Download Metamorphic Rocks

Stage 3 Revision
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Rock Cycle
3 R’s
Types of Metamorphism
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Regional
Contact
Dynamic
Metasomatic
Metamorphic Grades
 Increasing with Pressure/Temperature
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The presence of certain minerals in a
Metamorphic rock tells you the temp and the
pressure it was formed at
Many of these minerals are only present in
Metamorphic rocks
Some of these minerals are Polymorphs
 A polymorph is a compound that has differing
atomic structures eg Diamond/Graphite
Mineral crystallisation
occurs under specific
conditions.
 Diamonds exist under
the crust where the
pressure and temps
are very great, and
must be brought up
fast (in a volcano) too
slow and they will turn
into Graphite
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The best index
mineral in
Metamorphic rocks.
Kyanite
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Sillimanite
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Andalusite
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These minerals are all made from the same
elements/compound Al2SiO5
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But they only form at certain temperatures
The presence of any tells you the
approximate temp and pressure the rock
formed at.
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Pressure causing alignment of minerals
 Sheet like texture called Foliation
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Increased grain size
 Due to incresed temperatures and pressure for a
longer period
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Here you have minerals growing in a preferred
orientation in the rock. Some layers are thin
others are thick.
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Foliation can give a rock a striped or streaky
appearance
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Foliated rocks are distinguished by their grain
size and nature of the foliation.
Minerals such as Muscovite (Mica) and Chlorite
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The finest grained foliated rock
Parent rock (Protolith) Shale,
Mudstone
 LOW grade metamorphism (what
does this mean?)
 The rock has a slaty CLEAVAGE in
one direction. That comes from the
pressure above and below the rock.
 Minerals:
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Chlorite
Clay or Mica (Muscovite)
Quartz*
Feldspar*
* In smaller amounts
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Fine grained
Parent rock: Slate
Names after the greek
for leaf because the
rock has a leafy lustre.
Minerals:
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Mica
Chlorite
Feldspar*
Quartz*
* in smaller amounts
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Medium to Coarse grained rock
Schistocity: Preferred orientation of
larger Mica crystals (than Phyllite)
caused by stress (from the pressure
that would have caused the
metamorphism)
Also the formation of wavy or
distorted surfaces instead of flat
planes
Forms at a higher temp than Phyllite
Minerals:
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Kyanite
Feldspar
Quartz
Amphibole
Garnet (Sometimes as a
Porphyroblast/clast)
Coarse grained
Alternating layers of dark
and light coloured minerals
 Gneissic texture, high grade
metamorphism
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Banding, Banding can occur for
different reasons
Temperature causing minerals to
grow in bands
 Sheering helping the minerals
grow in bands
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Minerals
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Quartz
Biotite
Garnet
Feldspar
Sillimanite
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Lineation
 Minerals grow as prismatic
crystals facing a similar
direction
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Banding
 As mentioned previously
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Augen
 Quartz and similar silica
minerals can be squashed
into an ellipsoid shape (eye)
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Granular
 This is important!
 All mineral grains in a metamorpic rock are
approximately the SAME SIZE. Because
they have been exposed to the same
pressure/temp for the same time. These
occur in rocks with a simple chemistry (eg
Sandstone SiO2 and Limestone CaCO3)
 These are generally NOT foliated but have
had high temps!
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Folding
 Directional pressure can make rocks behave
plastic
Now for these types (and the others) what
types of metamorphism could they be
related to?
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Here the crystals have grown without
anything restricting their growth (or making
them grow in a preferred orientation)
Often formed without any pressure-only
temp
The crystals are interlocking
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Often made of only one mineral
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Parent rock: Shale,
mudstone, limestone
 Fine grained
 Minerals: Dependent on
Parent rock (can differ)
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 Shale or clay hornfels are
made of Biotite but have
Andalusite and Sillimanite
in it.
 What does that say about
its formation location?
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Parent rock:
Sandstone
Original loose SiO2
grains that have
recrystallised making
larger and interlocking
formation.
Minerals:
 SiO2 (Quartz)
Parent rock: Limestione
Original loose CaCO3
grains that have
recrystallised making
larger and interlocking
formation.
 All fossils destroyed
 Mineral:
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 CaCO3 (Calcium
Carbonate or Calcite)
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As pressure and/or temperature
increases so does the amount of
change a metamorphic rock
makes. This is the Grade
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As the Metamorphic Grade increases the
minerals in the rock changes as well.
They change in a predictable pattern
depending on Pressure and Temperature.
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These are a set of metamorphic mineral
assemblages (or ingredients) that relate to a
certain range of pressure or temperature
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Industrial
 Talc
 Graphite
 Garnet
 Asbestos
 Kyanite, Andalusite
& Sillimanite
 Building
 Slate
 Marble
 Quartzite
 Gneiss