Kaolinised Tourmaline Granite - Littlejohns (#16)
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Fact sheet
Description: 

This altered granite comes from Littlejohn's china clay pit, St Austell, Cornwall. China clay is mined in this area by high pressure water hoses and filtering to remove the remaining micas and quartz. Littlejohn's clay pit has been active since the 1800s and continues to supply fine clays suitable for use in paper making.

In thin section the rock contains highly altered plagioclase and orthoclase feldspars that are barely recognisable as they have been transformed to fine-grained clay minerals, mostly kaolinite. Mica is recognisable through the pleochroic haloes, although highly altered. It may have originally crystallised as siderophyllite (biotite), but it is now the Li-rich species, zinnwaldite. Topaz probably crystallised during the same alteration event.  Small grains of tourmaline are also present. They have brown cores (magmatic origin) and are rimmed with blue tourmaline (hydrothermal origin). It is only the quartz that remains unaltered.

In Place
In Time
Location description: 
Littlejohns china clay pit, near St Austell, 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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Rock-forming minerals:

Rock forming mineral:

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