Solid-State%20Microstructures

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Solid-State Microstructures

• Metamorphic rocks form the minerals that have the
  stable lowest energy paragenesis under the
  conditions of formation generally also have
  microstructures that minimise the excess energy
  associated with the grain boundaries.
• The energy differences involved in the reactions
  are gtgtgt than those that generate the types of
  grain boundaries.
• Grain boundary energy reduction is accomplished
  by either reducing the total area of grain
  boundaries and/or forming grain boundaries that
  have minimal excess energy (most atoms are bonded
  correctly).

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Reduction in grain-boundary energy

• The excess energy because of imperfect bonding of
  atoms in the grain-boundaries is reduced by
• Reducing the total area of grain-boundaries by
• Forming polygonal grains that have low surface
  area (the solid space filling 3D equivalent of
  spheres) and
• By increasing the grain size that also reduces
  the total area of grain-boundaries (1000 mm cubes
  have a surface of 60 cm2, one cm cube has the
  same volume and only 6 cm2 surface area).
• Or by forming crystal faces that have most of the
  bonds in one crystal satisfied as is the case for
  mica (001) faces.

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Minerals can be roughly subdivided into those
that have an isotropic structure and those that
are strongly structurally anisotropic

• Quartz, feldspar and calcite are isotropic
• Micas, chlorite, sillimanite are very
  anisotropic.
• Most single mineral metamorphic rocks are
  polygonal.
• With two or more it depends on the individual
  minerals.

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Isotropic minerals form polygonal or foam-like
aggregates

• Small grains tend to have fewer face, larger ones
  more.
• Small grains have more curved faces but all faces
  are curved and not related to crystallography
  (e.g. not cleavage parallel.
• Small grains are removed by the enlargement of
  large grains (process can be seen in foams). This
  is easier if the rock in monomineralic (e.g.
  marble).

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Micas are very anisotropic have very few bonds
that cross the (001) plane

• Aligned mica that has (001) faces that quartz
  just moulds onto. The mica (001) is so stable the
  quartz-quartz boundaries meet it at right angles.
• Quartz and feldspar forms a polygonal array
  except where biotite (001) faces control the
  shapes.

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Polygonal vs Polyhedral

• Apart from micas and sillimanite, most single
  mineral aggregates are polygonal.
• Quartz and feldspar are similar enough to form a
  polygonal aggregate.
• Olivine and pyroxene also form polygonal
  aggregates.
• Silicates enclosed in calcite are polyhedral.
• Even some fluid inclusions can be polyhedral
  (negative crystals) e.g. in fluorite

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Polyhedral Porphyroblasts
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INCLUSIONS

• Inclusions have grain boundaries and the same
  rules apply.
• Isotropic minerals generally form sub-spherical
  grains.
• The micas inclusions form the sheets (001) but
  have hemispherical ends.
• Hornblende forms some faces in quartz.

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Solid state growth twins

• Pre-impingement twins grow behind the advancing
  interface, post impingement twins develop at
  triple junctions. Sector twins form as a result
  of polymorphic transformation (e.g. cordierite).
• Plagioclase has sparse solid state growth twins
  (can have many deformation twins).
• The amphibole cummingtonite has abundant growth
  multiple twins.

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Radiating Aggregates

• Large grain boundary area with energy reduced by
  formation of crystal faces but still higher than
  equant aggregates.
• Form as a result of very low nucleation and
  diffusion rates (like spherulites).
• Formed by anisotropic minerals like chlorite and
  sillimanite.
• Imply absence of deformation during growth
  especially if three dimensional.

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Zoning in Metamorphic minerals

• Shown by zones of inclusions and/or chemical
  zoning.
• Mineral maps using EMP reveal zoning is common
  but it is rare to have oscillatory zoning.
• Almandine Garnet commonly has Mn-rich cores
  recording the garnet that forms in meta-mudstones
  at the lowest T surrounded by higher temperature
  more almandine pyrope-rich rims.

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Intergrowths (A)

• Symplectites pseudomorphous replacement and may
  form coronas. Generally post-deformation and
  indicate simultaneous growth of all minerals. If
  some of the original minerals remains as a core
  they indicate the reaction.
• Upper photo is symplectic intergrowth of biotite,
  quartz and andalusite that replaces cordierite.

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Degree of discordance for two grains of the same
mineral

• Grains that are close to having the same
  orientation have low energy (most atoms get to
  bond correctly).
• This explains how the radiating fibres in
  spherulites justify their existence.
• High angles have high energy unless you fluke a
  twin orientation. 

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INTERGROWTHS cont.

• Myrmekite an intergrowth of plagioclase and
  quartz that commonly develops on existing
  plagioclase and replaces adjacent K-feldspar. The
  plagioclase is all in optical continuity as is
  the quartz that forms worm-like inclusions.
• Occurs in granitic rocks especially if slightly
  deformed. Deformation allows in the H2O needed to
  bring the components Ca Na in and K out.

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Incomplete Metamorphic Reactions

• Common only in low temp. prograde and retrograde
  metamorphic rocks. At higher temp. reactions go
  to completion. Commonly reflect the slow entry of
  H2O into the rock. Photo of igneous pyroxene
  phenocryst partly replaced by hornblende. If not
  deformed igneous microstructures can be preserved.

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Preservation of Pre-metamorphic Structures

• Favoured by minimal deformation.
• Unreactive rock (e.g. quartz sandstone that can
  show cross-bedding at granulite facies.
• Several stages of growth of porphyroblasts that
  can preserve structure as inclusions.

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