Siderophyllite Granite - Luxulyan 2 (#201)
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

Siderophyllite Granite - Luxulyan 2 (#201)

This sample of granite comes from Luxulyan, St Austell, Cornwall. It is known locally as biotite granite, although the dark mica is the aluminous species siderophyllite. The granite actually contains two mica species - siderophyllite and muscovite. This granite represents part of the main mass of the St Austell granite, and it was this pluton that drove much of the heating and fluid infiltration of local rocks leading to further intrusions and mineralisation. Adjacent mineral deposits and the large clay pits were also the result of the hydrothermal fluid circulation surrounding the SW England pluton during its intrusion in the Permian period.

The thin section contains large plates of biotite showing alteration to chlorite, with lesser amounts of equally sized muscovite. Large plates of quartz show little deformation, orthoclase and plagioclase feldspar are both altered, plagioclase exhibits compositional zoning and orthoclase exhibits coarse perthite textures. 

Additional images
  • width 21 cm
  • width 4.3 cm
  • width 4.3 cm
  • width 21 cm
50.399232, -4.739914
Luxulyan, St Austell, Cornwall
About this collection

A case study of the St Austell granite complex in Cornwall, England, illustrating the range of rocks associated with a granite intrusion. The earliest part of the complex is a siderophyllite (biotite) granite containing muscovite and tourmaline typical of a SW England granite, with many primary magmatic features.

This early intrusion was followed by the intrusion of an evolved volatile-rich magma which was the driving force behind a series of intense hydrothermal processes as volatiles escaped from this magma and helped to establish an extensive alteration halo (aureole). Boron, fluorine and lithium (as well as water) played major roles in the formation of the second intrusion and in the associated hydrothermal processes. Igneous activity lasted around 18 million years from 282 Ma (siderophyllite granite) to 265 Ma (fluorite granite).


Sample details

Collection: St Austell Granite
Rock-forming mineral
Category guide  
Category Guide
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Accessory minerals
Minerals that occur in very low abundance in a rock. They are usually not visible with the naked eye and contribute perhapssver, they often dominate the rare elements such as platinum group metals.
Rock-forming minerals
Minerals that make up the bulk of all rock samples and are also the ones used in rock classi?cation.
Selecting one or more period, for example 'Jurassic'.
A term used to group together related samples that are not already gathered into a single Collection. For instance, there is a ‘SW England granites’ theme that includes such rock types as granite, hydrothermal breccia, skarn and vein samples.
A general term used to label a rock sample. It is a useful way of grouping similar samples throughout a collection. Category names are often, but not exclusively, common rock names (e.g. granite, basalt, dolerite, gabbro, greisen, skarn, gneiss, amphibolite, limestone, sandstone).
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We would like to thank the following for the use of this sample: