Growth II Twinning, defects, and polymorphism

Growth II Twinning, defects, and polymorphism

Growth II Twinning, defects, and polymorphism Jon Price Congratulationsits twins! Rational, symmetrical intergrowth of structures QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. 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 Chromite contact twins QuickTime anddec

a TIFF (Uncompressed) are needed to see this Staurolite penetration twins Common Twin Laws Triclinic Albite twinning: plagioclase feldspars (CaAl,NaSi)AlSi2O8 commonly show b-axis

perpendicular polysynthetic twinning QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. http://open-adit.com Pericline twinning: microcline, KAlSi3O8 , QuickTime and a develops twining around the [010] axis when it TIFF (Uncompressed) decompressor are needed to see this picture.

transforms from a monoclinic structure X-polar photomicrograph by K. Hollocher, Union Common Twin Laws Monoclinic Manebach twinning: orthoclase, KAlSi3O8 , contact twin. Very common. Formed from accidental growth. QuickTime and decompressor a TIFF are (Uncompressed)

needed to see this picture. Carlsbad twinning: orthoclase and sanidine, QuickTime andthis aO TIFF (Uncompressed) decompressor , develop penetration twining around the are needed toKAlSi

see 3 8picture. [001]. Formed from accidental growth. Baveno twinning: orthoclase, KAlSi3O8 , develops contact twin during accidental growth. QuickTime and adecomp TIFF are (Uncompressed) needed to see this pictur http://open-adit.com

Common Twin Laws Monoclinic QuickTime and a decompressor TIFF (Uncompressed) are needed to see this picture. Swallow tail twinning: gypsum, CaSO 4 2H2O ,

develops contact twin during accidental growth. Tetragonal Most are cyclical contacts on {011}. Rutile (TiO2) and cassiterite (SnO2) are examples. QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. http://open-adit.com

Common Twin Laws Hexagonal Calcite twinning: Common contact twins are QuickTime and a {0001} and rhombohedron. The right from can TIFF (Uncompressed) decompressor are needed to see this picture. also can be stress Induced. Brazil twinning and daupine twinning: Penetration quartz twins resulting from

transformation. QuickTime and adecompressor TIFF QuickTime and a(Uncompressed) TIFF (Uncompressed) decompressor are needed

to see this picture. are needed to see this picture. http://open-adit.com Common Twin Laws Isometric Spinel twinning: contact twin parallel to an and decompressor

a octahedron common to spinel (MgAl 2O4) TIFF QuickTime (Uncompressed) are needed to see this picture. Iron cross twin: Pyrite (FeS2), 2/m class, may have pentration twinning of the forms with appearant 3A4 symmetry. QuickTime and decompressor a TIFF

are (Uncompressed) needed to see this picture. http://open-adit.com Defects Missing atoms (vacancies) Impurities Edge dislocations Screw dislocations Interlayered structures Twins

Non-stoichiometric atoms Schottky defect Image from Perkins, 1998 Edge defect Frenkle defect Image from Perkins, 1998 QuickTime and a TIFF (Uncompressed) decompressor

are needed to see this picture. STM image of PtNi alloy edge defects Michael Schmid, IAP/ TU Wien Edge defect QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Screw dislocation

AFM image of growth spiral on graphite along [001]. MIT Importance of defects Incorporation of non-stoichiometric elements (non substitution) Color Incorporation of foreign materials inclusions Can produce diagnostic characteristics Twinning in feldspar

Energy Minimization Everything explained in the course thus far is the result of energy minimization! Examples? In nature, energy is the only commodity. Energy Minimization - a system will assume a state of minimum energy. Parameterizing energy - the Gibbs 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 So Free Energy is dependent on:

1. The nature of the bonding 2. Pressure 3. Temperature 4. Degree of disorder The Carbon System Graphite - steep dG/dP Diamond - higher initial G, shallow dG/dP Diamonds excited state Image modified from Zoltai and Stout, 1984

Poly morph - many forms These abound in Earth Materials and can be of great use in pinning down the conditions at which the mineral formed. 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! At P = 5 kbar

Image from Pauling, 1970 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 Reconstruction vs. displacement Displacement requires less transition energy because lattices are just tweaked Reconstruction requires substantial excess energy to move things to new

configuration Polymorphs Formula CaCO3 SiO2 Al2SiO5 Al2O3 H2O C Mineral Name Calcite

Aragonite Quartz L. Cristobolite H. Cristobolite L. Tridymite H. Tridymite Coesite Stishovite Kyanite Silimanite Andalusite Gibbsite Boehmite

Graphite Diamond Density 2.715 2.94 2.65 2.32 2.20 2.26 2.22 3.01 4.28

3.6 3.25 3.15 3.4 3.02 2.25 3.51 Symmetry Trigonal Orthorhombic Trigonal Tetragonal

Isometric Monocl/ortho Hexagonal Monoclinic Tetragonal Triclinic Orthorhombic Orthorhombic Orthorhombic Orthorhombic Trigonal Cubic

Coordination C3 Ca8 C3 Ca8 Si4 Si4 Si4 Si4 Si4 Si4 Si8 Al6Al6 Si4 Al6Al5 Si4 Al6Al4 Si4

Al6 H2 Al6 H2 C3 C4 From Blackburn & Dennen, 1998 Silica Polymorphs More morphs CaCO3

AlSiO5 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. Alkali Feldspar Order-disorder M

T T 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 . 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 In summary - Polymorphism is a reconfiguration of chemical components in response to changing energy. Polymorphs therefore have the same composition but differing structures.

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