Fluorite Granite - Rostowrack (#42)
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Fact sheet
Description: 

This lithium-enriched granite has been subjected to at least two major hydrothermal events. The first caused considerable alteration to the feldspars, to the extent that it is not always possible to discriminate between plagioclase and orthoclase. This was the first step to the creation of a china clay deposit as the rotted feldspar was replaced by secondary muscovite and later kaolinite.

The second alteration process involved fluorine-rich fluids which arrived in small veinlets and pervasively attacked the entire rock. Lithium mica was replaced along cleavages by fluorite and voids in feldspar were infilled with larger crystals of colour-zoned purple-white fluorite. Topaz (another fluorine-bearing species) is also present in this rock.

Rotation 2 shows opaque blades of columbite-tantalite - a species only usually found in extremely evolved granite and pegmatites. It's a mineral that cannot be positively identified with a transmitted light microscope, although its form is typical of the species. Electron microprobe analysis, however, confirms the species.

Further evidence of hydrothermal activity in this sample is shown in rotation 3, where a large quartz crystal is seen to contain many trains of tiny inclusions. These are fluid inclusions - trapped water and gases (such as carbon dioxide) sealed into the quartz as it crystallised.

In Place
In Time
Location description: 
Restowrack SW china clay pit, near St Dennis, Cornwall
Accuracy:
Location precision: 
Moderate
Timescale:
Timescale: 
Permian
Ma = Millions of years ago
In Focus
Collection:
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Type:
Type: 
igneous
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