Two mica granite - Rosemanowes
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Fact sheet
Description: 

This two mica granite sample comes from a British Geological Survey borehole that was drilled at the Rosemanowes quarry borehole, Cornwall as part of the 'Hot Dry Rock' project that ran from 1977 to 1980. The borehole penetrated the Carnmenellis Granite to over one kilometre depth. The site was chosen because the granite in the area has the highest heat flow in England (120 milliwatts per square metre). The borehole did not develop into an operational geothermal exploitation scheme but the research laid the foundations for similar projects elsewhere.

In thin section the rock contains 100–200 micron diameter orthoclase (which appears mottled and altered), rarer plagioclase grains, quartz, and both biotite and muscovite are common. Biotite is distinguished by its brown pleochroism and black pleochroic haloes caused by radiation damage from zircon grains that sit at the core of each halo. The high heat flow recorded in the granite is the result of natural radiation present in these small grains producing heat throughout the granite.

In Place
In Time
infoImage: 
Location description: 
Rosemanowes quarry borehole, Cornwall
Accuracy:
Location precision: 
Good
Timescale:
Timescale: 
Permian
Ma = Millions of years ago
In Focus
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Type:
Type: 
igneous
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