Topaz Greisen - St Mewan Beacon (#200)
Collection:
Click the microscope button to view a thin section for this sample.
Microscope
Click the microscope button to view a thin section for this sample.
Microscope
Click the object button to view an object rotation for this sample.
Object

Fact sheet

Topaz Greisen - St Mewan Beacon (#200)

This rock comes from St Mewan Beacon, Cornwall and was a loose block from a small quarry. The rock is a quartz-topaz tourmaline-rich greisen at the southern edge of the Permo-Carboniferous-age St Austell granite. The sample represents a late stage, highly silica-rich fraction of a hydrothermal saline fluid, from the roof zone of the granite, possibly trapped beneath an impermeable pelitic country rock.

The thin section contains tourmaline that changes from patchy blue and brown to colourless upon rotation — a very unusual pleochroic effect. When viewed in crossed polars the colours are bright but anomalous (not directly comparable with the standard birefringence colours) second order birefringence colours. Topaz is colourless in plane polarised light and grey in crossed polars but can be distinguished from quartz by its higher relief. Orthoclase and rare plagioclase are also present.

Additional images
  • Topaz greisen
  • width 3.3 cm
Map
50.330043, -4.811947
Description:
St Mewan Beacon, St Austell, Cornwall
Precision:
Moderate
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
topaz
tourmaline
quartz
orthoclase
feldspar
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: