Troctolite with chromite layer
Collection:
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Object

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
Map
56.9948, -6.2934
Description:
Cnapan Breaca, Isle of Rum, Scotland
Precision:
Poor
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
igneous
Category
gabbro
Rock-forming mineral
olivine
pyroxene
plagioclase
feldspar
chromite
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: