china clay

• China clay is known as kaolinite and is a residual deposit. 
• The process where feldspars are altered to kaolinite is called kaolinization. 
• Sodium rich feldspar is most susceptible to transformation to pure kaolinite. 
• Potassium rich feldspar is less susceptible to kaolinite transformation. 
• 285 ma stage one - intrusion of biotite granite to form st. Austell granite. 
• Stage 2 - pulses of magma move to form muscovite and lithium rich granite in St. Austell.
• Stage 3 + 4 - main stage of hydrothermal vein mineralisation with tin and quartz rich veins. 
• Stage 5 + 6 - vigorous radioactive driven convection draws surface water down into the granite causing kaolinization. Interrupted when the peninsula became submerged.
• At the end of the process the interlocking crystal structure of solid granite has been replaced by a mass of clay particles with quartz sand. 
• The muscovite and lithium rich granites are those that form best quality clay deposits. 
• Iron oxide staining due to iron ions releasing makes kaolinite red, so means its unmarketable. 
• Kaolinization of granite still happens today. 
• The ko'd zones are often in forms of tunnels pointing towards the surface.