Li-mica Tourmaline Granite - Gunheath (#21)
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Fact sheet
Description: 

The presence of purple fluorite on joint planes gives the first indication that this sample is unusual .......but can you see fluorite in thin section?

In thin section the pale brown mica is typical of the lithium-rich species zinnwaldite. It is associated with brown tourmaline, quartz and feldspar.  Note that the feldspar is cloudly in appearance - an indication that is has been subjected to a limited degree of hydrothermal alteration. Look closely to see topaz too - further evidence of an extremely evolved granite.

One interpretation for this rock is that it was a tourmaline granite similar to that in the west of the complex, but that it has been strongly affected by later magmatic-hydrothermal fluids (when Li-mica, fluorite and topaz appeared). Ask yourself - does the Li-mica look primary or secondary? ........and should the sample be grouped with the tourmaline granites to the west, or fluorite granites to the east? It really is a borderline case.

In Place
In Time
Location description: 
Gunheath china clay pit, near Stenalees, St Austell, Cornwall
Accuracy:
Timescale:
Timescale: 
Permian
Ma = Millions of years ago
In Focus
Collection:
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Type:
Type: 
igneous
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