Garnet gneiss - Roneval
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Fact sheet

Garnet gneiss - Roneval

This garnet-rich, high-grade Lewisian rock comes from Roneval on South Harris in the Western Isles of Scotland. The rock was subjected to deep burial and temperatures exceeding 800 °C during the Proterozoic Laxfordian orogeny. The origin of this rock is difficult to determine but it may have been a granite intruded into the Archean Lewisian prior to regional metamorphism. 

In thin section the rock is dominated by garnet, amphibole and plagioclase feldspar, with some scapolite. The garnet grains are surrounded by particularly good examples of symplectite (an intergrowth of two minerals at a scale of a few microns formed when another mineral breaks down) texture, and are strongly denuded by reactions that occurred as the rock travelled from its deepest point towards the surface, cooling and decompressing to form plagioclase and pyroxene (later altered to amphibole).

Additional images
  • Garnet gneiss
  • Garnet gneiss - width 3 cm
57.7607, -6.9771
Abhainn Thorro, SW side of Roneval (=Roinebhal), South Harris, Western Isles, Scotland
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: