Lecture11 twins-defects-creep

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Geo 3500 - Lecture 11

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  • Lecture11 twins-defects-creep

    1. 1. Twins and Defects
    2. 2. TwinningA twin is a symmetrical intergrowth of two or morecrystals of the same substanceTwinning can be considered a type of planar defectThe surface on which two individuals are united isknown as the composition surface. If this surface isa plane, it is called the composition plane and canbe irregular or regular
    3. 3. TwinningThe operations that relate a crystal to its twinnedcounterpart are symmetry operations:‣ twin plane - reflection by a mirror plane‣ twin axis - rotation about a crystal direction common to both, with the angular rotation normally 180°‣ twin center - inversion about a point
    4. 4. Twin Classification4 types of twins:✦ Contact ✦ Polysynthetic✦ Penetration ✦ Cyclic
    5. 5. Contact & Penetration Contact twins have a regular composition surface separating the two individuals; the twin is defined by a twin plane Penetration twins are made up of interpenetrating individuals having an irregular composition surface, and the twin law is usually defined by a twin axis direction
    6. 6. Polysynthetic & Cyclic Multiple twins that have three or more individual crystals twinned according to the same law and if all the composition surfaces are parallel, the resulting group is a polysynthetic twin If successive composition planes are not parallel, a cyclic twin results
    7. 7. Common TwinsSwallowtail Polysynthethic PenetrationTwin Gypsum Albite Twin Twin Galena
    8. 8. Common TwinsPenetration Penetration Carlsbad TwinTwin Pyrite Twins Orthoclase Staurolite
    9. 9. Causes of TwinningGrowth twins - error in stacking of unit cells whencrystal first start to growTransformation twin - variations in temperature andpressure stresses the lattice - lattice relieves thestress by “warping”Deformation twin - some kind of stress is put on thecrystal structure - crystal relieves stress by twinning
    10. 10. Structural Defects As a crystal grows, structural defects are incorporated into the mineral Such imperfections affect growth rate, crystal morphology, and basic properties of crystalline materials, such as strength, conductivity, mechanical deformation, and color
    11. 11. Structural Defects 3 Types: point line plane
    12. 12. Point Defects
    13. 13. Frenkel & Shottky DefectsFrenkel Shottky
    14. 14. Line Defects Edge Screw
    15. 15. Edge Dislocation
    16. 16. Screw Dislocation
    17. 17. Planar DefectsGrain BoundariesTwin PlanesSub-grain Boundaries
    18. 18. Grain Boundaries
    19. 19. Twin Planes
    20. 20. Subgrain Boundaries
    21. 21. Crystal Deformation
    22. 22. Crystal Deformation Dislocation Slip
    23. 23. Migration of Vacancies thru Crystal Lattice
    24. 24. Dislocations QuartzTransmission Electron Microscope Photos
    25. 25. Dislocations QuartzTransmission Electron Microscope Photos
    26. 26. Dislocations Quartz Strain-free Neoblasts
    27. 27. Subgrain Boundaries
    28. 28. Order-Disorder

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