2 metamorphic textures

Metamorphic Textures


Recrystallization: Minimization of Interfacial
Free Energy by Minimizing Surface Area
Sandstone

Quartzite

Crack
Recrystallization

Cement

Crack

Crack


Recrystallization of a Bi-minerallic Rock
 a) interfacial free energy of AA =
AB; dihedral angle (θ) is thus 120o
 b) interfacial free energy of AA >
AB; dihedral angle decreases to
decrease surface area of AA

Plagioclase
Cpx


Recrystallization of Quartz-Mica Rock

 Interfacial free energy of Quartz-Mica >> Quartz-Quartz
 Interfacial angle increases from 120o to 180o


Decussate Texture

Aggregate of mica grains in which most interfaces are
parallel to {001}. This occurs because the surface energy of
the {001} face is substantially lower than that of other faces,
e.g. {110}. The texture may also be developed by other
minerals.


Granoblastic/Decussate Textures (Characteristic
of Contact or Thermal Metamorphism)


Porphyroblast development
 Nucleation

of new crystals requires
excess free energy
 If

the nucleation energy is high,
growth is favoured and
porphyroblasts form
 Porphyroblasts will be euhedral
if their faces have high energy

Crystalloblastic Series
1.
2.
3.
4.
5.
6.
7.

Most Euhedral
Titanite, rutile, pyrite, spinel
Garnet, sillimanite, staurolite, tourmaline
Epidote, magnetite, ilmenite
Andalusite, pyroxene, amphibole
Mica, chlorite, dolomite, kyanite
Calcite, vesuvianite, scapolite
Feldspar, quartz, cordierite
Least Euhedral


Textural Effects of Differential Stress
Stressed grains store strain energy released during metamorphism
by clearing defects and dislocations (polygonization)
Undulose extinction

Subgrain development


Defects Dislocations and Deformation

Defects
Dislocation


Deformation by Dislocation


Pressure Solution
Differential stress produces
serated grain boundaries


Foliation Development

σ1
S1

Schistocity S1 flattened around
garnet porphyroblasts

S2
S1

Crenulation cleavage S2
developed by folding S1


Pressure Shadows

Chlorite in shadow
around garnet

Quartz in shadow
around staurolite


Pressure Fringes

Staurolite

Pyrite
Muscovite

Sillimanite


Metamorphic Mineral Growth
Relative to Deformation


Pre-tectonic Textures

Undulose extinction
Pressure shadows

Pressure fringes

Micro-boudinage

Crenulation cleavage

Deformation twins


Syn-tectonic Texture


Snowball Garnet with Pressure Shadows


Post-tectonic Textures

Helicitic folds

Chiastolite

Randomly oriented crystals

Overgrowth on snowball

Polygonal arc

Pseudomorph


Polymetamorphism


Stages in Polymetamorphism (S0, S1)

Stages in Polymetamorphism (S2)


Stages in Polymetamorphism (S3)


Corona Textures