A walkthrough identifying an igneous rock.

It is not enough to be able to identify and name a rock by site. (Some people actually manage to memorise, by sight, every sample in the rock collection.) You still need to be able to name the essential minerals and estimate their relative amounts to pass the test.

This skill requirement carries on into second year as well. So being able to identify Quartz, Plagioclase, Biotite etc in a hand sample under a lens is essential to doing well in Geology.

Just looking at the rocks without a lens you can make some important observations.

Looking at the left most rock we immediately notice the very large pink Phenocrysts. If you look carefully at the pink mineral you might even see two distinct planes within the crystal. This is an example of twinning (Carlsbad Twinning). If you remember there are two types of Feldspar in the lab samples, Orthoclase and Plagioclase. Orthoclase has this twinning and can be pink in colour. Plagioclase has multiple twinning (Albite twins) and is never pink.

Lets look at texture:

We see crystals all interlocking. I'll cover the differences between sedimentary and metamorphic rocks in another post. Some sedimentary rocks can be confused with igneous rocks initially.

What colour is the rock? Is it light or dark? Even though this sample has a lot blackish minerals in it its still a light coloured rock. So its most likely a felsic or intermediate rock. Can you see the grains with your eye? Or is the groundmass too fine to make out? Grains are clearly visible in the sample.
So lets make some notes, we have: A felsic or intermediate rock with a granitic texture made of

  • A pink mineral 
  • A blackish mineral 
  • A whitish mineral 
  • A beige mineral 

If you look with a hand lens you will see a white or beige mineral that has no discernible cleavage. It might be milky like this.

Or it might be glassy and smoky inside. As you move the sample under the lens try and observe the difference between the minerals that shine with cleavage and the ones that don't. We notice the milky mineral has no cleavage and the beige mineral does have cleavage. We notice also the Dark mineral has cleavage.

So lets make update notes, we have:

  • A pink mineral phenocryst with cleavage, probably felspar
  • A blackish mineral with cleavage, probably biotite
  • A whitish mineral without cleavage, probably feldspar
  • A beige mineral with cleavage, probably feldspar

Which of these minerals are mafic and which are felsic?
Do you remember which are essential minerals for Felsic rocks?

The Biotite is mafic and the feldspars and quartz are felsic. If you remember for large grained, felsic rocks Biotite is an essential mafic and Quartz and Feldspar are essential felsics.

Can we make some estimates of percentages of each of these minerals? Looking at the chart to the right of the samples. I think its quite tricky for the eye to focus on this task with all the clutter on the rock. The trick I used was to make a small circle the size of the ones on the card and compare just that area.

So here is a random circle from the rock about the same size as the samples on the card.

So how does the sample compare to these? You notice the biotite is probably less than you might initially have thought looking at the whole rock? Maybe 15-20% of the rock. This way is a lot less confusing than rolling the rock around in your hand trying to get a feel for it.

40%    Quartz 
40%    Feldspar
20%    Biotite

So what is this rock?

Could it be Syenite, as it is a coarse grained igneous rock that is a light colour? No, Syenite doesn't contain much quartz (if any). So that leaves us with Granite as the other large grained Felsic rock in the sample set.

So we need a name. Remember you cant write Granite and expect full marks. You should aim for at least a three word name.

This rock has phenocrysts, it has biotite, it has colours. When a rock has crystals of two distinct sizes we can call this texture Porphyritic.

Porphyritic Biotite Granite.

So what can we say about the history of this rock?

The phenocrysts imply a long, slow cooling period initially at depth, followed by a slightly faster cooling period, still at depth to form the coarse grains, maybe in a dike or batholith.  The rock has then been brought to the surface slowly by uplift and erosion. Not by volcanic eruption.

No comments:

Post a Comment