Siderophyllite Granite - Luxulyan 1 (#10)
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Fact sheet

Siderophyllite Granite - Luxulyan 1 (#10)

This sample of siderophyllite(biotite)-rich granite comes from Luxulyan (Tregarden) quarry to the north of Luxulyan village. This sample is a less evolved portion of the intrusion, which is associated with later the tourmaline and topaz-rich intrusions and mineralisation. Luxulyan valley is part of a World Heritage Site containing the remains of extensive early 19th century industrial activity, including mining and quarrying, which at its peak included several water wheels and steam engines.

The thin section illustrates the quartz-rich nature of the rock, which also contains altered orthoclase, plagioclase and both siderophyllite and muscovite. The siderophyllite is notably rich in pleochroic haloes, indicating a high proportion of zircon and monazite crystals. Rotation 1 displays a rare crystal of cordierite surrounded by typical pinite alteration.

Additional images
  • width 9.5 cm
  • width 2.8 cm
  • width 2.8 cm
  • width 9.5 cm
  • width 9.5 cm
50.399287, -4.738283
Luxulyan quarry, near 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
Accessory minerals
Category guide  
Category Guide
Refers to any word or phrase that appears in the individual rock names. Names are generally descriptive; they allow users to search for broad terms like ‘granite’ as well as more specific names such as ‘breccia’. However, the adjacent descriptions of the specimens captures a wider range of general words and phrases and is a more powerful search tool.
Refers to any word or phrase that appears anywhere in the descriptions of the specimens
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: