shells - high Z gtgt low Z
electronegativity - low ? gt high ?
chemical environment - bond length
oxidation state - anions gtgt cations
co-ordination number (CN) - high CN gtgt low CN
3
ERTH 2001 Crystal Chemistry
Sizes of Atoms and Ions atomic radius
(measured in ?) reflects of electron shells
- high Z gtgt low Z electronegativity - low ? gt
high ?
within periods
low ?
high ?
high Z
within groups
low Z
4
ERTH 2001 Crystal Chemistry
Sizes of Atoms and Ions atomic radius
(measured in ?) reflects
chemical environment - bond length
5
ERTH 2001 Crystal Chemistry
Sizes of Atoms and Ions atomic radius
(measured in ?) reflects oxidation
state - anions gtgt cations co-ordination
number (CN) - high CN gtgt low CN
anions
high CN
cations
low CN
6
ERTH 2001 Crystal Chemistry
Sizes of Atoms and Ions atomic radius
(measured in ?) reflects oxidation
state - anions gtgt cations co-ordination
number (CN) - high CN gtgt low CN
anions
high CN
what is CN?? see Nesse Ch. 4
cations
low CN
7
ERTH 2001 Crystal Chemistry
Ionic Potential charge / radius (z/r) -
important in mineral chemistry and structure -
influences the type of cation bonding with
oxygen - influences substitutions in the crystal
lattice - determines many geochemical properties
of elements e.g., ionic potential lt 2 (low
field strength) - elements tend to be
geochemically mobile ionic potential gt
2 (high field strength) - elements tend to be
geochemically immobile
8
ERTH 2001 Crystal Structure (Nesse, Ch. 4)

• Controls of Crystal Structure (p.57-65)
• Isostructural Minerals Polymorphs (p.65-69)
• Classification Compositional Variation
  (p.69-73)

9
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Crystal Structure
kinds of atoms present 3D arrangement in space
chemistry
geometry, symmetry
Controls
what determines how the atoms in minerals are
arranged in the crystal lattice?
- packing - bond type and geometry -
co-ordination number (CN)
10
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
11
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
which arrangement fills the most space?
A
B
12
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
which arrangement fills the most space?
A
hexagonal close packing (74 volume occupied)
B
13
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
how can hexagonal-close-packed (hcp) layers be
arranged in 3D?
14
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1
layer 2
how can hexagonal-close-packed (hcp) layers be
arranged in 3D?
15
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1
layer 2
layer 3 (option1)
how can hexagonal-close-packed (hcp) layers be
arranged in 3D?
16
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1
layer 2
layer 3 (option 2)
how can hexagonal-close-packed (hcp) layers be
arranged in 3D?
17
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
anions
layer 1
layer 2
cations
anions are larger than cations and therefore fill
most of the space in the crystal framework of
crystal therefore consists of anions with cations
fitting in between
18
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
where do the cations fit?
19
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 1
where do the cations fit?
20
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 1
how many anions surround each cation?
21
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 1
4
how many anions surround each cation?
22
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 2
where do the cations fit?
23
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 2
how many anions surround each cation?
24
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 2
6
how many anions surround each cation?
25
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
layer 1 anions
layer 2 anions
cations option 1
4
cations option 2
6
how many anions surround each cation?
26
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
co-ordination number, CN of anions
surrounding a given cation
layer 1 anions
layer 2 anions
CN 4
cations option 1
4
CN 6
cations option 2
6
how many anions surround each cation?
27
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
which arrangement fills the most space?
A
hexagonal close packing
B
28
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
what about this one?
A
29
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
what about this one?
cubic close packing (74 volume occupied)
A
30
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
what about this one?
cubic close packing (74 volume occupied)
A
layer 3 layer 2 layer 1
31
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
what about this one?
cubic close packing
A
layer 3 layer 2 layer 1
CN of cations?
32
ERTH 2001 Crystal Structure (Nesse, Ch. 4)
Controls of Crystal Structure
packing in crystal structures how to fill space
most efficiently?
what about this one?
cubic close packing
A
layer 3 layer 2 layer 1
6
CN of cations?
33
ERTH 2001 Crystal Structure
Sizes of Atoms and Ions
high CN gtgt low CN
high CN
common values of CN 3, 4, 6, 8, 12 4
"tetrahedral" 6 "octahedral" also
possible,uncommon 5, 9,10
low CN
34
ERTH 2001 Crystal Structure (Nesse, Ch. 4)

• Controls of Crystal Structure (p.57-65)
• Isostructural Minerals Polymorphs (p.65-69)
• Classification Compositional Variation
  (p.69-73)

35
ERTH 2001 Crystal Structure
Close-packing
hexagonal
cubic
face-centred
Nesse Fig. 4.1
36
ERTH 2001 Crystal Structure
Cubic close-packing
Nesse Fig. 4.1d
face-centred (74 space filled)
different stacking arrangements same overall
symmetry CN?
body-centred (68 space filled)
Nesse Fig. 4.2
37
ERTH 2001 Crystal Structure
Packing and co-ordination number co-ordination
polyhedra
CN 12 8 6 4 3 2
solid geometric shapes representing arrangement
of anions surrounding a central cation
38
ERTH 2001 Crystal Structure
Packing and co-ordination number co-ordination
polyhedra
CN 12 8 6 4 3 2
solid geometric shapes representing arrangement
of anions surrounding a central cation
12
8
cubic (bcc)
Nesse Fig. 4.3
39
ERTH 2001 Crystal Structure
Packing and co-ordination number co-ordination
polyhedra
CN 12 8 6 4 3 2
6
octahedral
4
tetrahedral
Nesse Fig. 4.3
40
ERTH 2001 Crystal Structure
Packing and co-ordination number co-ordination
polyhedra
CN 12 8 6 4 3 2
triangular
3
linear (not relevant to minerals)
2
Nesse Fig. 4.3
41
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
expanded ("stick") view
space-filling view
co-ordination polyhedron
SiO44-
Nesse Fig. 11.1
42
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
expanded ("stick") view
space-filling view
co-ordination polyhedron
basal view
top view
SiO44-
Nesse Fig. 11.1
schematic
43
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
SiO44- linked in different ways to form variety
of silicate structures 6 subclasses of
silicates (more later!!)
Nesse Fig. 11.2
44
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
SiO44- linked in different ways to form variety
of silicate structures 6 subgroups of
silicates (more later!!)
schematic disilicate structure (e.g., epidote)
Nesse Fig. 11.2
45
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
SiO44- linked in different ways to form variety
of silicate structures 6 subgroups of
silicates (more later!!)
Nesse Fig. 11.2
46
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
SiO44- linked in different ways to form variety
of silicate structures 6 subclasses of
silicates (more later!!)
- all 6 subclasses have characteristic SiO
ratios (net charge) - determines geometry of
structure and types of cations linked to SiO44-
groups (chemistry)
Nesse Fig. 11.2
47
ERTH 2001 Crystal Structure
Silicate tetrahedron the single most important
structural element in minerals
SiO44- linked in different ways to form variety
of silicate structures 6 subclasses of
silicates (more later!!)
- all 6 subclasses have characteristic SiO
ratios (net charge) - determines geometry of
structure and types of cations linked to SiO44-
groups (chemistry)
orthosilicates isolated tetrahedra
linked by cations SiO?
net charge?
Nesse Fig. 11.2
48
ERTH 2001 Crystal Structure
Representing mineral structures
olivine Mg2SiO4 (orthosilicate)
Nesse Fig 4.9
49
ERTH 2001 Crystal Structure
Representing mineral structures
space-filling view
expanded (stick) view
Mg2
O2-
Si4
atomic radius (?) CN Mg2 0.72 Si4 0.26 O2
- 1.40
olivine Mg2SiO4 (orthosilicate)
Nesse Fig 4.9
50
ERTH 2001 Crystal Structure
Representing mineral structures
polyhedra sticks
co-ordination polyhedra
tetrahedra
Mg2
SiO4-
octahedra
atomic radius (?) CN Mg2 0.72
6 Si4 0.26 4 O2- 1.40
olivine Mg2SiO4 (orthosilicate)
Nesse Fig 4.9
51
ERTH 2001 Crystal Structure
Representing mineral structures
distances between atoms shown as unit cell
dimensions (distances along a, b,
c crystallographic axes) 0 bottom 100 top 50
half-way
atomic radius (?) CN Mg2 0.72
6 Si4 0.26 4 O2- 1.40
olivine Mg2SiO4 (orthosilicate)
Nesse Fig 4.9
52
ERTH 2001 Crystal Structure (Nesse, Ch. 4)

• Controls of Crystal Structure (p.57-65)
• Isostructural Minerals Polymorphs (p.65-69)
• Classification Compositional Variation
  (p.69-73)

53
ERTH 2001 Crystal Structure
Representing mineral structures
most common view used in textbooks
atomic radius (?) CN Mg2 0.72
6 Si4 0.26 4 O2- 1.40
olivine Mg2SiO4 (orthosilicate)
Nesse Fig 4.9
54
ERTH 2001 Crystal Structure
Isostructural minerals same crystal structure
different
chemical composition
55
ERTH 2001 Crystal Structure
Isostructural minerals same crystal structure
different
chemical composition
halite structure (fcc) NaCl PbS
56
ERTH 2001 Crystal Structure
Isostructural minerals same crystal structure
different
chemical composition
Polymorphs same chemical composition differ
ent crystal structure
halite structure (fcc) NaCl PbS
57
ERTH 2001 Crystal Structure
Isostructural minerals same crystal structure
different
chemical composition
Polymorphs same chemical composition differ
ent crystal structure
a-SiO2
ß-SiO2
halite structure (fcc) NaCl PbS
58
ERTH 2001 Crystal Structure
Isostructural minerals same crystal structure
different
chemical composition
Polymorphs same chemical composition differ
ent crystal structure
a-SiO2
ß-SiO2
reconstructive polymorphism displacive
polymorphism order-disorder polymorphism
displacive polymorphism
59
ERTH 2001 Crystal Structure
Solid Solution and End Members
Cations with similar size and/or charge can
substitute for each other in suitable lattice
sites without significantly distorting the
crystal structure. K (1.55 ?) ? Na (1.24 ?)
Mg2 (0.72 ?) ? Fe2 (0.78 ?)
60
ERTH 2001 Crystal Structure
Solid Solution and End Members
Cations with similar size and/or charge can
substitute for each other in suitable lattice
sites without significantly distorting the
crystal structure. Minerals that exhibit a
continuous change in chemical composition without
a significant change in crystal structure are
said to show solid solution.
61
ERTH 2001 Crystal Structure
Solid Solution and End Members
Cations with similar size and/or charge can
substitute for each other in suitable lattice
sites without significantly distorting the
crystal structure. Minerals that exhibit a
continuous change in chemical composition without
a significant change in crystal structure are
said to show solid solution. In the chemical
formula, cations that form solid solutions on any
given site are listed together in
parentheses (K,Na)AlSi3O8 (Mg,Fe)2SiO4 K
(1.55 ?) ? Na (1.24 ?) Mg2 (0.72 ?) ?
Fe2 (0.78 ?)
62
ERTH 2001 Crystal Structure
Solid Solution and End Members
In the chemical formula, cations that form solid
solutions on any given site are listed together
in parentheses (K,Na)AlSi3O8 (Mg,Fe)2SiO4
K (1.55 ?) ? Na (1.24 ?) Mg2 (0.72 ?)
? Fe2 (0.78 ?) For convenience, solid solution
series may described in terms of chemically
pure end members, which may or may not exist as
minerals in nature. alkali feldspar
olivine orthoclase
(Or) KAlSi3O8 forsterite (Fo) Mg2SiO4 albite
(Ab) NaAlSi3O8 fayalite (Fa) Fe2SiO4
63
ERTH 2001 Crystal Structure
Solid Solution and End Members
Coupled Substitutions Cations with similar
ionic radii but different valences can form solid
solutions. This is particularly common for major
elements, eg Na (1.24 ?) ? Ca2 (1.18
?) Al3 (0.39 ?) ? Si4 (0.26
?) BUT this requires an additional coupled
substitution elsewhere in the lattice to maintain
charge balance e.g., plagioclase NaAlSi3O8 ?
CaAl2Si2O8 albite (Ab)
anorthite (An)
64
ERTH 2001 Crystal Structure
Solid Solution and End Members
Order - disorder in solid solutions Within the
crystal lattice, substituting cations can be
distributed - randomly both cations equally
likely to be found in a given site in the
lattice (disordered) tends to be favoured by
high T leads to higher symmetry non-randomly
each cation "prefers" a specific site in
the lattice (ordered) tends to be favoured
by low T leads to lower symmetry
65
ERTH 2001 Crystal Structure
Solid Solution and End Members
Order - disorder in solid solutions Within the
crystal lattice, substituting cations can be
ordered or disordered
e.g., K-feldspar KAlSi3O8 Al ?
Si in tetrahedral sites in ratio 13
66
ERTH 2001 Crystal Structure
Solid Solution and End Members
Order - disorder in solid solutions Within the
crystal lattice, substituting cations can be
ordered or disordered
disordered completely random distribution of Al,
Si (sanidine)
e.g., K-feldspar KAlSi3O8 Al ?
Si in tetrahedral sites in ratio 13
67
ERTH 2001 Crystal Structure
Solid Solution and End Members
Order - disorder in solid solutions Within the
crystal lattice, substituting cations can be
ordered or disordered
disordered completely random distribution of Al,
Si (sanidine)
e.g., K-feldspar KAlSi3O8 Al ?
Si in tetrahedral sites in ratio 13
ordered non-random distribution of Al,
Si (microcline)
68
ERTH 2001 Crystal Structure
Solid Solution and End Members
Order - disorder in solid solutions Within the
crystal lattice, substituting cations can be
ordered or disordered
disordered completely random distribution of Al,
Si (sanidine)
e.g., K-feldspar KAlSi3O8 Al ?
Si in tetrahedral sites in ratio 13
ordered non-random distribution of Al,
Si (microcline)
order-disorder polymorphism
69
ERTH 2001 Crystal Structure
Solid Solution and End Members
Chemical compositions of minerals are written in
terms of chemical formulas (balanced) Structural
formula (Nesse, p.71-73, 172-174, 183-186) -
chemical formula written in a way that also
conveys information on crystal structure -
written in a standard form for each major mineral
group
70
ERTH 2001 Crystal Structure
Solid Solution and End Members
Chemical compositions of minerals are written in
terms of chemical formulas (balanced) Structural
formula (Nesse, p.71-73, 172-174, 183-186) -
chemical formula written in a way that also
conveys information on crystal structure -
written in a standard form for each major mineral
group e.g., structural formulas of silicates
have the following general format A...
(W,X)... (Y,Z)... (Al,Si)... O... (OH)...
71
ERTH 2001 Crystal Structure
Solid Solution and End Members
Structural formulas of silicates have the
following general format A... (W,X)...
(Y,Z)... (Al,Si)... O... (OH)... A monovalent
cations (e.g., K, Na), typically large, in 8- or
12- fold co-ordination (W,X) divalent cations
(e.g., Mg, Fe2, Mn, Ca) that substitute for each
other, typically in octahedral (6-fold)
co-ordination (Y,Z) trivalent (or higher
valance) cations (e.g. Al, Fe3, Ti4) that
substitute for each other, typically in
octahedral co-ordination (Al,Si) cations
occupying the tetrahedral site (4-fold
co-ordination) in the silicate tetrahedron it is
common for some Al to substitute for Si in this
site O oxygen at apices of silicate
tetrahedron this value is generally regarded as
fixed for any given mineral or mineral type (OH)
additional anions or anionic groups (e.g., OH,
F, Cl, O) that are chemically bound within the
crystal structure but generally do not occupy
apical sites in silicate tetrahedron
72
ERTH 2001 Crystal Structure