August 2017

Great balls of magma

I found this fossil on the weekend while gold detecting. I would like to know what kind of fossil it is, and its’ approximate age.


A weathered chunk of rare orbicular graniteA weathered chunk of rare orbicular graniteClose up of one of the orbicules, showing layered structure.Close up of one of the orbicules, showing layered structure.

The specimen in your photos is something rather extraordinary. It’s not a fossil, but is a rare kind of igneous rock called an ‘orbicular granite’. These rocks are characterised by semi-spherical, layered nodules (‘orbicules’) within a more typical granite matrix. Granites are plutonic rocks, meaning that they form deep underground when magma (molten rock) slowly cools. In most cases this slow cooling results in relatively large crystals (visible between the orbs in your specimen). Our curator of mineralogy, Dr Andrew Christy, explains how the orbicules are thought to form: “… the orbicules start as squishy, half-crystallized droplets of slightly higher-melting-temperature, denser, more mafic [rich in iron and magnesium minerals] composition than the surrounding granite melt, and gradually settle out to form relatively small  (< tens of metres) lens-, pod- or layer-like concentrations in a texturally more normal granite host.” Bear in mind that this is the prevalent idea of how these rocks form; there’s still some debate among geologists.

Orbicular granites are rather rare worldwide. There’s a well known source in Western Australia near Mount Magnet that is quarried commercially, but as far as I know yours is the first record from Queensland.

Unfortunately, I don’t have a precise age for your specimen. Granites from your part of Queensland are mostly from the Ordovician to the Devonian in age, a range of 480-360 million years ago (not very precise!).

Orbicular granites are one of serval kinds of rocks that may have a layered, ‘onion-like’ structure. Stromatolites are layered, commonly dome-shaped structures that slowly accrete over time when particles of sand and mud become trapped by sticky films of bacteria. The oldest fossils in Queensland are stromatolites near Mt Isa that date to roughly 1.65 billion years ago. At that time stromatolites may have been the most obvious evidence of life on Earth, but they rarely form today.

Concretions are another kind of layered rock, and one that is commonly brought to the museum for identification. These are sedimentary structures that form when loose sediments start to become cemented together to form sedimentary rock. There is more information about concretions on our Pseudofossils fact sheet (239 KB) pdf document icon.

Orbicular granites can be distinguished from both stromatolites and concretions by their mineral composition and the relatively large crystals that form the matrix between the orbicules.

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