Siderophyllite Granite - Colcerrow (#33)
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Fact sheet
Description: 

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?

In Place
In Time
Location description: 
Colcerrow quarry, near Luxulyan, St Austell, Cornwall
Accuracy:
Location precision: 
Good
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:

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Accessory minerals:

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