Altered granite with tourmaline and cassiterite vein
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Fact sheet

Altered granite with tourmaline and cassiterite vein

This sample comes from reclaimed land near the village of Trethurgy, close to the giant Carclaze china clay pit, St Austell, Cornwall. The rock is a hydrothermally altered granite cut by a cassiterite bearing tourmaline vein that formed asboron-rich late-stage magmatic-hydrothermal fluids from which the tourmaline crystallised, also carried tin. It is from this paragenesis that the entire tin industry of Cornwall is based - an industry that can trace its beginnings from Roman times or possibly even earlier.

The thin section shows a coarse grained igneous texture with large grains of quartz, muscovite mica, and altered plagioclase feldspar. Blue/brown igneous tourmaline is present in the granitic portion of the rock but the majority of the tourmaline grains are found in the vein. This hydrothermal tourmaline is darker in colour with blue/brown pelchroism and the vein also contains cassiterite (tin oxide), distinguished by its higher relief and dark brown colour, although unlike many oxides, it is not opaque.

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
Accessory minerals
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: