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Pegmatite |
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Pegmatite |
Photographed from the Geological Museum Perth |
Pegamatite is the name given to an igneous rock which has very large crystals of the individual minerals. Usually the rock is basically granite, so that there are very large crystals of quartz, feldspar and mica, but other mineral compositions are possible. Pegmatite generally occur as veins or small bodies associated with a lager igneous intrusion. Why do the crystals grow so large? One reason is that as crystals form in the main intrusion, the hot liquid which is left becomes concentrated in gases, fluids and unusual chemical elements. It might squeeze into cracks or form masses around the margins of the intrusions. The composition of the residual liquid and the environment appear to be particularly suitable for the rapid growth of crystals. |
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This specimen has
crystallised from a liquid rich in lithium. The purplish flaky mineral
here is a member of the mica family, like muscovite and biotite, but it is
lepidolite, and contains the chemical element lithium in the parts of the
crystal where muscovite would have aluminium. The feldspar (white) is also
a little different from the plagioclase in ordinary granite: it has much more of
the chemical element sodium. The pink mineral seen in this photo is
Tourmaline which is often used as a gemstone.
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Muscovite pegmatite »»»»»»»»»»»» Photographed from the Geological Museum Perth |
We would like to thank the "Geological Museum of the University of Western Australia" for the information supplied. |
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