ORBICULAR GRANITE
Orbicular granites are characterized by having large,
concentrically layered, spheroidal to subspheroidal masses with radiating
crystals. The rounded masses are called orbs, or orbicles. Shown below
are samples of a Precambrian orbicular granodiorite from Western Australia.
The matrix is granodiorite (K-feldspar, plagioclase
feldspar, quartz, mafic mineral(s)), and the orbicles are composed of
plagioclase feldspars, hornblende amphibole, and biotite mica.
This material is mid-Neoarchean in age (2.687 billion
years old) and comes from a quarry in the vicinity of Warroan Hill, a little
west of Boogardie, west of Mt. Magnet & east-northeast of Geraldton, in the
western part of Western Australia.
