Metamorphic Rocks - Suitable to print back to back

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  • Grain Size: Small, normally under 2mm
  • Colour: Very pure Limestones metamorphose to a pure white marble, though this is rare. There can be many variations in colour can be caused by muds/silts/sands/iron oxides or possibly chert
  • Protolith: Limestones/Carbonates
  • Geological Description: Recrystallised carbonate rocks, normally displaying a granoblastic/ equigranular texture, normally all traces of fossils or sedimentary structures have been destroyed, but can remain as  ghost fossil which is normally a grey "shadow".
  • Texture: Granoblastic
  • Type of Metamorphism: Regional and Contact
  • Minerals: Calcite
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  • Grain Size: Normally under 2mm, depending on grain size of protolith
  • Colour: Can be white to grey, sometimes varying shades of pink depending on the amount of iron in the protolith. 
  • Protolith: Sandstone - quartzite, orthoquartzite, quartz arenite. 
  • Geological Description: A metamorphosed sandstone, normally displaying an equigranular/Granoblastic texture. 
  • Texture: Granoblastic
  • Type of Metamorphism: Contact or regional metamorphism
  • Minerals: Quartz.
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  • Grain Size: Normally fine grained.
  • Colour: Normally Dark brown to black, can show some banding as a relic of bedding.
  • Protolith: Sandstones, shale, slate and some limestones.
  • Geological Description: A high grade metamorphic rock, normally very hard and splintery, with the protolith's fissility removed by baking. The dominant mineral is normally a mica, especially a biotite in Black Hornfels. It is formed by the partial melting and recrystallising of a range of rocks within a metamorphic aureole. 
  • Texture: Fine Grained, though the coarser the texture the more thorough the melting and slower the recrystallisation
  • Type of Metamorphism: Contact Metamorphism. 
  • Minerals: Quartz, Feldspar, biotite, muscovite, pyroxenes, garnet, and calcite are common ingredients of hornfels.
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  • Grain Size: Very fine grained
  • Colour: Normally dark grey, but displays a very varied range of colours, even within one outcrop. Can be purple, Green or Cyan. 
  • Protolith: Shale or Mudstone
  • Geological Description: A low grade metamorphic rock, with fine grains, commonly displaying a foliated texture
  • Texture: Foliated, demonstrates slatey cleavage
  • Type of Metamorphism: Low Grade Regional Metamorphism
  • Minerals: Quartz, Muscovite, Secondary minerals are Biotite, Haematite, Chlorite and Pyrite.
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  • Grain Size: Fine Grained - larger than slate.
  • Colour: Silvery, sometimes tints of green. 
  • Protolith: Shale/Mudstone (Pelite)
  • Geological Description: A low grade metamorphic rock showing a foliated texture that is normally wavy. This is normally formed by the continued metamorphism of slate. 
  • Texture: Phyllitic, Foliated
  • Type of Metamorphism: Low Grade Regional Metamorphism
  • Minerals: Quartz, Mica and Chlorite
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  • Grain Size: Fine to Medium Grained - mineral crystals normally visible with the naked eye.
  • Colour: Often metallic, silvery from the inclusion of Mica, and spotted with darker minerals such as garnet. Colour can be very varied, and some minerals can grow quite large (several centimetres across).
  • Protolith: Slate or Phyllite (pelites)
  • Geological Description: A medium grade regional metamorphic rock largelly composed of lammellar minerals giving it a flaky texture
  • Texture: Foliated (Sometimes called schistose)
  • Type of Metamorphism: Medium Grade regional
  • Minerals: Commonly Muscovite, quartz, feldspar and Garnet, but can contain a wide variety of minerals such as chlorite (makes it green), tourmaline, glaucophane etc. 
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  • Grain Size: Large - often several mm to cm
  • Colour: Very varied across different bands, but commonly include transparent greys, black, reds, pinks and sometimes white.
  • Protolith: Pelites but gneiss is the highest grade in the slate-phyllite-schist-gneis sequence, beyond the temperature and pressure zones of gneiss rocks start to melt and become migmatites with a different mineralogical composition. 
  • Geological Description: A high grade metamorphic rock displaying an extreme foliated texture where recrystallisation has occurred and similar minerals have grouped together to form bands.
  • Texture: coarse, foliated, banded.
  • Type of Metamorphism: High grade regional metamorphism
  • Minerals: Platy minerals are normally destroyed by this heat and pressure. 
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Spotted Rock

  • Grain Size: Fine
  • Colour: Dark grey, can be very varied and minerals can be a range of colours.
  • Protolith: Pelites (Mudrocks)
  • Geological Description: A slate that has developed "spots" that are minerals that have grown into poikiloblasts. 
  • Texture: Poikiloblastic
  • Type of Metamorphism: Low Grade Contact
  • Minerals: Clay minerals, pyrite, chiastolite, chlorite, graphite, muscovite.
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Describing Metamorphic Rocks

There are some key rules to identifying metamorphic rocks, you need to be familiar with the minerals listed on the next card, and the metamorphic terms for texture.

Foliated - minerals aligned perpendicular to pressure

Non-Foliated - minerals do not show alignment but have recrystallised in random orientations.

Granoblastic - equigranular

Poikiloblastic - One mineral grows much larger, similar to porphyritic in igneous

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The Syllabus

*Identification in hand specimen of the following metamorphic rocks from their composition, texture and other diagnostic features: marble, metaquartzite, spotted rock, hornfels, slate, schist, gneiss, contact aureoles, metamorphic foliations.

Metamorphism involves mineralogical and/or textural change of pre-existing rocks in response to changes in temperature and/or pressure. Contact (thermal) and regional metamorphism produce distinctive mineralogical and textural changes: non-foliated in contact metamorphism: foliation (slaty cleavage, schistosity and gneissose banding) in regional metamorphism.

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Metamorphic Grade


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Aureole Diagram


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Contact Metamorphism Classification

Contact Metamorphism
(based on mineral content)
Parent RockMetamorphic rockDominant MineralsCharacteristics Limestone Marble Calcite Interlocking grains. Fizzes in weak acid Quartz Sandstone Quartzite Quartz Sugary texture Shale Hornfels (Spotted Rock) Micas Dark colour

Caused by heating from an external source. Contact metamorphism occurs next to an igneous body. The degree of metamorphism decreases away from the body. This occurs at fairly shallow depths (<10km), as temperature not pressure is the dominating factor.

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Regional Metamorphism Classification

Regional Metamorphism
(name based on degree of metamorphism)
TextureRock NameCharacteristics Slatey Slate Splits easily into sheets Between slate and schistose Phylite Silky lustre, splits into wavy sheets Schistose Schist Pearly looking. Silky to touch Gneissic Gneiss Wavy, white and dark layers

Regional metamorphism is caused by high pressure and temperatures usually during mountain building (oregenesis). The extremes of regionally metamorphic rocks are a high pressure, low temperature rock (called a blueschist) and a high pressure and very high temperature rock (called a granulite). If the rock is heated to the point of melting, but doesn't actually melt, it is called a migmatite.

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Metamorphic Grade and Aureole

One of the key things to work into any description of metamorphic rocks, is to indicate if you know how high the grade of metamorphic rocks. 

Having a high grade, denotes that the rock has undergone metamorphism at either/or high temperature and/or pressure. For instance, in the pelite (mudrock) sequence of metamorphism, slate is the lowest grade, and as metamorphism continues, the grade increases (both t and p) and the rocks progress through a well defined sequence of slate-phyllite-schist-gneiss. 


An Aureole is an eye shaped region around an intrusion, and is the region of rock that has been affected by the heat of the intrusion. 

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