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2.5 Structural Defects and twins

2.5 Structural Defects and twins - Postcrystallization...

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Unformatted text preview: Postcrystallization process Changes in structure and/or composition following crystallization Examples Ordering Exsolution another example of phase diagram Recrystallization Radioactive decay Structural defects Twinning e.g. in the Kfeldspars Changes result from cooling Exsolution Common in alkali feldspars, also occurs in the plagioclase feldspars High T: complete solid solution between K and Na Low T: limited solid solution Distribution of solid solution shown on phase diagram Solid homogeneous alkali feldspars Albite matrix K spar matrix Homogeneous compositions not allow Split into two separate phases Fig. 523 Exsolution occurs in solid state Perthite term for albite exsolution lamellae in Kspar matrix Antiperthite Kspar exsolution lamellae in albite matrix Time and temperature depending Most have sufficient time for diffusion to move ions Examples of postcrystallization Ordering Exsolution another example of phase diagram Recrystallization Radioactive decay Structural defects Twinning e.g. in the Kfeldspars Changes result from cooling Recrystallization Surfaces are high energy environment because of terminated bonds Minerals will change to minimize the surface area Grains become larger Edges become smoother Smoother boundaries from recrystallization Larger grain size from recystallization Fig. 522 Pseudomorphism Replacement of one mineral by another Preserves the external form of original mineral Example: Goethite (orthorhombic) replacing pyrite (isometric) Radioactivity Generate new elements cause substitution defects Decay of 40K to 40Ca and 40Ar Below closing T, Ar trapped, used for dating Alpha decay Alpha particle dislodges atoms Metamict minerals form if long enough time and high enough radioactivity Change physical properties because loss of long range order Causes defect in crystal structure Also may change physical properties of surrounding minerals Less dense Darker Optical properties change Structural Defects Disruptions in ordered arrangement of crystals Occur as point, line, or plane defect Different from compositional variation Common in natural minerals I will only talk about types of point defects Systematic throughout crystal lattice Point Defects Schottky Defect Vacant Sites Frenkel defect Atoms out of correct position Impurity defects: Extraneous atoms or ions Substituted atoms or ions Similar to solid solution series or substitutions Difference is magnitude of substitution Schottky defects Vacancy i.e. both cation and anion missing 1:1 ratio vacancy if similar charge e.g. Halite Can be more complex with higher charge Frenkel Defects Dislocation defects Generally cations because they are smaller No change in the charge balance Frenkel and Schottky Mechanism for changes in solid state Diffusion through minerals Allows metamorphism Impurity Defects Interstitial defects Substitution defects Ions or atoms in sites not normally occupied Requires charge balance of mineral Substitution of one ion for another ion in the structure Identical to "substitution", but depends on expectation of pure composition Interstitial defect foreign cation located in structure Substitution defect foreign cation substitutes for normal cation Fig. 511 Twinning Intergrowth of two or more crystals Related by symmetry element not present in original single mineral Several twin operations: "Twin Law" describes twin operation and axis or plane of symmetry Reflection Rotation Inversion (rare) Reflection Two or more segments of crystal Related by mirror that is along a common crystallographic plane Can not be a mirror in the original mineral Rutile TiO2 Crystallographic axes Twin law: Reflection on (011) Reflection on {011} Fig. 516 Rotation Two or more segments of crystal Related by rotation of crystallographic axis common to all Usually 2fold Can not duplicate rotation in original mineral Twin Law: Rotation on [001] Very common in K spars called "Carlsbad twins" Fig. 516 Twin terminology Composition surface plane joining twins, may be irregular or planar Composition plane if composition surface is planar; referred to by miller index Contact twin no intergrowth across composition plane Contact Twins Spinel reflected on {111} Gypsum reflected on {100} Calcite reflected on {001} Fig. 517 Penetration twin intergrown twins, typically irregular composition surfaces Pyrite 90 rotation on [001] Staurolite reflection on {231} Fig. 518 Simple twins two twin segments Multiple twins three or more segments repeated by same twin law Polysynthetic twins succession of parallel composition planes (plagioclase) Cyclic twins succession of composition planes that are not parallel Polysynthetic Twins Cyclic Twins Plagioclase repeated reflection on {010} Rutile repeated reflection on {011} Fig. 519 Mechanism forming twins Growth occur during growth of minerals Transformation displacive polymorphs Occurs during cooling of minerals E.g. leucite, transforms from cubic to tetragonal system @ 665 C Space change accommodated by twins Isometric above 665 C Tetragonal below 665 C Can be elongate along any three directions Leucite KAlSi2O6 A feldspathoid Twinned crystals can fill all available space Fig. 520 Deformation twinning Result from application of shear stress Lattice obtains new orientation by displacement along successive planes Fig. 520 ...
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  • Spring '11
  • Staff
  • Crystal, Feldspar, Crystal twinning, Albite, Radioactive decay Structural, Recrystallization Radioactive decay

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