Garnet-biotite augen gneiss
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Fact sheet
Description: 

Darwin wrote: “Mica, quartz and feldspar (porphyritic with large crystals of do.) arranged in plane; containing garnets and joining to 467. (It is a true gneiss, a most beautiful rock).”

The rock is a garnet-biotite augen gneiss. Some individual augen have cores of plagioclase feldspar mantled by quartz. Other augen are entirely quartz.

In Place
In Time
Location description: 
Rio de Janeiro, Brazil
Accuracy:
Location precision: 
Moderate
Timescale:
Not known
Ma = Millions of years ago
In Focus
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Type:
Type: 
metamorphic
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