6. Textures 1

Question 1

The driving force for metamorphism is reduction in Gibbs free energy. How does this occur?

Answer 1

Neomineralisation: drive towards chemical equilibrium → stable paragenesis

Recrystallisation: drive towards textural equilibrium by:

• Increase in grain size (reduce grain boundary surface area)
• eliminate stored elastic energy produced during deformation

Question 2

Minerals in contact metamorphism are equidimensional or in random orientation, and have equilibrium grain shapes. Why?

Answer 2

Low P → low deviatoric stress → equidimensional/random orientation

Heat from igneous intrusion → recrystallisation → reduction in surface energy through a reduction in grain boundary surface area → equilibrium grain shapes

Question 3

Define isotropic

Answer 3

A mineral in which the surface energy of the grain boundaries is equal in all directions is texturally isotropic e.g Garnet

Question 4

Define anisotropic

Answer 4

A mineral in which the surface energy of the grain boundaries depends in the orientation of the boundary to the crystal lattice is texturally anisotropic e.g. mica

Question 5

A monomineralic rock is made of isotropic/minor anisotropic minerals.

(e.g. quartz, calcite, feldspar, garnet)

Describe it’s possible texture

Answer 5

polygonal grain shapes, 120° triple junctions, granoblastic polygonal texture

Question 6

A monomineralic rock is made of anisotropic minerals.

(e.g. amphibole, pyroxene, mica)

Name it’s possible texture

Answer 6

Decussate texture

Question 7

Name two textural features of a polymineralic rocks

Answer 7

Porphyroblasts and Poikiloblasts

Question 8

Describe a Granofels

Answer 8

massive, non-schistose

granoblastic, decussate or radiating textures

‘static crystallisation’ - no deviatoric stress

Question 9

Describe a Hornfels

Answer 9

a fine-grained granofels with granoblastic texture

Question 10

Describe deformation mechanisms at low T

Answer 10

brittle deformation (cataclasis) → Fault breccia (cataclasite)

Cataclasis may be modified by pressure solution or diffusive mass transfer, causing redistribution of material

Question 11

Describe deformation mechanisms at High T

Answer 11

intracrystalline plasticity (crystal plastic deformation)

• high strain → dislocations and distorted crystal lattice → undulose extinction
• Dislocations migrate → recovery produces two strain-free subgrains → smaller grainsize

Recrystallisation

• new grains no sub grains so no reduction in grainsize
• combined deformation and recrystallisation = dynamic recrystallisation
• Post-deformational recrystallisation = annealing

Question 12

What are the three mechanisms of foliation formation?

Answer 12

Oriented growth of new minerals

crystal plastic deformation and recrystallisation

Diffusive mass transfer (pressure solution)

Question 13

Crenulation vs Crenulation cleavage

Answer 13

crenulation is the small scale folding of a pre-existing cleavage

crenulation cleavage is when diffusive mass transfer(pressure solution) and sometimes neomineralisation occur along the second cleavage resulting in a spaces cleavage = spaced schistosity

Question 14

Describe foliation development by crenulation

Answer 14

Question 15

Define Mineral Lineation

Answer 15

a lineation defined by a preferred orientation of prismatic or acicular crystals

Question 16

Describe cataclasite

Answer 16

Low T fault texture, occurs in shallow fault zones

broken, bent or crushed crystals with little recrystallisation

finer matrix

no preferred orientation of grains

undulose extinction of quartz grains

Question 17

Define a Fault breccia

Answer 17

unaltered cataclastic rock, or coarser grained examples

Question 18

Define Fault gouge

Answer 18

alteration of fault breccia to clay minerals, resulting from interaction with water percolating along the fault plane

Question 19

Describe Mylonite

Answer 19

High T → crystal plastic deformation and dynamic recrystallisation

a fine-grained recrystallised fault rock, with a very intense tectonic fabric (both planar and linear) and, usually, numerous porphyroclasts

• quartz is often deformed into elongate bands

Question 20

Define Pseudotachylite

Answer 20

formed under extreme amounts of high strain deformation → melting as a result of frictional heating

forms thin, commonly interconnected, seams of glassy igneous-like rock