Fluorite Granite - Rostowrack (#45)
Collection:
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Fact sheet

Fluorite Granite - Rostowrack (#45)

This lithium-fluorine-enriched granite was collected close to the other fluorite granite in this collection, but has been included here because it shows veining and kaolinite development better than sample #42. Both rocks have been subjected to at least two major hydrothermal events. The first phase of hydrothermal alteration caused breakdown of the feldspars, and the second, driven by fluorine-rich fluids ended with fluorite crystallisation (replacement) along cleavages in lithium mica and in voids within feldspar.

Although the hydrothermal fluids pervasively attacked this rock, some were also channelled through veinlets such as the one crossing the centre of the thin section. Here needles of tourmaline crystallised at the vein margins prior to fluorite precipitation indication a change in conditions from boron to fluorine-dominated fluids. 

Kaolinite is notoriously difficult to prepare in thin section (its a soft white clay) and is often lost during sample preparation, however, in rotation 2 the bluish-orange patch (XPL) it has been preserved. This field of view also shows fluorite alteration along cleavages in a lithium mica and two low birefringent crystals of fluorapatite partially enclosed by the lithium mica.

This rock is close to being a china clay resource! 

Additional images
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
fluorite
Accessory minerals
kaolinite
tourmaline
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: