Biotite granite - St Austell
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

Biotite granite - St Austell

This sample of granite comes from Luxulyan, St Austell, Cornwall. It is known locally as biotite granite, although the granite actually contains both muscovite and biotite. This granite represents part of the main mass of the St Austell granite, and it was this pluton that drove much of the heating and fluid infiltration of local rocks leading to intrusions and mineralisation. Adjacent mineral deposits and the large clay pits were also the result of the hydrothermal fluid circulation surrounding the SW England pluton during its intrusion in the Permian period.

The thin section contains large plates of biotite showing alteration to chlorite, with lesser amounts of equally sized muscovite. Large plates of quartz show little deformation, orthoclase and plagioclase feldspar are both altered, plagioclase exhibits compositional zoning and orthoclase exhibits coarse perthite textures. 

Additional images
  • biotite granite - width 4.3 cm
  • orthoclase in biotite granite - width 6 cm
  • biotite granite - width 21 cm
50.3986, -4.7409
Luxulyan, 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: