Li-mica Tourmaline Granite - Gunheath (#21)
Collection:
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Fact sheet

Li-mica Tourmaline Granite - Gunheath (#21)

The presence of purple fluorite on joint planes gives the first indication that this sample is unusual .......but can you see fluorite in thin section?

In thin section the pale brown mica is typical of the lithium-rich species zinnwaldite. It is associated with brown tourmaline, quartz and feldspar.  Note that the feldspar is cloudly in appearance - an indication that is has been subjected to a limited degree of hydrothermal alteration. Look closely to see topaz too - further evidence of an extremely evolved granite.

One interpretation for this rock is that it was a tourmaline granite similar to that in the west of the complex, but that it has been strongly affected by later magmatic-hydrothermal fluids (when Li-mica, fluorite and topaz appeared). Ask yourself - does the Li-mica look primary or secondary? ........and should the sample be grouped with the tourmaline granites to the west, or fluorite granites to the east? It really is a borderline case.

Additional images
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  • width 2.8 cm
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  • width 9 cm
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Map
50.375934, -4.809952
Description:
Gunheath china clay pit, near Stenalees, 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
Type
igneous
Rock-forming mineral
quartz
feldspar
li-mica
tourmaline
topaz
Accessory minerals
fluorite
Category guide  
Category Guide
Title
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.
Description
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.
Timescale
Selecting one or more period, for example 'Jurassic'.
Theme
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.
Category
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).
Owner
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: