SLIDE 1
Growth II
Twinning, defects, and polymorphism
Jon Price
SLIDE 2
Rational, symmetrical intergrowth of structures This raises the internal energy
- Growth twins - free growth accidents, where
a lattice becomes offset during nucleation
- Transformation twins - movement of parts of
the lattice when internal symmetry changes
- These may be contact (planar face) or
penetration (throughgoing) twins.
- Gliding twins - offsets in the lattice as a strain
(in response to a stress). Polysynthetic
hromi ontac
tauroli
SLIDE 3
SLIDE 4 Common Twin Laws Triclinic
Albite twinning: plagioclase feldspars (CaAl,NaSi)AlSi2O8 commonly show b-axis perpendicular polysynthetic twinning Pericline twinning: microcline, KAlSi3O8 , develops twining around the [010] axis when it transforms from a monoclinic structure X-polar photomicrograph by K. Hollocher, Union
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SLIDE 5 Common Twin Laws Monoclinic
Manebach twinning: orthoclase, KAlSi3O8 , contact twin. Very common. Formed from accidental growth. Carlsbad twinning: orthoclase and sanidine, KAlSi3O8 , develop penetration twining around the [001]. Formed from accidental growth. Baveno twinning: orthoclase, KAlSi3O8 , develops contact twin during accidental growth.
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SLIDE 6 Common Twin Laws Monoclinic
Swallow tail twinning: gypsum, CaSO4 2H2O , develops contact twin during accidental growth. Most are cyclical contacts
- n {011}. Rutile (TiO2) and
cassiterite (SnO2) are examples.
Tetragonal
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SLIDE 7 Common Twin Laws Hexagonal
Calcite twinning: Common contact twins are {0001} and rhombohedron. The right from can also can be stress Induced. Brazil twinning and daupine twinning: Penetration quartz twins resulting from transformation.
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SLIDE 8 Common Twin Laws Isometric
Spinel twinning: contact twin parallel to an
- ctahedron common to spinel (MgAl2O4)
Iron cross twin: Pyrite (FeS2), 2/m class, may have pentration twinning
- f the forms with appearant 3A4
symmetry.
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SLIDE 9
Defects
Missing atoms (vacancies) Impurities Edge dislocations Screw dislocations Interlayered structures Twins
SLIDE 10 Non-stoichiometric atoms Schottky defect
Image from Perkins, 1998
SLIDE 11 Frenkle defect Edge defect
Image from Perkins, 1998
SLIDE 12 Edge defect Screw dislocation AFM image of growth spiral on graphite along [001]. MIT STM image of PtNi alloy edge defects Michael Schmid,
IAP/TU Wien
SLIDE 13
Importance of defects
Incorporation of non-stoichiometric elements (non substitution) Color Incorporation of foreign materials inclusions Can produce diagnostic characteristics Twinning in feldspar
SLIDE 14 Energy Minimization
Everything explained in the course thus far is the result
Examples?
In nature, energy is the only commodity.
SLIDE 15 Energy Minimization - a system will assume a state of minimum energy. Parameterizing energy - the Gibb’s Free Energy equation
G = E + PV - TS
E is a measurement of lattice energy, or the sum
P is pressure V is molar volume T is temp S is entropy note: E + PV = H
SLIDE 16 So Free Energy is dependent
- n:
- 1. The nature of the bonding
- 2. Pressure
- 3. Temperature
- 4. Degree of disorder
SLIDE 17
The Carbon System
Graphite - steep dG/dP Diamond - higher initial G, shallow dG/dP
SLIDE 18 Image modified from Zoltai and Stout, 1984
Diamond’s excited state
SLIDE 19
Poly morph - µ many forms
These abound in Earth Materials and can be of great use in pinning down the conditions at which the mineral formed.
SLIDE 20
Why can we observe graphite and diamond at the same time? There is a place where both phases share the same G, but at room T, this is ~14 kbar!
SLIDE 21
At P = 5 kbar
SLIDE 22 Image from Pauling, 1970
SLIDE 23
Phase Diagram
Recall that as you go into the Earth, both P and T increase These two variables control phase stability of compositions in the earth. On the left is a map for phases of carbon
SLIDE 24
Reconstruction vs. displacement
Displacement requires less transition energy because lattices are just “tweaked” Reconstruction requires substantial excess energy to move things to new configuration
SLIDE 25
Polymorphs
SLIDE 26
From Blackburn & Dennen, 1998
SLIDE 27
Silica Polymorphs
SLIDE 28
More ‘morphs
CaCO3 AlSiO5
SLIDE 29
Order-disorder
Reorganization of atoms into more ordered arrangements Decrease in T produces higher order
G = E + PV - TS
Change in structure accompanies change in order.
SLIDE 30 Alkali Feldspar Order-disorder
M T T
SLIDE 31
Polytypism
Polymorphs that differ only in the stacking of identical, two-dimensional sheets or layers. Cell dimensions parallel to sheets are identical Spacing between sheets is related by multiples .
SLIDE 32
- 1. Increasing P results in structures with
high densities and large CN are favored
- 2. Increasing T favors low density and CN
- 3. High-T modifications often has highest
symmetry
SLIDE 33
In summary - Polymorphism is a reconfiguration of chemical components in response to changing energy. Polymorphs therefore have the same composition but differing structures.
