Growth II Twinning, defects, and polymorphism. Jon Price
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- Daniel Waters
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1 Growth II Twinning, defects, and polymorphism Jon Price
2 !ongratula"on# i$%& twins! Rational, symmetrical intergrowth of structures This raises the internal energy Growth twins - free growth accidents, where a lattice becomes offset during nucleation!hromi' (ontac$ )win# 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 *tauroli' +ene,a"on twin#
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4 Common Twin Laws Triclinic Albite twinning: plagioclase feldspars (CaAl,NaSi)AlSi 2 O 8 commonly show b-axis perpendicular polysynthetic twinning Pericline twinning: microcline, KAlSi 3 O 8, develops twining around the [010] axis when it transforms from a monoclinic structure X-polar photomicrograph by K. Hollocher, Union
5 Common Twin Laws Monoclinic Manebach twinning: orthoclase, KAlSi 3 O 8, contact twin. Very common. Formed from accidental growth. Carlsbad twinning: orthoclase and sanidine, KAlSi 3 O 8, develop penetration twining around the [001]. Formed from accidental growth. Baveno twinning: orthoclase, KAlSi 3 O 8, develops contact twin during accidental growth.
6 Common Twin Laws Monoclinic Swallow tail twinning: gypsum, CaSO 4 2H 2 O, develops contact twin during accidental growth. Tetragonal Most are cyclical contacts on {011}. Rutile (TiO 2 ) and cassiterite (SnO 2 ) are examples.
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.
8 Common Twin Laws Isometric Spinel twinning: contact twin parallel to an octahedron common to spinel (MgAl 2 O 4 ) Iron cross twin: Pyrite (FeS 2 ), 2/m class, may have pentration twinning of the forms with appearant 3A 4 symmetry.
9 Defects Missing atoms (vacancies) Impurities Edge dislocations Screw dislocations Interlayered structures Twins
10 Non-stoichiometric atoms Schottky defect Image from Perkins, 1998
11 Frenkle defect Edge defect Image from Perkins, 1998
12 Edge defect STM image of PtNi alloy edge defects Michael Schmid, IAP/TU Wien Screw dislocation AFM image of growth spiral on graphite along [001]. MIT
13 Importance of defects Incorporation of non-stoichiometric elements (non substitution) Color Incorporation of foreign materials inclusions Can produce diagnostic characteristics Twinning in feldspar
14 Energy Minimization Everything explained in the course thus far is the result of energy minimization! Examples? In nature, energy is the only commodity.
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 of bond energy P is pressure V is molar volume T is temp S is entropy note: E + PV = H
16 So Free Energy is dependent on: 1. The nature of the bonding 2. Pressure 3. Temperature 4. Degree of disorder
17 The Carbon System Graphite - steep dg/dp Diamond - higher initial G, shallow dg/dp
18 Diamond s excited state Image modified from Zoltai and Stout, 1984
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.
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!
21 At P = 5 kbar
22 Image from Pauling, 1970
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
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
25 Polymorphs
26 From Blackburn & Dennen, 1998
27 Silica Polymorphs
28 More morphs CaCO 3 AlSiO 5
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.
30 Alkali Feldspar Order-disorder T M T
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.
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
33 In summary - Polymorphism is a reconfiguration of chemical components in response to changing energy. Polymorphs therefore have the same composition but differing structures.