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]
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!
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.
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.
This is a good checklist to follow: