Siderophyllite Granite - Colcerrow (#33)
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Fact sheet

Siderophyllite Granite - Colcerrow (#33)

Large and abundant orthoclase feldspar crystals dominate this sample (the thin section in particular). Such phenocrysts indicate that crystallisation of this magma began prior to intrusion at the current crustal level. The finer-grained siderophyllite (biotite), tourmaline and plagioclase all crystallised later. Note the small amount of cordierite in this sample - evidence for magma contamination. The cordierite probably recrystallised from digested Devonian metasediments.

Look for characteristic pale green and yellow alteration in and around the cordierite - termed pinite. Rotation 3 should help you get started. Note the higher relief of cordierite compared to quartz.

Other features of note - sericitic alteration of plagioclase, veinlets of muscovite invading the orthoclase, and the exceptionally abundant inclusion suite in the siderophyllite (biotite) of the granite. In comparison, the siderophyllite that occurs as inclusions in orthoclase is different!

A tricky question - which crystallised first - siderophyllite or tourmaline?

Additional images
  • width 17.5 cm
  • width 2.8 cm
  • width 2.8 cm
  • width 2.8 cmm
  • width 7.5 cm
50.39099, -4.726996
Colcerrow 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).
The owner of the sample that appears in the collection. For example, NASA owns all the samples that appear in the Moon Rocks collection
We would like to thank the following for the use of this sample: