Topaz greisen - St Mewan Beacon
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Fact sheet

Topaz greisen - St Mewan Beacon

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, rare plagioclase and feldspars are also present.

Additional images
  • topaz greisen - width 3.3 cm
50.33, -4.812
St Mewan Beacon, St Austell, Cornwall
About this collection

The United Kingdom Virtual Microscope (UKVM) collection consists of igneous, sedimentary and metamorphic rocks from around the UK.

It is intended as a teaching resource, helping to tell the story of the common rock types and how they form, and reflecting the history of the UK at the margins of the continent of Europe. The collection is a series of teaching sets, for example igneous rocks from the North Atlantic Igneous Province and SW England; high-temperature metamorphic rocks from Scotland and low-temperature metamorphic rocks from Wales; and sedimentary rocks, including English limestones and sandstones.

Sample details

Rock-forming mineral
Category guide  
Category Guide
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.
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.
Selecting one or more period, for example 'Jurassic'.
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.
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).
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: