Topazfels in hornfels
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

Topazfels in hornfels

This sample is part of a sedimentary xenolith that became incorporated into the late stage intrusion at Castle-an-Dinas, St Austell, Cornwall. It derived from the main St Austell granite, which was intruded during the Permian period, and was probably the result of an intrusion of the most evolved fluid remaining in the magma chamber at the end of a sequence of crystallisation. Castle-an-Dinas was the site of widespread mineralisation and was once home to Cornwall’s largest tungsten mine.

The thin section exhibits three distinct areas. The left-hand portion was originally fine-grained sedimentary layers that would have contained clay and detrital minerals such as quartz, but an influx of boron-enriched fluid has converted it to bands of tourmaline and quartz. Below the layered section is a quartz vein, and to the right of the tourmaline-rich zones is a sample of the intruding body, which crystallised to form a mixture of topaz, quartz and some small tourmaline grains.

Additional images
50.4279, -4.8926
Castle-an-Dinas, 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: