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How to Write a Rock Description for Igneous, Sedimentary and Metamorphic Rocks (Geology)

Many of us, through our time at sixth form or university, have to go through the tedious process of writing a rock description. It is, let's be honest, not the most exciting thing to do yet not something you can often escape. Might as well get it over and done with!

I have split igneous, sedimentary and metamorphic rock descriptions into separate posts to make it simpler to read. I will add links to the others as I write them:

Igneous: [this post]
Sedimentary:
Metamorphic:

At the bottom there is some more information on types of properties like crystal habit, hardness and lustre.
If requested enough, I will add each mineral's properties to the blog post too!


IGNEOUS

The general process for writing rock descriptions is to start off with a general description (general colour, grain size, texture..) , followed by identification of the minerals within the rock (the mineral assemblage), the name of the rock (deduced from the assemblage) and then finally your best guess on how and where it was formed.

(Source - http://movingtoward.com/images/igneous_canterbury.ac.nz_sm.jpg)

All this is based on igneous rocks where the mineral grains are visible. If none are (it is way too fine grained) then it is a volcanic glass, formed in a very violent eruption resulting in fast cooling magma that didn't have time to crystallise. If it is a compact glass it is an obsidian and if it is a 'frothy' glass then it is a pumice. Obsidian is much denser than pumice.

There are two different types of rock description, which either may be done together or separately. The first is a hand specimen rock description which is where you only have a rock, a hand lens and your eyes with which to try to deduce the type of rock. The second is a thin section rock description where you have a slice of the rock which is on a glass slide making it possible to view the rock under a microscope.

Hand specimen descriptions focus on physical properties of the rocks whereas the thin section descriptions looks at optical properties instead.

Then, once you have completed the description (either or both of the types of description) you use the information you have to make a decision on what type of rock it is and how it was formed.


Hand Specimen

The first thing to do is to simply look at the rock and describe what you see. In specimen, you are looking at physical properties of the rocks.

(Source: http://minerva.union.edu/hollochk/c_physicalgeology/images/Granite.jpeg)

This is a good checklist to follow:
  • General colour
  • Grain size
  • Variation in grain size - is it porphyritic?
  • Mention any groundmass
  • Identify minerals - one at a time, starting with the darkest minerals first
    • Colour
    • Cleavage planes
    • Lustre
    • Hardness (according to Mohs' scale)
    • Crystal habit
    • Name of the mineral
  • Relative abundance of minerals (mention if one is particularly common etc)
[More information on types of each property are given at the bottom]

For example:
This is a large grained (1-21mm), light coloured rock with the largest phenocrysts being of a colourless mineral. It is a porphyritic rock. 
The black mineral is smallest in grain size (1-5mm) and has a vitreous lustre. It seems to have one perfect cleavage plane and when scratched it comes off in flakes. It is therefore biotite mica. 
One of the colourless minerals is of medium grain size (3-12mm), it is vitreous and has obvious cleavage in one plane but I think I can see a second plane of cleavage too. It has a massive crystal habit (no distinct shape). This is plagioclase feldspar. There is quite a large amount of this mineral in the sample. 
The phenocrysts of the specimen are another colourless mineral of 6-21mm in diameter. It seems to have a massive crystal habit as it shows no clear formations or crystal shape and does not show any cleavage planes. This is clearly quartz. 
When looking back at the black minerals I spotted that a minority seem to have two cleavage planes, when looking through the hand lens I noticed that they were not at 90 degrees to one another suggesting amphibole. The mineral was vitreous in lustre and was flaky when scratched. It being amphibole would fit with the other minerals present as they all appear in a felsic rock. 
This rock is felsic and is coarse grained with phenocrysts, meaning it is a quartz-mica porphyritic granite. It was intrusively cooled within a pluton, sill or dyke where the slow cooling allowed the large mineral grains to form.


Thin Section

Thin sections require more skill to describe as there are more optical properties and not all properties are always visible. If a hand specimen rock description has already been done then the thin section description is to back up previous decisions with more evidence. Firstly, hold the thin section up to the light and look at it to find a good area to concentrate on under the microscope. This area should be representative of the rock as a whole.

(Source - http://geology.gsapubs.org/content/33/10/761/F2.large.jpg)


Often a thin section description will be done in the format of a diagram where you draw the thin section as seen under the microscope. The left half is drawn as seen in PPL (plane polarised light - without the analyser) and the right half is drawn in XPL (cross-polarised - with the analyser). This is annotated with all the different optical properties, once for each mineral present on each side of the diagram. Make sure to add a scale so the size of grains can be deduced.

When writing a description or annotating a diagram, make sure you include the following:
  • Cleavage planes
  • PPL (plane-polarised) colour
  • Relief
  • Interference colours (or isotropic?)
  • Twinning
  • Pleochroism
  • Extinction angle
  • Mineral shape
  • Mention any inclusions present
  • Mention any opaque minerals (permanently black minerals)
  • Mineral name

For example:

A reasonable attempt at a thin section drawing and annotations.


Type of Rock

Now it is time to make a decision on what the rock type is and how it was formed.

The mineral assemblage will give whether it is a felsic, intermediate, mafic or (in rare cases) ultramafic rock:

(Source - http://www.laniainakai.com/Images/gg103/ign_rock_chart.png)
Then, depending on the grain size, you choose what type of rock it is within the compositional group.

Felsic (also known as silicic or acidic):
  • Coarse grained = Granite
  • Medium grained = Microgranite
  • Fine grained = Rhyolite

Intermediate:
  • Coarse grained = Diorite
  • Medium grained = Microdiorite
  • Fine grained = Andesite

Mafic (also known as basic):
  • Coarse grained = Gabbro
  • Medium grained = Dolerite
  • Fine grained = Basalt

Ultramafic:
  • Coarse grained = Peridotite
  • Fine grained = Komatitie (rare)


Then you add to this name with descriptive words relevant to the rock in question. This is usually 'porphyritic' if the rock has large phenocrysts and then (occasionally) the dominant mineral type (or two). Some minerals have a descriptive term to use instead - the main one is for feldspar, 'felsparic'. It is really just a way to describe the rock more accurately, in a way that flows off the tongue. If it is easier to say 'amphibole-rich diorite' instead of 'amphibole diorite' then write that!

For example:
  • Quartz-mica porphyritic granite
  • Felsparic gabbro
  • Porphyritic diorite
  • Amphibole-rich andesite


*Extra Information to add on*

If all the mineral grains are fragments then it is likely the rock was formed in a pyroclastic eruption, this then gives another name to any of the above:

Fragments less than 2mm = Tuff
Fragments greater than 2mm = Volcanic brecca

These link onto the end of the rock name after it has been adjusted to be an adjective:
  • Granite --> Granitic
  • Rhyolite --> Rhyolitic
  • Diorite --> Dioritic
  • Andesite --> Andesitic
  • Gabbro --> Gabbroic
  • Basalt --> Basaltic

For example:
  • Felsparic gabbroic tuff
  • Quartz-mica granitic (volcanic) brecca


*Other*

Descriptions of cleavage planes:
  • Basal = one cleavage plane
  • Prismatic = two cleavage planes
  • Rhombohedral = 3 cleavage planes
  • Cubic = 3 cleavage planes (all at 90 degrees to eachother)

Most common lustres:
  • Metallic
  • Waxy
  • Vitreous
  • Sub-vitreous
  • Resinous
  • Silky
  • Pearly
  • Earthy

Most common of crystal habits:
  • Acicular = needle like
  • Banded = narrow bands of different colours
  • Bladed = flattened needle like shape
  • Drusy = thin 'sheen' over a mineral surface
  • Equant = short and stubby
  • Foliated = easily separated into plates
  • Massive = no distinct shape
  • Tabular = flat and rectangular shaped
  • Fibrous = fine thread-like strands

Mohs' Scale of Hardness:
  • 1 = Talc
  • 2 = Gypsum
  • 2.5 = Fingernail
  • 3 = Calcite
  • 3.5 = Copper coin
  • 4 = Fluorite
  • 5 = Apatite
  • 5.5 = Penknife blade
  • 6 = Orthoclase
  • 6.5 = streak plate
  • 7 = Quartz
  • 8 =  Topaz
  • 9 = Corundum / ruby / sapphire
  • 10 = Diamond



The GeoMessenger

As always, please do not copy my work without referencing back to me! 
Comment if you found it useful and any additions that would be useful!

12 comments:

  1. Thankyou!!! I'm a first year Geo student and this is a big help.

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    Replies
    1. Excellent exposition on the way to look at rock as a hand specimen and in thin section.
      It appears you have profound knowledge. In the Dimensional Stone Industry mineral identification and nomenclature will greatly help Dimensional Stone Industry as well as Earth Sciences Community.
      It would be of great contribution if a compendium of hand specimens of various dimensional stones particularly Granite along with a comment on the elements and mineral assemblage that gives the background color such as Brownish, Bluish ,Blackish , Greenish etc which variation adds or diminishes the value for Granite.
      It would be an eternal contribution if a compendium of common defects , color variations , close set joints from surface to deeper layers as gathered from wire saw cut faces and how better qualities whether can be expected or to close the quarry with minimum expenses and use the knowledge as a loss preventing tool with minimum invasion and not seriously polluting the Environment about which every person on the Globe expressing concern and Mining and Quarrying is looked down upon as dangerous culprits because of less scientific inputs are getting in to Mining Industry and the practice is hit and run with the advice of quarry masters who are not Geologists..Excellent exposition on the way to look at rock as a hand specimen and in thin section.
      It appears you have profound knowledge. In the Dimensional Stone Industry mineral identification and nomenclature will greatly help Dimensional Stone Industry as well as Earth Sciences Community.
      It would be of great contribution if a compendium of hand specimens of various dimensional stones particularly Granite along with a comment on the elements and mineral assemblage that gives the background color such as Brownish, Bluish ,Blackish , Greenish etc which variation adds or diminishes the value for Granite.
      It would be an eternal contribution if a compendium of common defects , color variations , close set joints from surface to deeper layers as gathered from wire saw cut faces and how better qualities whether can be expected or to close the quarry with minimum expenses and use the knowledge as a loss preventing tool with minimum invasion and not seriously polluting the Environment about which every person on the Globe expressing concern and Mining and Quarrying is looked down upon as dangerous culprits because of less scientific inputs are getting in to Mining Industry and the practice is hit and run with the advice of quarry masters who are not Geologists.
      Regards with reverence for such contribution without expecting any reward or remuneration as young Geologists would get motivated iimmensly..
      N.Subrahmanyam

      Delete