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Intrusive igneous rocks

The following links show 3D examples of intrusive igneous rocks.

The models are organised in rough order from the most silicon-rich rocks to the least silicon rich rocks.

Click on the models to view them in 3D and click on the numbered hotlinks in the models for more information on each model.

 

∗ Granite

Granite is an intrusive igneous rock with large crystals that contains a high proportion of silicon and is mainly made up of light-coloured minerals.

This hand specimen comes from northeast Tasmania.

 

∗ Pegmatite

Pegmatite is an intrusive igneous rock with extremely large crystals sometimes greater than 10cm. Pegmatites are usually composed of feldspars, quartz and micas but may also contain minerals such as tourmaline (a silicate mineral that contains boron).

Pegmatites are often only small bodies and hence would be expected to cool rapidly and have small crystals but the high concentration of water and elements such as flourine in these magmas leads to rapid large crystal growth.

This outcrop is at Bluestone Bay in eastern Tasmania.

 

∗ Porphyry

Porphyries are intrusive igneous rocks that have both large and small crystals.

They form when magmas partially crystallise deep within the Earth creating initial large crystals in the magma. If the magma is then forced upwards to shallower/cooler levels the remaining liquid magma crystallises rapidly to form small crystals that surround the original large crystals.

This sample of sanidine porphyry comes from near Cygnet in southern Tasmania. The large crystals are sanidine ( a type of potassium feldspar). The dark green groundmass of the rock has extremely small crystals that can only be seen under a microscope.

 

 

∗ Diorite

Diorite contains higher proportions of iron and magnesium than granite. The iron and magnesium is contained in the dark-coloured minerals biotite and hornblende. Diorite contains only plagioclase feldspar and does not contain potassium feldspar.

This example of quartz diorite comes from Bingi-Bingi Point in NSW.

 

∗ Gabbro

Gabbro is a dark-coloured rock that has low silicon, high iron and high magnesium.

The rock consists almost entirely of plagioclase feldspar  ((Na,Ca)Al2Si2O8) and the dark pyroxene mineral augite ( (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) which contains the iron and magnesium.

This example shows the westhered surface of a gabbro outcrop near the Gibb River Road in the Kimberley, WA.

 

 

Compare the weathered rock above to the unweathered rock, from the TESEP Rock Kit, below. Why would the weathered rock have a distinct red colour?

∗ Ultramafic rocks

Hartzburgite is a dark ultramafic rock that has very low concentrations of silicon and high concentrations of iron and magnesium. It consists primarily of the minerals olivine ((Mg, Fe)2SiO4 ) and  the pyroxene mineral enstatite  (MgSiO3 ).

Ultramafic rocks often react with hot subsurface water and the olivine is converted to a group of soft green minerals called  serpentine ((Mg, Fe)3Si2O5(OH)4).

This example from Serpentine Hill in western Tasmania shows the contact between brown unaltered Hartzburgite and green serpentine minerals.

 

 

Now explore extrusive igneous rocks!

 

Minerals Rock Cycle Igneous Rocks Sedimentary Rocks Metamorphic Rocks More ...

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This educational product is designed for Yr 7-10 secondary students to complement the earth and space component
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The content and design of this educational product is based upon materials previously published by AusGeol.org