Orbicular Granite

from Western Australia

Of all the rocks in the world, orbicular igneous rocks are some of the most extraordinary. Igneous means produced under intense heat or fire: in practice that means crystallised from a molten magma. Normally the crystals would grow in all directions, but very occasionally they grow in alternating layers of dark-coloured and light-coloured minerals, to make the shape of a ball. These ‘orbicules’, as they are called, sink to the bottom of the magma and squash together to create the unusual ‘orbicular’ rock.

This orbicular granite is nearly 2.7 billion years old and comes from Boogardie Station, near Mount Magnet in Western Australia. The whole Boogardie deposit was a 40m-wide oval flying saucer shape. It was large enough to be sawn and polished for use in tables and decorative inlays, as well as in sculptures.

Our ‘cube’ is a fantastic example of this rare kind of rock.

Listen to… Dave Waters – Curator, Mineralogical Collections

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In 2013, while the Oxford University Museum of Natural History is closed to have the roof fixed, some of the exhibits have sneaked away to the town centre!
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