DUCTILE
STRUCTURES
Ductile Behavior-
Permanent deformation
No
Mesoscopic (brittle) grain breakage.
Achieved through:
Grain boundary sliding
Kinking
Dissolution
Crystal
plasticity.
Dependent
upon:
Temperature
Stress
Strain
rate
Grain size
Rock composition
Fluid pore pressure
Ductile
mechanisms:
1.Cataclastic
Flow- mesoscopic
ductile behavior facilitated by microscopic brittle fracturing and frictional
sliding. Occurs at low lithostatic pressures (shallow crust).
Low confining pressure
High strain rates
High stress
2. Diffusional Mass Transfer-
flow
of material through the crystals
Temperature
dependent process
Thermal energy necessary to break bonds by vibration of crystal lattice structure.
Pressolution-
(pressure solution)-
Involves a fluid phase and may result in mineral differentiation
(quartz vs. mica rich zones; calcite vs. clays).
May involve substantial volume loss.
Examples: stylolites in limestones
Cleavage,
pressure shadows in slates
Grain overgrowths in sandstones
Solid State Diffusion
(Grain-Boundary or Volume Diffusion)-
Solid
diffusion process involving migration of vacancies to sites of greatest stress
and atoms to sites of minimal stress
3. Crystal Defects-
Errors in crystal lattice structure
Occurs at elevated temperatures.
a.
Point Defects-
i. Vacancies-
empty sites in crystal lattice
ii. Impurity
atoms-
substitutions
replace original atom
iii. Interstitial-
atom
occupies non-lattice site
b. Line Defects (Dislocations)-
linear array of lattice imperfections
i. Edge Dislocation-
extra 1/2 plane of atoms in lattice
ii. Screw Dislocation-
cork-screw distortion of lattice
4.Crystal Plasticity-
Strain
achieved by bending of the lattice Gliding on weak planes
i. Mechanical Twinning-
common in calcite, feldspars
ii. Kinking-
common
in micas and clays
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