Topaz greisen - St Mewan Beacon
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Fact sheet
Description: 

This rock comes from St Mewan Beacon, Cornwall and was a loose block from a small quarry. The rock is a quartz-topaz tourmaline-rich greisen at the southern edge of the Permo-Carboniferous-age St Austell granite. The sample represents a late stage, highly silica-rich fraction of a hydrothermal saline fluid, from the roof zone of the granite, possibly trapped beneath an impermeable pelitic country rock.

The thin section contains tourmaline that changes from patchy blue and brown to colourless upon rotation — a very unusual pleochroic effect. When viewed in crossed polars the colours are bright but anomalous (not directly comparable with the standard birefringence colours) second order birefringence colours. Topaz is colourless in plane polarised light and grey in crossed polars but can be distinguished from quartz by its higher relief. Orthoclase, rare plagioclase and feldspars are also present.

In Place
In Time
infoImage: 
Location description: 
St Mewan Beacon, St Austell, Cornwall
Accuracy:
Location precision: 
Moderate
Timescale:
Timescale: 
Permian
Ma = Millions of years ago
In Focus
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Type:
Type: 
igneous
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