Post-crystallization process

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

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

3. Idealized feldspar structure Fig. 12-6 Si or Al K (or Na, Ca) Si or Al Al migrates through structure with cooling: Sanidine to Orthoclase to Microcline as Al restricted Fig. 4-13

4. 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

5. Alkali Feldspar – complete phase diagram PH2O = 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

6. Solid homogeneous alkali feldspars Fig. 5-27 Albite matrix K-spar matrix Start Homogeneous compositions not allow Split into two separate phases

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

8. Alkali Feldspar – phase diagram PH2O = 5 kb Solvus line intersects the Liquidus and Solidus curves Crystallization continues as usual until point d – eutectic, Ks53 and Ks19 crystallize until solid With more cooling, Albite and K-spar “unmix” and become more “pure” phases. Still limited solid solution.

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

10. Fig. 5-26 Smoother boundaries from recrystallization Minimize surface area

11. Contact metamorphism Larger grain size from recrystallization

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

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

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

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

16. 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

17. Schottky defects • 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+ • Can be more complex with higher charge Fig. 5-15a

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

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

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

21. Fig. 5-11 Substitution defect – (1) foreign cation substitutes for normal cation (2) Radioactive decay Interstitial defect – foreign cation located in structure

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

23. 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

24. Rutile TiO2 - Tetrahedral Fig. 5-20 Crystallographic axes Twin law: Reflection on (011) Reflection on {011}

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

26. Fig. 5-16 Twin Law: Rotation on [001] Very common in K-spars – called “Carlsbad twins”

27. 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

28. Fig. 5-21 Contact Twins Spinel isometric – reflected on {111} Gypsum Monoclinic – reflected on {100} Calcite hexagonal – reflected on {001}

29. Fig. 5-22 • Penetration twin – inter-grown twins, typically irregular composition surfaces Staurolite Monoclinic – reflection on {231} Pyrite Isometric – 180º rotation on [001]

30. 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

31. Polysynthetic Twins Fig. 5-23 Cyclic Twins Rutile – repeated reflection on {011} Plagioclase: Albite twinning: repeated reflection on {010} Allows Michel – Levy technique

32. 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 - @ 665º C • Space change accommodated by twins

33. Isometric above 665º C Tetragonal below 665º C Fig. 5-20 Can be elongate along any three directions Leucite KAlSi2O6 A feldspathoid Twinned crystals can fill all available space

34. Fig. 5-20 • Deformation twinning • Result from application of shear stress • Lattice obtains new orientation by displacement along successive planes