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Pegmatitic granite (Precambrian; Alexandra Mine, Broken Hill Block, New South Wales, Australia) 1 | by James St. John
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Pegmatitic granite (Precambrian; Alexandra Mine, Broken Hill Block, New South Wales, Australia) 1

Pegmatitic granite from the Precambrian of Australia. (~10.3 centimeters across at its widest)

 

Pegmatitic granite (a.k.a pegmatite) is a very coarsely-crystalline textured granite, formed by cooling of a relatively water-rich magma. The pegmatite shown above is apparently an early Mesoproterozoic intrusion that intrudes metamorphic host rocks such as amphibolite (www.flickr.com/photos/jsjgeology/26393781216). This rock has grayish quartz (SiO2), pale yellowish to pinkish-orange colored K-feldspar (KAlSi3O8), black schorl tourmaline, and glittery, pale greenish mica (= retrograde fine-grained muscovite mica or sericite?).

 

Locality: Alexandra Mine (= locality "SN5" of Barnes, 1988 - New South Wales Geological Survey Bulletin 32: 210), 3 kilometers southeast of the town of Silverton & southeast of the confluence of Black Mill Creek and Umberumberka Creek, west-northwest of the city of Broken Hill, Broken Hill Block, far-western New South Wales, southeastern Australia

 

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Taken on April 17, 2016