Garnet sillimanite gneiss - Isle of Harris
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Fact sheet

Garnet sillimanite gneiss - Isle of Harris

This coarse-grained, Lewisian metamorphosed granite comes from Rodel on South Harris in the Western Isles of Scotland. The rock probably originated as a siltstone sediment in the Archean, but was metamorphosed when it was subjected to temperatures in excess of 800°C during the Laxfordian orogeny in the Proterozoic period.

In thin section the coarse-grained texture is dominated by two types of layer. The first layer type is composed of garnet (slightly lower relief and isotropic) and coarse-grained sillimanite (slightly lower relief and medium birefringence colours). The remainder of the rock is a finer-grained matrix of quartz, biotite and feldspars. The garnet was highly cracked by deformation and pressure changes as the rock returned to the Earth’s surface.

Additional images
  • garnet sillimanite gneiss - width 4 cm
  • garnet sillimanite gneiss - width 13 cm
57.7421, -6.9652
Rodel, 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
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.
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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).
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We would like to thank the following for the use of this sample: