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Your specimen is probably a piece of granite. Granite
forms when molten rock (magma) cools in underground chambers called magma
chambers. These chambers can lie under volcanoes or can be trapped deep in the
earth's crust. Large chambers cool slowly as there is lots of molten rock which
keeps the temperature high for a long time: this slow cooling causes large
mineral crystals to form. Small chambers usually cool quickly producing smaller
crystals. Granite is usually made up of quartz, biotite and feldspar crystals.
Granite can be lots of different colours depending on the minerals it contains
and is a hard, strong rock. It is often used for decorative features in
buildings or for making monuments.
A number of igneous rocks from The Potteries Museum &
Art Gallery collections are featured on the
Virtual
Store.
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