Tips for Identifying Igneous Rocks from Hand Samples

How to distinguish between Igneous Rocks in the Sample Set.

First thing you should do before the pratical exam is visit the lab and use the study sets as often as you can. Even if its only 10 minutes each morning before first class, time spent with the samples in hand is exam gold.

You need to be able to use your hand lens to identify the common minerals in the hand sample. If you can't tell feldspar from Biotite you need to talk to someone urgently who can show you how.

You need to remember the order from Felsic to Ultra Mafic of the samples.
So memorise the mnemonic. Real Geologists Tell Stories And Deliver Big Gains

Rhyolite, Granite, Trachyte, Syenite, Andesite, Diorite, Basalt, Gabro

The Chart in the lab manual with all the rock types and their associated essential minerals should be heavily studied. I mean read it every night before you sleep, and try recall all those little details.

Igneous rocks can be classified by their silica content and grain size. Felsic rocks being the most contaminated with silica and Ultramafic rocks having the least amount of Silica present. High silica rocks are generally lighter in colour than mafic-ultramafic rocks. When identifying hand samples you need to identify the essential felsic minerals to name the rock.

The easiest rocks to identify are:

Scoria (hard and full of holes lab samples are red)

Pumice (very light and full of holes)

Obsidian (black and glass like)

Ignimbrite is fragmental and light coloured.

The difference between Granite and Rhyolite

The colour and mineralogy of Granite changes significantly over the lab samples. But it has clearly visible crystal grains. Some of the granite has very large Feldspar phenocrysts.

The rhyolite in the sample set is a light colour and flow banded. Individual grains can not be seen with the naked eye. You can see this sample is much lighter than the other fine grained rocks in the set like Trachyte and Andesite. Its not this easy in the real world unfortunately so to be an outstanding geoligist you should remember the general rules as the colours will change. 

Differentiating between Syenite/Diorite/Trachyte/Andesite

Differentiating between Syenite/Diorite.

 Syenite/Diorite are both coarse grained but Diorite has Sodic Plagioclase as an essential Felsic. (You will be told the hand sample has this in the test so its a no brainer to get this correct) Its also darker in colour than Syenite.

Differentiating between Trachyte/Andesite

Trachyte/Andesite are both fine grained but Andesite has Sodic Plagioclase as an essential Felsic. (You will be told the hand sample has this in the test so its a no brainer to get this correct) Its also darker in colour than Trachyte.

If the Essential Felsic is given as Calcic Plagioslacse then you just ned to decide if its coarse, medium or fine grained. Coarse gives us Gabbro and Gabbro has Augite as an essential Mafic mineral. Basalt has Olivine as a more common mineral.

Remember Pyroxene is essential in Gabbro and Basalt. Watch out for Microgabbro which has small but visible grains whereas Basalt is aphyric.

If the rock given is very dark and no essential flesics are given then you have Pyroxenite, Dunite or Peridotite. There is no peridotite sample in the lab. Dunite looks almost like an olivine crystal, whereas the Pyroxenite is very, very dark with some calcic plagioclase crystals mixed in.

Volcanic History of the rocks is important to be able to deduce from a given sample. So pay attention to the implications of grain size on cooling time and presence or absence of phenocrysts.

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