Post-crystallization process Changes in structure and/or composition following crystallization Changes in structure and/or composition following crystallization

Examples Ordering Ordering e.g. in the K-feldspars e.g. in the K-feldspars Changes result from cooling Changes result from cooling Exsolution – another example of phase diagram Exsolution – another example of phase diagram Recrystallization Recrystallization Radioactive decay Radioactive decay Structural defects Structural defects Twinning Twinning

Idealized feldspar structure Fig Si or Al K (or Na, Ca) Si or Al Fig Al migrates through structure with cooling: Sanidine to Orthoclase to Microcline as Al restricted

Exsolution Common in alkali feldspars, also occurs in the plagioclase feldspars Common in alkali feldspars, also occurs in the plagioclase feldspars High T: complete solid solution between K and Na High T: complete solid solution between K and Na Low T: limited solid solution Low T: limited solid solution Distribution of solid solution shown on phase diagram Distribution of solid solution shown on phase diagram

Alkali Feldspar – complete phase diagram P H2O = 1.96 kb Only limited temperature range with complete solid solution (770 to 680) Works exactly like the plagioclase feldspar except binary minimum Fig. 5-7a

Fig Solid homogeneous alkali feldspars Homogeneous compositions not allow Split into two separate phases Albite matrix K-spar matrix Start

Exsolution occurs in solid state Exsolution occurs in solid state Time and temperature dependent Time and temperature dependent Most have sufficient time for diffusion to move ions, separate two phases Most have sufficient time for diffusion to move ions, separate two phases Perthite – term for albite exsolution lamellae in K-spar matrix Perthite – term for albite exsolution lamellae in K-spar matrix Antiperthite – K-spar exsolution lamellae in albite matrix Antiperthite – K-spar exsolution lamellae in albite matrix

P H2O = 5 kb Solvus line intersects the Liquidus and Solidus curves Crystallization continues as usual until point d – eutectic, Ks 53 and Ks 19 crystallize until solid With more cooling, Albite and K-spar “unmix” and become more “pure” phases. Still limited solid solution. Alkali Feldspar – phase diagram

Recrystallization Surfaces are high energy environment because of terminated bonds Surfaces are high energy environment because of terminated bonds Minerals change to minimize the surface area Minerals change to minimize the surface area Edges become smoother Edges become smoother Grains become larger Grains become larger

Fig Smoother boundaries from recrystallization Minimize surface area

Contact metamorphism Larger grain size from recrystallization

Pseudomorphism Replacement of one mineral by another Replacement of one mineral by another Low – T phenomenon usually, weathering Low – T phenomenon usually, weathering Preserves the external form of original mineral Preserves the external form of original mineral Example: Example: quartz (hexagonal) replacing fluorite (isometric) quartz (hexagonal) replacing fluorite (isometric) Cubic Quartz??

Radioactivity – Beta decay Generate new elements cause substitution defects Generate new elements cause substitution defects Decay of 40 K to 40 Ca and 40 Ar Decay of 40 K to 40 Ca and 40 Ar Beta decay (electron or positron emitted) Beta decay (electron or positron emitted) The newly created elements are not same size or charge as the original element The newly created elements are not same size or charge as the original element Not typically substituted in mineral Not typically substituted in mineral Below closing T, Ar trapped, used for dating Below closing T, Ar trapped, used for dating

Radioactivity - Alpha decay Alpha particle dislodges atoms Alpha particle dislodges atoms Causes defect in crystal structure Causes defect in crystal structure Metamict minerals form if long enough time and high enough radioactivity Metamict minerals form if long enough time and high enough radioactivity Change physical properties because loss of long range order Change physical properties because loss of long range order Less dense Less dense Darker Darker Optical properties change Optical properties change Also may change physical properties of surrounding minerals Also may change physical properties of surrounding minerals

Structural Defects Disruptions in ordered arrangement of atoms within crystals Disruptions in ordered arrangement of atoms within crystals Common in natural minerals Common in natural minerals Occur as point, line, or plane defect Occur as point, line, or plane defect Different from compositional variation Different from compositional variation Systematic throughout crystal lattice Systematic throughout crystal lattice I will only talk about types of point defects I will only talk about types of point defects

Point Defects Schottky Defect - Vacant Sites Schottky Defect - Vacant Sites Frenkel defect - Atoms out of correct position – Frenkel defect - Atoms out of correct position – Impurity defects: Impurity defects: Extraneous atoms or ions Extraneous atoms or ions Substituted atoms or ions Substituted atoms or ions Similar to solid solution series or substitutions Similar to solid solution series or substitutions Difference is magnitude of substitution Difference is magnitude of substitution

Schottky defects Vacancy – i.e. both cation and anion missing Vacancy – i.e. both cation and anion missing 1:1 ratio vacancy if similar charge – e.g. Halite missing equal amount of Cl - and Na + 1:1 ratio vacancy if similar charge – e.g. Halite missing equal amount of Cl - and Na + Can be more complex with higher charge Can be more complex with higher charge Fig. 5-15a

Frenkel Defects Dislocation defects Dislocation defects Generally cations because they are smaller Generally cations because they are smaller No change in the charge balance No change in the charge balance Fig. 5-15b

Frenkel and Schottky Mechanisms for changes in solid state Mechanisms for changes in solid state Diffusion through minerals Diffusion through minerals Allows metamorphism Allows metamorphism

Impurity Defects Interstitial defects Interstitial defects Ions or atoms in sites not normally occupied Ions or atoms in sites not normally occupied Requires charge balance of mineral Requires charge balance of mineral Substitution defects Substitution defects Substitution of one ion for another ion in the structure Substitution of one ion for another ion in the structure Identical to “substitution”, but depends on expectation of pure composition Identical to “substitution”, but depends on expectation of pure composition Example – radioactive decay, 40 K to 40 Ar Example – radioactive decay, 40 K to 40 Ar

Fig Interstitial defect – foreign cation located in structure Substitution defect – (1) foreign cation substitutes for normal cation (2) Radioactive decay

Twinning Intergrowth of two or more crystals Intergrowth of two or more crystals Related by symmetry element not present in original single mineral Related by symmetry element not present in original single mineral Several twin operations (i.e. symmetry element): Several twin operations (i.e. symmetry element): Reflection Reflection Rotation Rotation Inversion (rare) Inversion (rare) “Twin Law” – describes twin operation and axis or plane of symmetry “Twin Law” – describes twin operation and axis or plane of symmetry

Reflection Two or more segments of crystal Two or more segments of crystal Related by mirror that is along a common crystallographic plane Related by mirror that is along a common crystallographic plane Can not be a mirror in the original mineral Can not be a mirror in the original mineral

Fig Crystallographic axes Reflection on {011} Twin law: Reflection on (011) Rutile TiO 2 - Tetrahedral

Rotation Two or more segments of crystal Two or more segments of crystal Related by rotation of crystallographic axis common to all Related by rotation of crystallographic axis common to all Usually 2-fold Usually 2-fold Can not duplicate rotation in original mineral Can not duplicate rotation in original mineral

Fig Twin Law: Rotation on [001] Very common in K- spars – called “Carlsbad twins”

Twin terminology Composition surface – plane joining twins, may be irregular or planar Composition surface – plane joining twins, may be irregular or planar Composition plane – if composition surface is planar; referred to by miller index Composition plane – if composition surface is planar; referred to by miller index Contact twin – no intergrowth across composition plane Contact twin – no intergrowth across composition plane

Fig Contact Twins Spinel isometric – reflected on {111} – reflected on {111} Gypsum Monoclinic – reflected on {100} – reflected on {100} Calcite hexagonal – reflected on {001} – reflected on {001}

Fig Penetration twin – inter-grown twins, typically irregular composition surfaces Penetration twin – inter-grown twins, typically irregular composition surfaces Pyrite Isometric – 180º rotation on [001] Staurolite Monoclinic – reflection on {231}

Simple twins – two twin segments Simple twins – two twin segments Multiple twins – three or more segments repeated by same twin law Multiple twins – three or more segments repeated by same twin law Polysynthetic twins – succession of parallel composition planes (plagioclase) Polysynthetic twins – succession of parallel composition planes (plagioclase) Cyclic twins – succession of composition planes that are not parallel Cyclic twins – succession of composition planes that are not parallel

Fig Polysynthetic Twins Plagioclase: Albite twinning: repeated reflection on {010} Allows Michel – Levy technique Cyclic Twins Rutile – repeated reflection on {011}

Mechanism forming twins Growth – occur during growth of minerals Growth – occur during growth of minerals Transformation – displacive polymorphs Transformation – displacive polymorphs Occurs during cooling of minerals Occurs during cooling of minerals E.g. leucite, transforms from cubic to tetragonal system 665º C E.g. leucite, transforms from cubic to tetragonal system 665º C Space change accommodated by twins Space change accommodated by twins

Fig Isometric above 665º CTetragonal below 665º C Can be elongate along any three directions Twinned crystals can fill all available space Leucite KAlSi 2 O 6 A feldspathoid

Fig Deformation twinning Deformation twinning Result from application of shear stress Result from application of shear stress Lattice obtains new orientation by displacement along successive planes Lattice obtains new orientation by displacement along successive planes