Defects and Twinning

7/30/2019 Defects and Twinning

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Structural defects and

twinning


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No crystal has 100% flawless structure


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Crystal defects

Defects can affect

Strength

Conductivity

Deformation style

Color


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Crystal DefectsSteel spheres:

a) Regular packed array

with 3 point defects

b) Point and line defects

c) Mosaic (ordomains)

separated by defect

boundaries

Fig 3.50 of Klein

and Hurlbut, Manual

of Mineralogy,

John Wiley and Sons


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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


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Crystal Defects

1. Point Defectsa) Schottky (vacancy) - seen with

steel balls in last frame

a) 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

a. Schottky defect

b. Interstitial (impurity) defect


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Crystal Defects

1. Point Defects

c) Frenkel (cation hops from lattice

site to interstitial)

= a + b combination

b. Frenkel defect


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Line defects

Crystal deformation controlled by crystalstructure

Planes/locations are favored fordeformation based on bond strength

Bond breakage doesnt happen

throughout entire structuresimultaneously

Lump in carpet


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Crystal Defects2. Line Defects

d) Edge dislocation

Migration aids ductile deformation

Fig 10-4 of

Bloss,

Crystallography

and CrystalChemistry.

MSA


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Crystal Defects

2. Line Defectse) Screw dislocation (aids mineral growth)

Fig 10-5 of

Bloss,

Crystallographyand Crystal

Chemistry.

MSA


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Planar defects

Mismatch of the crystal structure across

a surface

Officially grain boundaries count asplanar defect


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Crystal Defects3. Plane Defects

f) Domain structure (antiphase domains)Has short-range but not long-range order

Fig 10-2 of Bloss, Crystallography and Crystal Chemistry. MSA


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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


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Twinning

Rational symmetrically-related

intergrowth

Lattices of each orientation havedefinite crystallographic relation to each

other

A variety of planar structural defect


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Twinning

Aragonite twin

Note zone at twin

plane which iscommon to each

part

Redrawn from Fig 2-69 of Berry,

Mason and Dietrich, Mineralogy,

Freeman & Co.

Although aragonite is

orthorhombic, the twin lookshexagonal due to the 120o O-C-O

angle in the CO3 group


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Twinning 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 axiscommon to both (twin axis): normal andparallel twins.

Examples: carlsbad twin

3) Inversion (twin center)


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Contact & Penetration twins

Both are simple twins only two parts


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Multiple twins (> 2 segments repeated by same law)

Cyclic twins - successive planes not parallel

Polysynthetic twinsAlbite Law

in plagioclase


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Twinning

Mechanisms:1) Growth

Growth increment cluster adds w/ twin

orientationEpitaxial more stable than random

Not all epitaxis twins

Usually simple & penetrationsynneusis a special case


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Twinning

Mechanisms:

1) Growth

Feldspars:

Plagioclase: Triclinic Albite-law-striations

b

a-c

b

a-c


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Twinning

Mechanisms:

1) Growth

Feldspars:

Plagioclase: Triclinic Albite-law-striations


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Twinning

Mechanisms:

2) Transformation (secondary)

SiO2: High T is higher symmetry

High Quartz P6222 Low Quartz P3221

cyclic twinning in

inverted low quartz


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Twinning

Mechanisms:

2) Transformation (secondary twins)

Feldspars:

Orthoclase (monoclinic) microcline(triclinic)

Monoclinic

(high-T)

b

a-c Triclinic

(low-T)

b

a-c


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Twinning

Mechanisms:2) Transformation (secondary)

Feldspars:

K-feldspar: large K lower T oftransformation

tartan twins


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Twinning

Mechanisms:3) Deformation (secondary)

Results from shear stress

greater stress gliding, and finally ruptureAlso in feldspars.

Looks like transformation, but the difference

in interpretation is tremendous


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Mechanisms:

3) Deformation (secondary)

Results from shear stress. Plagioclase

Mechanisms:


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Mechanisms:

3) Deformation (secondary)

Results from shear stress. Calcite


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Isostructural minerals

2 minerals with identical structure

NaCl, PbS

Different chemical and physicalproperties, identical symmetry,

cleavage, habit


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Isostructural group

Group of isostructural minerals realted

by common anion or anionic group

Calcite group: calcite, magnesite,rhodochrosite, siderite


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polymorphism

Ability of a chemical compound to

crystallize with more than 1 structure

SiO2, Al2SiO5, KAlSi3O8


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polymorphism

Ability of a chemical

compound to

crystallize with morethan 1 structure

SiO2, Al2SiO5,

KAlSi3O8

Stishovite

Coesite

- quartz

- quartz

Liquid

Tridymite

Cristobalite

600 1000 1400 1800 2200 2600

2

4

6

8

10

Pressure(GPa)

Temperature oC


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Polymorphism

1. Displacivepolymorphism

quartz at 573oC

at atmospheric

pressure

2

4

High-QuartzLow-Quartz

500

Temperature

0

Coesite

Pressure(G

Pa)


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Polymorphism

1. Displacivepolymorphism

Note: higher T higher

symmetry due to more

thermal energy (may twinas lower T)

Transition involves small

adjustments and no

breaking of bonds

Easily reversed and non-

quenchable (low E

barrier)

High

Low

P6222

P3221


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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


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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 mineralsretain 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

Defects and Twinning

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