1. Structural defects and twinning

2. No crystal has 100% flawless structure

3. Crystal defects Defects can affect Strength Conductivity
Deformation style
Color

4. Crystal Defects Steel spheres:
a) Regular packed array with 3 point defects
b) Point and line defects
c) Mosaic (or domains) separated by defect boundaries
Fig of Klein and Hurlbut, Manual of Mineralogy, © John Wiley and Sons

5. Point defects Higher density of defects at higher T
Defects represent disorder - disorder favored at higher T
Point defects
Vacant sites
Atoms out of correct position
Extraneous atoms
Substituted atoms

6. Crystal Defects 1. Point Defects
a. Schottky defect
1. Point Defects
Schottky (vacancy) - seen with steel balls in last frame
Need to maintain charge balance!
b) Impurity
Foreign ion replaces normal one (solid solution)
Not considered a defect
Foreign ion is added (interstitial)
Both combined
b. Interstitial (impurity) defect

7. Crystal Defects 1. Point Defects
c) Frenkel (cation hops from lattice site to interstitial)
= a + b combination
b. Frenkel defect

8. Line defects Crystal deformation controlled by crystal structure
Planes/locations are favored for deformation based on bond strength…
Bond breakage doesn’t happen throughout entire structure simultaneously
Lump in carpet

9. Crystal Defects 2. Line Defects d) Edge dislocation
Migration aids ductile deformation
Fig of Bloss, Crystallography and Crystal Chemistry.© MSA
Dislocation direction is perpendicular to the dislocation line

10. Crystal Defects 2. Line Defects
e) Screw dislocation (aids mineral growth)
Fig of Bloss, Crystallography and Crystal Chemistry. © MSA
Dislocation direction is parallel to the dislocation line, vertical dislocation line

11. Planar defects Mismatch of the crystal structure across a surface
Officially grain boundaries count as planar defect

12. Crystal Defects 3. Plane Defects
f) Domain structure (antiphase domains)
Has short-range but not long-range order
Fig of Bloss, Crystallography and Crystal Chemistry. © MSA

13. Crystal Defects 3. Plane Defects g) Stacking faults
Common in clays and low-T disequilibrium
A - B - C layers may be various clay types (illite, smectite, etc.)
ABCABCABCABABCABC
AAAAAABAAAAAAA
ABABABABABCABABAB

14. Twinning Rational symmetrically-related intergrowth
Lattices of each orientation have definite crystallographic relation to each other
A variety of planar structural defect

15. Twinning Aragonite twin
Note zone at twin plane which is common to each part
Although aragonite is orthorhombic, the twin looks hexagonal due to the 120o O-C-O angle in the CO3 group
Redrawn from Fig of Berry, Mason and Dietrich, Mineralogy, Freeman & Co.

16. Twinning 1) Reflection (twin plane) Examples: gypsum “fish-tail”
Twin Operation is the symmetry operation which relates the two (or more) parts (twin mirror, rot. axis)
1) Reflection (twin plane)
Examples: gypsum “fish-tail”
2) Rotation (usually 180o) about an axis common to both (twin axis): normal and parallel twins.
Examples: carlsbad twin
3) Inversion (twin center)

17. Contact & Penetration twins Both are simple twins only two parts
Pass around staurolite, muscovite, selenite, plag
Can’t ID them 2006 since we haven’t seen them all yet

18. Polysynthetic twins Albite Law in plagioclase
Multiple twins (> 2 segments repeated by same law)
Cyclic twins - successive planes not parallel
Polysynthetic twins
Albite Law
in plagioclase

19. Twinning Mechanisms: 1) Growth
Growth increment cluster adds w/ twin orientation
Epitaxial more stable than random
Not all epitaxis  twins
Usually simple & penetration
synneusis a special case

20. Twinning Mechanisms: 1) Growth Feldspars:
Plagioclase: Triclinic Albite-law-striations
a-c
a-c b b

21. Twinning Mechanisms: 1) Growth Feldspars:
Plagioclase: Triclinic Albite-law-striations

22. cyclic twinning in inverted low quartz
Mechanisms:
2) Transformation (secondary)
SiO2: High T is higher symmetry
High Quartz P6222
Low Quartz P3221

23. Twinning Mechanisms: 2) Transformation (secondary twins) Feldspars:
Orthoclase (monoclinic)  microcline (triclinic)
a-c
a-c
Monoclinic
(high-T)
Triclinic
(low-T) b b

24. Twinning Mechanisms: 2) Transformation (secondary) Feldspars:
K-feldspar: large K  lower T of transformation
“tartan twins”
Interpretation wrt petrology!

25. Twinning Mechanisms: 3) Deformation (secondary)
Results from shear stress
greater stress  gliding, and finally rupture Also in feldspars.
Looks like transformation, but the difference in interpretation is tremendous

26. Mechanisms: 3) Deformation (secondary)
Results from shear stress. Plagioclase

27. Mechanisms: 3) Deformation (secondary)
Results from shear stress. Calcite

28. Isostructural minerals
2 minerals with identical structure
NaCl, PbS
Different chemical and physical properties, identical symmetry, cleavage, habit

29. Isostructural group
Group of isostructural minerals realted by common anion or anionic group
Calcite group: calcite, magnesite, rhodochrosite, siderite

30. polymorphism
Ability of a chemical compound to crystallize with more than 1 structure
SiO2, Al2SiO5, KAlSi3O8

31. polymorphism
Ability of a chemical compound to crystallize with more than 1 structure
SiO2, Al2SiO5, KAlSi3O8

32. Polymorphism 1. Displacive polymorphism
   quartz at 573oC at atmospheric pressure
1000 2 4
High-Quartz
Low-Quartz
500
Temperature
Coesite
Pressure (GPa)

33. Polymorphism 1. Displacive polymorphism
High
1. Displacive polymorphism
Note: higher T  higher symmetry due to more thermal energy (may twin as lower T)
Transition involves small adjustments and no breaking of bonds
Easily reversed and non- quenchable (low E barrier)
P6222
Low
P3221

34. Polymorphism 2. Reconstructive polymorphs
More common: other quartz polymorphs, graphite- diamond, calcite-aragonite, sillimanite-kyanite- andalusite
Transition involves extensive adjustments, including breaking and reformation of bonds
High E barrier, so quenchable and not easily reversed (still find Precambrian tridymite)
Stable
Unstable
Metastable

35. Pseudorphism May be confused with polymorphs
A completely different thing
Complete replacement of one mineral by one or more other minerals such that the new minerals retain the external shape of the original one
Limonite after pyrite
Chlorite after garnet
Brucite after periclase
Forsterite after tremolite
Can use the shape to infer the original mineral
Very useful in petrogenetic interpretations