5.15" Polished Orbicular Granite Section - Western Australia

Orbicular granite is the name given to an unusual kind of plutonic granite formation. The granite forms as orbicules, spherical formations of concentric bands of minerals around a grain serving as a nucleus. They form as fluid-rich dioritic magma cools quickly around these nucleus grains. Some deposits' orbicules show gravitational deformation: they are deformed by each other, seemingly by stacking as they precipitated out of magma.

Orbicular granite occurs only rarely, and their occurrences are often very small. Most deposits are found in Finland, but notable deposits can also be found in Chile and Australia. This material is from the Australian Boogardie Orbicular Granite deposit, near Mount Magnet in Western Australia. It is characterized by light andesine with darker hornblende and biotite radiating from plagioclase or hornblende cores. This orbicular granite is touted as the oldest in the world, at almost 2.7 billion years!