orbicular granite - hand/display specimen of a very unusual plutonic rock from Western Australia
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Mt. Magnet, Boogardie Station, Western Australia
Orbicular granite is often not a true granite. This specimen has a granodiorite-tonalite matrix. The orbicules initially crystallized from a fluid-rich dioritic magma where nucleation centers were sparse. Some have nucleated on seed crystals. Others nucleated on amphibolitic xenoliths or on fragments of previously-formed orbicules, then appear to have settled by gravity while still a skeletal mesh of crystals, since they are frequently deformed or molded against one another.
The orbicules are composed of andesine, hornblende and biotite arranged around a core of plagioclase and/or hornblende. Many of the orbicules exhibit a comb structure, where the axis of the hornblende crystals are perpendicular to the margin of the orbicule.
Uranium/lead radiometric dating gives an age of 2687 ± 5 million years, making this the oldest known orbicular granitoid. Over one third of known exposures are in Finland and most are not large.
Boogardie Station is a large cattle ranch, and until recently has not restricted access to the quarry that provided this specimen. As of February, 2020, access is no longer being granted.
Cut flat on both sides, 1/2" thick and unpolished.
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