Igneous Rocks

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Classification of Igneous Rocks

Igneous rocks make up about 95% of the Earth's surface. There are sedimentary rocks are the surface, metamorphic rocks are also found in the crust.

Crystal Grain Size

If no crystals can be seen, even using a hand lens, the rock has a glassy texture showing it cooled too quickly for any crystals to form.

Fine grained crystals are less than 1mm in size so that individual crystals cannot be identified by eye.

Medium grained crystals are 1mm to 5mm in size and can be seen by eye.

Coarse grained crystals are greater than 5mm in size and can be seen and identified by eye.

Silica Percentage

Chemical analyses give the overall chemical composition of a rock, rather than which minerals are present. The majority of SiO2 in igneous rocks is not present as quartz (SiOs), but is contained within other silicate minerals such as feldspar and olivine.

It is easier to link the SiO2 percentage to colour as a short cut to identification. The mafic rocks are rich in dark-coloured ferromagnesian minerals such as olivine and augite. Silicic rocks are rich in the light coloured minerals quartz and feldspars. But colour can be misleading in rocks such as obsidian, which looks black but is clear in thin section and is a silicic rock.

Mineral Composition

Igneous rocks usually contain many different rock forming minerals but only a few essential minerals are used in classification.

Felsic Minerals

The felsic minerals are quartz and feldspar, which are both rich in silica. The term felsic comes from the words feldspar and silica.

Mafic Minerals

These are minerals that contain both magnesium and iron. These minerals are also referred to as ferromagnesian.

Quartz

an important essential mineral found in only silicic or intermediate rocks. The magma must be oversaturated with silica so that excess silica is left after the other rock forming minerals crystallise to form the quartz

Recognising quartz - looks grey in colour and has an amorphous shape as the crystals grew in an enclosed space. If you look at it with a hand lens you will see the glassy appearance and lack of any smooth cleavage surfaces. You will also find that quartz crystals resist scratching with a steel nail

Hardness 7

Colour white, grey or transparent

No cleavage

Vitreous lustre (glassy)

Composition SiO2

Feldspars

most common rock forming minerals in igneous rocks. They form about 60% of the minerals in these rocks so are essential to igneous classification. The main types of felspar are potassium feldspar and plagioclase feldspar.

Potassium feldspars (orthoclase) is only found in silicic and intermediate igneous rocks so is a good mineral to use for classification. It is often found in large phenocrysts as well as in the groundmass of granite.

The pink colour of most K feldspars makes it very easy to identify. Grey or white feldspar is plagioclase. If you look at it with a hand lens you will see the glassy appearance and smooth cleavage surfaces. Feldspar crystals can just be scratched with a steel nail.

Hardness 6

Colour pink for K, white/grey for plagioclase

Good cleavage in two directions

Vitreous lustre (glassy)

Composition KAlSi3O8

Biotite and Muscovite Mica

the micas are biotite, found in silicic or intermediate rocks and muscovite, found in silicic rocks only

shiny black crystals of biotite can be easy to identify though the silvery flakes of muscovite are more difficult to pick out and need a hand lens. Micas can be scratched with a copper coin and are the only minerals in igneous rocks that are soft enough for this test

Hardness 2.5

Colour of biotite is black and muscovite is silvery

One perfect cleavage that makes flakes

Vitreous lustre (glassy)

Hornblende, augite and olivine

Hornblende is one of a group of minerals called amphiboles, which are particularly common in intermediate rocks. They are not easy to identify in hand specimen and can easily be confused with augite.

Hardness 5.5

Colour black

Two cleavages at 120 degrees forming six sided minerals

Vitreous lustre (glassy)

Augite belongs to the pyroxene group of minerals and is one of the main minerals in mafic and ultramafic rocks.

Hardness 5.5

Colour black to dark green

Two cleavages at 90 degrees, forming eight sided minerals

Vitreous lustre (glassy)

Olivine is found in some mafic and ultramafic rocks, it will only form when the magma is undersaturated in silica

Hardness 6.5

Colour light green

No cleavage

Vitreous lustre (glassy

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Felsic Minerals

Mafic Minerals