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 >>> 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”).
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.
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.
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).
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.
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
Polyhedral Porphyroblasts
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.
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.
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.
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.
• Symplectites:
pseudomorphous replacement Intergrowths
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.
(A)
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.
• 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.
INTERGROWTHS
cont.
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.
Preservation of Pre-metamorphic Structures
• Favoured by minimal
deformation.
• Unreactive rock (e.g.
quartz sandstone that
can show crossbedding at granulite
facies.
• Several stages of
growth of
porphyroblasts that
can preserve structure
as inclusions.