Andalusite-cordierite hornfels
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

Fact sheet

Andalusite-cordierite hornfels

This sample of andalusite-cordierite hornfels comes from the aureole of the Skiddaw granite in Cumbria, and is part of the Skiddaw Group, a series of deformed pelitic rocks that were thermally metamorphosed by the Skiddaw granite around 400 million years ago. The Skiddaw Group were originally a thick sequence of Ordovician mudstones and siltstones. Recrystallisation of the original aluminous sediments occurred during the contact metamorphism associated with granite intrusion forming the andalusite crystals that are visible in hand specimen. 

In thin section the square and elongate andalusite crystals are white in ppl and low first order colours between crossed polars. These andalusite crystals exhibit unusual crystal shapes with darker inclusion rich zones forming star shapes at the corners of the square cross sections. This variety of andalusite with carbonaceous material preferentially concentrated along crystallographic planes is known as chiastolite. Accompanying the andalusite is another metamorphic mineral, cordierite which forms indistinct, rounded crystals containing many inclusions of biotite and quartz, giving it a mottled appearance. The thin section also contains fine grained quartz, feldspar and biotite. 

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

Type
metamorphic
Category
hornfels
Rock-forming mineral
andalusite
cordierite
biotite
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: