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Thunder Eggs are spherical objects found in silica-rich volcanic rocks such as Rhyolite. They are often found in diffuse bands within volcanic lava and are evidence of trapped steam and other gases which form expanding bubbles as the lava cools. Silica and feldspar minerals can crystallise around the bubble or grow outwards from the centre. Mineral-filled bubbles with a radiating structure are called spherulites (Thunder Eggs).

Thunder eggs - Copyright Tweed Shire Council



Internal gas pressure can force the spherulite apart to form a central hollow, or cavity, and this can be filled with solutions containing other minerals such as chalcedony, banded agate, or various forms of quartz. Infilling of the cavity can occur over time, leaving layered patterns or shapes with wedge shaped segments that produce internal structures and interesting patterns of mineral deposit.

Thunder eggs - Copyright Tweed Shire Council


Solutions of different composition seep in at various times and the resulting layering of minerals can be recognised as characteristic to a particular locality. For example, in the Tweed, Thunder Eggs found within volcanic rocks in the Chillingham area contain characteristic patterns that can be distinguished from those found in different volcanic rocks at Doon Doon, which are different again from those found in the Perch Creek area.

This TRM selection includes examples from the different localities as well as some internal or “vertebra” types, where the surrounding spherulite has decomposed and eroded away.

Sources:
http://australianmuseum.net.au/Concretions-Thunder-Eggs-and-Geodes#sthash.lV1UJJcs.dpuf
Dictionary of Geology and Mineralology, 1994, published by McGraw-Hill, S. Parker editor-in-chief
Practical Guide to Rocks and Minerals, 1998, published by Chancellor Press, D. Wright and P. Bell authors

Last Updated: 22 October 2014