Gabbro with chromite band
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Fact sheet

Gabbro with chromite band

This sample of gabbro from the Isle of Rum is a remnant from a deeply eroded volcano formed in the Paleogene period. The volcano is sufficiently deeply eroded on Rum that the magma chamber is visible in the hillsides, and exhibits spectacular layering on many scales, from metres to microns. The layering reflects the changing local composition of the magma as it crystallised, and is by far the best location in the UK (and one of the best in the world) to see such a layered magma chamber. The layers we see formed in liquid magma as crystals of different composition and settled to the bottom of the chamber. In most cases the crystals continued to grow as they settled and, following their settling, the liquid filled pore spaces, which also crystallised, resulting in complex textures that allow geologists to track the evolution of the liquid magma composition in great detail.

The magma chamber of the Rum volcano has been mapped and analysed in great detail so we know that this sample comes from the uppermost part of one of 15 rhythmically layered units at the contact between units 11 and 12 on the mountain of Hallival.

The thin section shows just how fine the layers can be. In this case, a chromite-rich seam (the opaque band to the left-hand side of the sample) lies between anorthosite gabbro (rich in plagioclase but with interstitial pyroxene) and a feldspathic peridotite (dominated by pyroxene and interstitial plagioclase).

56.9819, -6.28333
Isle of Rum, 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: