Boogardie Orbicular Granite — the oldest in the world
Geology Explorer
What is it?
It was the Geological Society of Australia’s Rock of the Month for March 2013 and there are large blocks of it at Geoscience Australia in Canberra, waiting to be inducted into the National Rock Garden. The National Museum of Scotland has a hefty slab on display, and spheres of it, nearly a metre in diameter, rotate on plinths around the city of Perth.
It's an unusual type of granite. Granite is a rock many of us are familiar with as kitchen bench tops or as decorative stone in buildings. Most granite is light coloured with an even distribution of crystals ranging from a few millimetres to about 2 cm in size. Quartz, feldspar, biotite and hornblende are the most common minerals that make up the rock.
Orbicular granite is a relatively rare igneous rock only found in a few locations around the world. It is a very distinctive granitic rock that contains abundant subspherical inclusions called orbicules.
The orbicules are composed mainly of concentric shells of radiating hornblende and feldspar crystals, densely packed within a much larger body of host granite.
‘Orbicular’ is a term that describes the texture of various rock types and it is generally taken to mean ‘circular’ or ‘orb-like’, as in containing nodules or spherical features.
Although orbicular granites are relatively rare, there can be localized examples found on almost every continent. Recognizable by their eye-catching texture, the different occurrences nevertheless vary greatly in age and have unique appearances and compositions.
Here is a map of some notable sites around the world where similar rocks occur. Almost one-third of all known instances of orbicular granites are located in Finland.
Acknowledgement of country
We acknowledge that the story told here takes place on lands of the Badimia, Wajarri Yamatji and Yugunga-Nya Peoples, who are the traditional owners and custodians of these lands, and we pay our respects to the elders of these peoples, past, present and emerging.
Western Australia's own
Here in Western Australia we have the only known example of an orbicular granite in Australia. It is called the Boogardie Orbicular Granite after the pastoral station on which it was discovered and, with an age of about 2692 million years old, it is the oldest known orbicular granite in the world!
The Boogardie Orbicular Granite is one of the most famous and attractive of the orbicular granites. It is highly prized as an ornamental dimension stone where blocks of the stone are cut and polished to be used as decorative pieces.
A dimension stone is a natural stone or rock that has been selected and finished to a specific size or shape. To be selected it must be attractive, durable, and able to maintain its distinctive features over time.
Specimens of Boogardie Orbicular Granite are displayed at various locations in Perth. Spheres about 900 mm in diameter, and weighing close to 1000 kg, freely rotate on a layer of water that flows from beneath. This is both impressive to witness, and allows the beautiful texture and colour of the rock to be shown in its full glory.
When placed in the National Rock Garden , the Boogardie Orbicular Granite will undoubtedly be a star exhibit among the permanent displays of rocks, fossils and minerals. The aim of the garden is to showcase Australia’s geological heritage and iconic rocks in an attractive parkland setting.
Geological setting
The Boogardie Orbicular Granite is located on Boogardie Station, 35 km west of Mount Magnet in the Mid West of Western Australia. You can click on the map to learn more about the geology and different rock types.
Boogardie locality
Boogardie is a local Aboriginal name, although its meaning is unknown. The townsite was gazetted on 28 January 1898 and a little community burst into life shortly after.
Boogardie Station was established by the Jones family around 1880, and the family still operates it as a sheep station today. The outcrop has been called ‘Pretty Rock’ since the 1880s and was a well-established landmark and meeting place for local cattle musterers.
There are two main locations where the orbicular granite outcrops; one where the quarry has been established, and one to the northeast of this. It is believed they are part of the same continuous buried intrusion extending for about 300 m beneath the surface. During the late 1980s and 1990s a small quarry was developed at the southwestern end and the deposit has two separate mining leases over it.
NOTE this area is a State Geoheritage Site (full definition here ) which means the site includes a natural feature or features of special interest or exceptional geoscientific significance. It also means permission is required from the Boogardie Station owner to visit the site, and from the mining lease owners to collect any rock samples.
GSWA has generated a 3D model of the quarry using drone photogrammetry.
The orbicular granite deposit is a relatively shallow intrusion, and drilling has indicated that it is up to 15 m thick. It is hosted by a much larger body of medium- to coarse-grained porphyritic monzogranite.
The orbicule textures
The subspherical orbicules are typically 5 to 20 cm across and set in a groundmass of medium- to coarse-grained granitic rock. The orbicules are fairly evenly distributed through the groundmass, typically spaced up to 10 cm apart. There are zones where they appear to have accumulated more densely, and some orbicules are deformed against others.
The orbicules themselves are classified as hornblende diorite in composition. They consist almost entirely of black hornblende and white plagioclase feldspar, with some biotite and very rare titanite only in the outermost layer. The granitic groundmass that surrounds the orbicules ranges in composition from monzogranite to tonalite. The Big Bell Suite host rock has a composition of monzogranite to granodiorite.
These compositions are shown on a classification diagram for granitic rocks called a QAP diagram. This triangular graph can be used to classify the rocks based on their relative proportions of the minerals quartz (Q), alkali feldspar (A) and plagioclase feldspar (P).
The internal structures and layering of different orbicules are quite variable, although they all grew outwards around a central core. The most common type of core is a small cluster of radiating hornblende and plagioclase feldspar crystals; however, other cores consist of older rock fragments, broken orbicules, or fine-grained granular diorite.
Some of the most common orbicule core structures
Some cores are sharply defined, and are surrounded by a millimetre-thick granular layer. Around the core, most orbicules consist of five to seven, but in some cases up to 12, well-defined concentric layers or shells of variable thickness. Each shell is composed of elongate, alternating black hornblende and white plagioclase feldspar crystals that are at right angles to the surface of the underlying shell, giving the appearance of radiating outwards from the centre. The shells of radiating crystals are commonly separated by fine-grained granular layers composed mainly of plagioclase feldspar. The outermost shell is mid to dark grey, typically composed mainly of biotite, and can be several millimetres thick.
The internal structure and texture of an orbicule compared to the texture of the granitic groundmass
The radial arrangement of crystals contrasts significantly with the typically random crystal organization seen in the granitic groundmass around the orbicules. The groundmass shows a much more even, and equigranular texture (grains roughly the same size) with no obvious orientation in the crystals.
Surface weathering of the Boogardie Orbicular Granite
The biotite-rich outermost layer is quite soft and can be easily degraded by water. On weathered surfaces the rock sometimes appears pockmarked where the softer outermost layer has been eaten away over time and the orbicules have been loosened.
How did it form?
Quite a bit of research has been done to understand how orbicular granites form, and why they are somewhat rare.
Granitic rocks are typically medium- to coarse-grained igneous rocks that form from magma that cooled slowly in a magma chamber deep underground. Normally, as the magma cools, minerals begin to crystallize around nucleation sites. The relatively slow cooling process allows the steady precipitation of a sequence of different minerals that eventually solidify into a rock body with a relatively even distribution of randomly arranged crystals. The nucleation sites can be just a few molecules, but these are the seeds that ‘start off’ the whole crystallization process.
However, at Boogardie, something different occurred.
1. Normal crystallization was delayed, probably due to an absence of nucleation sites. It is possible that the nucleation sites were destroyed by an influx of water given off during crystallization of the host Big Bell Suite monzogranite.
2. The absence of nucleation sites means the magma was then able to cool below the temperature at which crystallization into a solid would otherwise have occurred (this is known as supercooling). Normal crystallization wasn’t then able to proceed.
The idea of supercooling is illustrated in this video where the beer is able to remain liquid despite cooling well below the temperature where it would normally become a solid, i.e. freeze. However, as soon as the bottle is banged on the worktop and opened it allows a few ice crystals to begin to form. These crystals act as nucleation sites and the beer immediately freezes and is able to crystallize to form a solid.
Video illustrating the process of supercooling
3. Eventually, when conditions cross a critical threshold below the 'typical' crystallization temperature, the minerals plagioclase and hornblende began to form around rock fragments and other ‘seed’ sites. Once the process starts it becomes a lot quicker and easier because less energy is required to nucleate on a solid.
4. The concentric layers of the orbicules were added progressively around the cores. The variations in texture and composition likely reflect local variations in temperature and saturation in different elements. It is also likely that during this stage, the orbicules moved freely around the magma chamber, driven by convection currents. The partially crystallized orbicules may have settled through the fluid magma, and some may have accumulated at the bottom of the magma chamber while still relatively soft. This can be seen where some orbicules are deformed, or others are joined together.
Examples of deformed orbicules
5. Finally, the remaining melt crystallized between the orbicules to form the granitic groundmass, locking the orbicules in place.
Dating the orbicular granite
Most granitic igneous rocks contain the mineral zircon which can be dated accurately. Zircon contains trace amounts of uranium which decays slowly to lead at a precisely known rate. The ratios between the amounts of these elements allow us to calculate how much time has passed since each zircon crystal formed and therefore date the rock it came from.
The rock is first crushed and milled to a fine powder, then sieved and washed to remove the finest powder. Magnetic minerals are then removed using strong magnets and the remaining material is placed in a high-density liquid where the zircon crystals then sink to the bottom. Once separated out, these very small crystals (about 100–200 microns long) are mounted in an epoxy disc and polished for analysis.
Microscope image of separated zircon crystals and a photograph of zircon crystals mounted and ready for analysis
The SHRIMP in the John de Laeter Centre at Curtin University, Perth
Zircon crystals from rocks in the Boogardie quarry were dated using the sensitive high-resolution ion microprobe (SHRIMP) in the John de Laeter Centre at Curtin University in Perth. The SHRIMP is a very large mass spectrometer which uses a finely focused beam of oxygen ions to enable us to measure the uranium and lead from within individual zircon crystals. There are only a few of these machines around the world.
Using the SHRIMP, GSWA scientists have determined that the Boogardie Orbicular Granite is 2692 million years old. The host biotite monzogranite of the Big Bell Suite is only slightly older, dated at about 2700 million years.