Orbicular granite is a rare type of igneous rock characterized by its
distinctive appearance. It is composed of concentrically layered,
spheroidal structures called orbicules, which are typically made up of
alternating bands of dark and light minerals. Orbicules can range in
size from a few centimeters to several meters in diameter.
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| What Is Orbicular Granite? |
The orbicules are
formed by crystallisation from a fluid-rich supercooled dioritic magma nucleated on seed crystals. The orbicules appear to have settled under gravity while each was still a skeletal mesh of crystals, since the orbicules are frequently deformed or moulded against one another.
The accumulation of orbicules depletes the magma in mafic elements, and increases the silica and fluid content, and the
granite nature of the matrix. Biotite flakes cut primary crystals and are thought to be secondary.
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| Orbicular granite, an unusual type of granite, near the town of Caldera, northern Chile |
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| Orbicular granite cut by a co-magmatic intrusion of granite and fine-grained diorite. |
Orbicular granite is found in a variety of locations around the world, but some of the most notable examples are found in Finland, Australia, and the United States. Orbicular granite is prized for its beauty and rarity, and it is often used as a decorative stone or in jewelry.
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