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# 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