Troctolite with chromite layer
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Fact sheet

Troctolite with chromite layer

This sample of troctolite comes from Cnapan Breaca on the Isle of Rum, which is a deeply eroded volcano formed in the Paleogene period. The remains of the volcano magma chamber exhibit layering reflecting the slow crystallisation of the magma as it cooled. The layers we see today formed 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. This sample comes from the uppermost part of one of 15 rhythmically layered units at the contact between units 7 and 8.

The thin section shows olivine grains apparently grading and settling, forming almost 100 per cent of the layer at one point just above a layer of smaller chromite grains (a denser and thus more rapidly settling mineral phase). Plagioclase also forms a cumulate phase intergrown with olivine. Pyroxene apparently grew late in the crystallisation sequence, filling spaces remaining between olivine grains.

Additional images
  • chromite layer - width 2.6 cm
  • chromite layer (wet) - width 8 cm
56.9948, -6.2934
Cnapan Breaca, 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
Category guide  
Category Guide
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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: