Principles of Mineral Behaviour (L7-13) Flashcards
(104 cards)
1
Q
What does a phase diagram show?
A
Which structure is stable at a given P, T
Which structure has lowest free energy = eq^m state
2
Q
What causes a phase transition?
A
Change in P, T
Change in eq^m state
Phase transition
3
Q
For SiO2, which phase transitions are displacive and which are reconstructive?
A
Displacive: α-β Qz, high-low crist, high-low trid
Reconstructive: Qz-trid, trid-crist, Qz-coesite
4
Q
Where does β cristobalite occur?
A
Rarely
In volcanic ash
5
Q
Where does high P tridymite occur?
A
Thermal aureoles around granite/gabbro intrusions
6
Q
What form is Si in, in α quartz?
A
Si in tetrahedra
7
Q
What form is Si in, in stishovite?
A
Si in octahedra
8
Q
Where does coesite occur?
A
As inclusions in garnet in ultrahigh pressure metamorphic rocks
9
Q
What can be said about the properties of an individual tetrahedron of SiO2?
A
Si-O bonds are strong
Rigid tetrahedron
10
Q
How do tetrahedra link?
What can be said about the angle?
A
Sharing corner oxygen, forming chains, sheets, or frameworks
θ = 180 = purely ionic, θ = 109 = purely covalent
θ observed 140-180: partly covalent
11
Q
What can be said about the variation of bond energy w.r.t Si-O bond length and Si-O-Si bond angle?
A
Si-O bond length has a narrow range
Changing θ doesn’t change energy by much, allowing variety in frameworks
12
Q
High β quartz: Crystal system? Space group? When is it stable? What does a view down the c axis show?
A
Hexagonal
P6_2 22
Stable above 573°C
Spirals of tetrehedra around screw nodes
13
Q
Low α quartz:
Crystal system?
Space group?
When is it stable?
A
Trigonal
P3_2 21
Stable below 573°C
14
Q
How can the structures of tridymite and cristobalite be described?
A
Sheets of tetrahedra stacked in different ways
15
Q
High tridymite:
Crystal system?
Space group?
Stacking of layers?
A
Hexagonal
P 6_3 /mmc
ABAB
16
Q
High cristobalite:
Crystal system?
Space group?
Stacking of layers?
A
Cubic
Fd3m
ABCABC
17
Q
Define polytypes
A
Different structure of identical layers
18
Q
Coesite:
Crystal system?
Space group?
Framework?
A
Monoclinic
C2/c
3D, contains 4-fold rings of tetrahedra
19
Q
Stishovite: Crystal system? Space group? Structure? When is it stable? Why so dense?
A
Tetragonal P4_2 /mnm Chains of edge sharing SiO6 octahedra Stable at v high P Dense as increased coordination of Si
20
Q
Displacive transition of quartz from β to α:
How many orientations?
What happens to screw hexad?
What name is given to the product?
A
2
Becomes screw triad
Dauphiné twins
21
Q
How can the SiO2 variants be told apart in thin section?
A
Quartz: uniaxial positive, dusty inclusions, undulose extinction
Tridymite: biaxial positive, arrow-head twinning
Cristobalite: uniaxial negative, fish scale texture
22
Q
What happens in a displacive transition?
A
A high-symmetry structure transforms to a closely-related low-symmetry structure via small displacements of atoms
Displacements often described in terms of rotations of structural units
23
Q
Displacive transitions:
Quenchable?
What happens if there’s a change in crystal system?
A
No, transition is too fast
Transformation twins appear
24
Q
What happens in a reconstructive transition?
A
One polymorph transforms to another polymorph with an unrelated structure
25
```
Reconstructive transitions:
Why is activation energy large?
Quenchable?
Any twins?
Why does it cause seismic anomalies?
```
Bonds broken and reformed, diffusion of atoms, very slow process
Easily, because transformation is slow, metastability is possible
No since structures are unrelated
Because all physical properties change
26
What is the equation to determine free energy?
G = H_0 - TS + PV
27
Enthalpy:
What is it?
When does it dominate free energy?
What causes high enthalpy?
Bond energies + kinetic energy from atomic vibrations
When P and T are low
Electrostatic repulsion, unfavourable bond angles, strain, cation size mismatch, charge mismatch
28
Entropy:
What is it?
When does it dominate free energy?
What causes high entropy?
Measure of disorder in a crystal
When T is high
Large vibrational degree of freedom, configurational disorder
29
Volume:
What is it?
How is molar volume determined?
What does high P favour?
Efficiency with which atoms in structures fill space
Volume of unit cell and the number of formula units in it
Structures with small molar volume
30
Why is it unnecessary that H, S and V are a function of P, T?
ΔH, ΔS, ΔV are usually constant for solid-solid reactions
31
Temperature dependence of free energy:
Which phases are stable at low T and high T?
dG/dT = ?
Change at Ttr = ?
Low enthalpy phase stable at low T
High entropy phase stable at high T
dG/dT = -S
ΔS
32
Pressure dependence of free energy:
Which phases are stable at low P and high P?
dG/dP = ?
Change at Ptr = ?
Low enthalpy phase stable at low P
Low volume phase stable at high P
dG/dP = V
ΔV
33
How do first-order and second-order phase transitions differ?
First-order: discontinuous dG/dT at transition point
| Second-order: continuous dG/dT, discontinuous d2G/dT2 at transition point
34
What kind of phase transition is Qz-trid?
Reconstructive
35
What determines the transition point in a phase transition?
ΔG = 0
36
Geological importance:
What is the SiO2 polymorph present indicative of?
What does the presence of transformation twins indicate?
How do reconstructive transitions relate to seismics?
How do displacive transitions relate to seismics?
PT conditions of formation
The cooling history
Reconstructive transitions -> Δρ -> Δv shear + body wave
Displacive transitions -> elastic property changes -> changes in v_s + v_p
37
Olivine:
General formula
Crystal system
Space group
R2SiO4
Orthorhombic
Pbnm
38
Where is olivine common?
Upper mantle
| Basic igneous rocks
39
What is the structure of olivine?
Isolated SiO4 tetrahedra
Linked by cation sites (m1, m2) with 6 fold coordination
O(2-) arranged in close packed layers
40
What are the Goldschmidt rules for solid solution?
Cation differs in size by < 15%
| Cations differ in charge by =< +/- 1
41
When is solid solution viable for olivine compositions, and when is it not?
Complete s.s. between Forsterite (Mg2SiO4) and Fayalite (Fe2SiO4)
No s.s. between Monticellite (CaMgSiO4) and Forsterite (Mg2SiO4)
42
Why can some structures containing octahedra not do displacive transitions?
Shared edges = not flexible
43
Outline the olivine-spinel reconstructive transition
Both have O(2-) in c.p. layers
Olivine layers ABABAB, spinel layers ABCABC
Olivine hcp, spinel ccp
44
How do the m1 and m2 cation sites differ in olivine's structure?
M1 sites form a chain sharing edges
M2 sites attached to sides of chains
M2 sites are on mirror planes
M1 sites are between mirror planes
45
What is the best way to distinguish forsterite and fayalite in thin section?
Forsterite: biaxial positive
Fayalite: biaxial negative
46
What are the distinguishing features of olivine in thin section?
Moderate/high relief
Moderate/high birefringence
Often has curvy serpentine cracks
47
Why does Fo and Fa form a s.s.?
Free energy of mixing is at a minimum between the two pure compositions
48
How can the proportion of melt or xal be determined in a T against X plot?
Lever rule
49
What develops if crystallisation is too fast for equilibrium compositions to develop?
How would it look for olivine?
Zoning
| Mg rich core, Fe rich rim
50
G-X curves for liquid and s.s. stability are a function of what?
T
51
What is exsolution?
Lowering of energy by phase separation into two phase compositions
52
When does exsolution typically occur?
| Why?
Low T
| ΔH(mix) value makes intermediate compositions unstable
53
Define binary system
A chemical system which can be described in terms of 2 chemical components
54
How do phase diagrams underpin petrology and geophysics?
Melting of mantle -> basalt
| Melting of seds -> granite
55
What are pyroxenes important in?
Basic igneous
| High/med grade matemorphic
56
What is the general formula for pyroxenes?
| What can fill each role?
```
W(1-p) (x,y)(1+p) Z2O6
W: Ca, Na
x: Mg2+, Fe2+
y: Al3+, Fe3+
Z: Al3+, Si4+
```
57
What is the structure of pyroxenes?
Single chains of SiO4 tetrahedra
| Linked laterally by cations in 6 or 8-fold coordination
58
What are the three main pyroxenes, and what are their space groups and crystal systems?
Diopside, C2/c, monoclinic
Orthopyroxene, pbca, orthorhombic
Pigeonite, p2(1)/c, monoclinic
59
What does the quadrilateral of pyroxenes look like?
```
Trapezium shape
Top left: diopside CaMgSi2O6
Top right: hedenbergite CaFeSi2O6
Bottom left: enstatite Mg2Si2O6
Bottom right: ferrosilite Fe2Si2O6
Hypersthene = midpoint of enstatite and ferrosilite
Augite along the top
Opx along the bottom
Pigeonite a bit above opx
```
60
Sodic pyroxenes:
Important in what?
Typically undergo what conditions of metamorphism?
Typical metamorphism product?
How do they differ from quadrilateral pyroxenes?
Important in alkali granites
High P, low T
Blueschists
Na+ instead of Ca2+, Al3+/Fe3+ instead of Mg/Fe2+
61
What are the distinguishing features of all pyroxenes?
2 cleavages at 90°
Colourless (expect for 3 rare types)
Moderate relief
62
How would cpx, opx and pig be distinguished in thin section?
cpx has inclined extinction, opx has straight
opx has low birefringence, cpx and pig have moderate
pig has inclined extinction + low 2V
63
How do pyroxene chains adjust to the different sizes of cation?
Rotation of tetrahedra
64
How do the structures of opx and cpx differ?
Cpx: all m1 site octahedra are in the same orientation
Opx: different orientations of m1 octahedra
65
How does high and low pigeonite differ?
High: C2/c space group, extended chains almost 180° between tetrahedra
Low: P2(1)/c, 2 types of chains with different tetrahedral rotation angles
66
Where does the large miscibility gap arise from between opx + pig and augite in the pyroxene quadrilateral?
Different cation sizes:
Ca2+ 1.00Å, Mg2+ 0.72Å, Fe2+ 0.78Å
Large ΔH(mix)
67
Why is Ca2Si2O6 not a pyroxene?
Can't substitute Ca on m1 sites
68
What does the sodic pyroxene triangle look like?
Top: Aegerine NaFeSi2O6
Bottom left: Jadeite NaAlSi2O6
Bottom right: Augite Ca(Mg,Fe)Si2O6
Complete s.s. between Aegerine and Jadeite
Complete s.s. between Aegerine and Augite
Not a complete s.s. between Jadeite and Augite
Stable omphacite phase between Jadeite and Augite
69
What is the structure of omphacite w.r.t. cation sites?
Ordered arrangement of alternating Al/Mg on m1
| Ca/Na on m2
70
How could omphacite be identified in thin section?
2 cleavages at 90°
| Green in ppl
71
There are three types of processes in quadrilateral pyroxenes, state and outline them
Entropy associated with thermal motions:
Open structures at high T, collapsed at low T
C2/c P2(1)/c displacive transition in pigeonite
Entropy associated with distribution of cations in structure:
Favours s.s. at high T, limited s.s. at low T
-> miscibility gap
-> exsolution of pig from aug and reverse, exsolution of opx from aug and reverse
Reconstructive transition pig opx:
-> expect distinctive textures in thin section
indicative of cooling rates
72
Exsolution of pig from aug:
When do lamellae of pig grow?
Why is the interface planar?
What is the interface plane?
When xal cools below solvus by diffusion of Ca & Mg/Fe2+
Minimise strain energy
// (001)
73
How can exsolution lamellae be used to determine cooling rate?
```
x^2 = D*t
x = diffusion distance ~ thickness of lamellae
D = diffusion coefficient
t = time
```
74
What does the diffusion coefficient depend on?
| What does this mean for exsolution rate?
Strong T dependence
| Faster at high T
75
How do exsolution lamellae depend on T besides growth rate?
Orientation of growth
76
Fast cooling:
Which rock types?
Which phase transitions?
Volcanics: basalt/andesite
Too fast for reconstructive transition or exsolution
Displacive transitions occur
77
What would be the result of fast cooling of:
Augite?
Pigeonite?
Augite: homogeneous augite xals
Pigeonite: homogeneous pig xals, displacive transition from high pig to low pig
78
Intermediate cooling:
Typical settings?
Which phase transitions?
Thick lava flow or thin sill
Too fast for reconstructive transition
Time for Ca/Mg diffusion -> exsolution
79
What would be the result of intermediate cooling of:
Augite?
Pigeonite?
Augite: exsolution of pig // (001)
Pigeonite: exsolution of aug // (001)
80
What's the typical setting of slow cooling?
Large intrusions of basic magma
81
What would be the process of slow cooling pigeonite?
Twinned xal of pigeonite
Cool to pig+aug solvus: exsolve aug // (001)
Cool to eutectoid point: exsolution lamellae coarsen + reconstructive transition pig -> opx (+ aug)
Further cooling: aug exsolving from opx // (100)
"inverted pigeonite" texture
82
What would be the result of slow cooling of augite?
Aug crystals with pig // (001)
83
What would be the result of slow cooling opx?
Opx crystals with aug // (100)
84
What is the chemistry of the alkali feldspar s.s.?
What is the extent at various T?
Why?
NaAlSi3O8 - KAlSi3O8
Complete s.s. at high T
Limited s.s. at low T
K+ Na+, large Δr, Δz = 0
85
What is the chemistry of the plagioclase feldspar s.s.?
What is the extent at various T?
Why?
NaAlSi3O8 - CaAl2Si2O8
Complete s.s. at all T
Na+ + Si4+ Ca2+ + Al3+, Δr small, Δz = 1
86
What is the structure of feldspars?
3D framework of corner sharing AlO4 & SiO4 tetrahedra
| Na, K, Ca in large cavity sites
87
What kind of phase transitions are seen in feldspars?
Displacive
Exsolution textures
Al/Si ordering
88
Displacive transition in Na-rich feldspar:
What happens?
Space group change?
Crystal system change?
Framework collapses around Na+
C2/m C(-)1
Monoclinic triclinic
89
```
Displacive transition in Na-rich feldspar:
What develops?
What is this known as?
What are the two types of interfaces?
Combining the two interface types gives?
```
Shear orientations of equal proportions
Transformation twinning
// old m plane = Albite twin, // old diad = pericline twin
Cross hatched twins, albite + pericline 90° apart
90
```
Exsolution in alkali feldspars:
What occurs?
Under what condition?
Thermodynamics + effect?
Product?
```
Na+ K+ diffusion
Intermediate cooling rate
Large +ve ΔH(mix) -> solvus, peak T = 700°C
Exsolution textures
91
Al/Si ordering (alkali feldspar):
Cooling rate?
What happens at high T?
What happens at low T?
Where is the transition between high and low T?
How can the high and low T transition be visible?
Very slow cooling rate
Configurational entropy favours disordered distribution of Al & Si among tetrahedral sites
Enthalpy reduction due to removal of local strains favours ordered distribution of Al & Si
Transition at 480°C
Sanidine -> microcline in contact metamorphic aureoles
92
Al avoidance:
Other name?
What is it?
Purpose?
Loewenstein's rule
Al & Al try to avoid occupying adjacent tetrahedra
Reduces elastic energy
93
What catalyses Al/Si diffusion?
| What conclusion can be drawn from this?
H+
| Wet granite Al/Si ordering faster than dry granite
94
What would be the process of intermediate cooling of sanidine?
Cools until solvus hit
Exsolution of Na-rich feldspar begins
Exsolution continues until eutectoid point hit
Na feldspar transformation: mono -> tri + twinning within lamellae (only albite twins)
"perthite texture" = irregular lamellae of albite in K feldspar
95
What is the product of slow cooling of an intermediate alkali feldspar composition?
Low albite + microcline showing coarse scale exsolution
96
Al/Si ordering (plag feldspar):
Purpose?
Problem?
Lowers enthalpy by reducing elastic energy
| Al:Si ratio changes across s.s.
97
Feldspar names:
NaAlSi3O8
CaAl2Si2O8
KAlSi3O8
NaAlSi3O8: albite
CaAl2Si2O8: anorthite
KAlSi3O8: sanidine/orthoclase/microcline
98
```
Plag composition in rocks:
Basalt?
Anorthosite?
Granite?
Andesite/granodiorite?
Metamorphic?
```
Basalt: An 65-70%
Anorthosite: An 80-90%
Granite: Ab-rich
Andesite/granodiorite: complex zoning at intermediate compositions
Metamorphic: An content increases with grade
99
Why do andesites & granodiorites have complex zoning of plag feldspars compositions?
Effect of p(H2O) on liquidus/solidus temperatures
100
What are the characteristic features of plag feldspars in thin section?
Abundant lamellar twinning (growth twins)
Low relief
Low birefringence
101
What is the effect of having 2 Al in anorthite compared to albite's 1 Al:
w. r.t structure
w. r.t Al avoidance
An has doubled c repeat in comparison with Ab
Add Al to ordered Ab: puts Al next to Al, unfavourable
-> get alternative ordering schemes & miscibility gaps
102
How many miscibility gaps are there in plag feldspar s.s.?
| Why?
3: peristerite, Boggild, Huttenlocher
| 4 ordered structures
103
Why does the peristerite miscibility gap exist for plag feldspars?
Increasing Al/Si order -> decreased free energy
Narrow G curve because ordered Ab can't accept much Al in s.s.
-> coexisting ordered Ab and disordered An_27
104
How can plag composition be determined from extinction angles?
Find a twinned crystal with sharp twin boundaries
Align twins N-S: same colour in both twins
Check twin interface is vertical: use medium power, twin plane doesn't move on focus change
Extinction angle symmetrical for both twins
Measure two extinction angles for 5 crystals
Use maximum value
Compare to chart
