Siderophyllite Granite - Pelyn (#32)
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Fact sheet
Description: 

This sample of siderophyllite (biotite) granite is from the eastern margin of the St Austell granite complex. In many respects it is a normal tourmaline-mica granite, however, it is quite unusual in also containing a mineral thought to be cordierite. This magnesium, aluminium silicate forms a solid solution series with an iron-rich member sekaninaite. Without an analysis the two species cannot be discriminated.

The presence of cordierite is evidence that the granitic magma incorporated (and digested) surrounding host-rocks (Devonian metasediments) before final consolidation. Cordierite is known from the margins of the Dartmoor granite, but this is possibly the first report of the mineral from the St Austell granite complex.

Cordierite usually resembles quartz, orthoclase or plagioclase. It can be distinguished from quartz by its cleavage and the presence of pinite (a characteristic form of alteration involving muscovite). It can be distinguished from orthoclase since its cleavage is less well developed.

How many cordierite crystals can you find?

In Place
In Time
Location description: 
Wooded area above old quarry workings, Pelyn, near Lostwithiel, 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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