Tourmaline Vein in Granite - Trethurgy (#25)
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Fact sheet
Description: 

This sample comes from reclaimed land near the village of Trethurgy, close to the giant Carclaze china clay pit, St Austell, Cornwall. The rock is a hydrothermally altered granite cut by a cassiterite bearing tourmaline vein that formed as boron-rich late-stage magmatic-hydrothermal fluids from which the tourmaline crystallised, also carried tin. It is from this paragenesis that the entire tin industry of Cornwall is based - an industry that can trace its beginnings from Roman times or possibly even earlier.

The thin section shows a coarse grained igneous texture with large grains of quartz, muscovite mica, and altered plagioclase feldspar. Blue/brown igneous tourmaline is present in the granitic portion of the rock but the majority of the tourmaline grains are found in the vein. This hydrothermal tourmaline is darker in colour with blue/brown pleochroism and the vein also contains cassiterite (tin oxide), distinguished by its higher relief and dark brown colour, although unlike many oxides, it is not opaque.

In Place
In Time
Location description: 
Trethurgy, near Carclaze china clay pit, St Austell, Cornwall
Accuracy:
Location precision: 
Good
Timescale:
Timescale: 
Permian
Ma = Millions of years ago
In Focus
Collection:
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Type:
Type: 
igneous
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Rock-forming minerals:

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Accessory minerals:

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