Riebeckite microgranite
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Fact sheet

Riebeckite microgranite

The Isle of Ailsa Craig lies in the Firth of Clyde, Scotland. It is the plug of an ancient volcano, formed during the Paleogene period during the eruption of the North Atlantic volcanic province. Ailsa Craig is composed almost entirely of one rock type: a homogeneous fine-grained microgranite that has made Ailsa Craig famous as a source of curling stones. The stones from this island came to prominence in the early 1800s when it was recognised that the very hard, fine-grained microgranite has a texture that resists fracture and water absorption, making it ideal for curling stones.

In thin section the sample is very fine-grained for a rock characterised as a granite. The thin section contains small grains of riebeckite (a blue-green amphibole) and arfvedsonite (a blue-green amphibole). Both minerals are abundant and visible in the thin section but the grains are too small to allow the user to recognise the key discriminating indicator of cleavage angle. The rock also contains quartz, plagioclase and orthoclase feldspar.

Additional images
  • coarse variety - width 5 cm
  • microgranite - width 2.4 cm
  • coarse variety - width 5 cm
  • microgranite - width 7 cm
  • coarse variety - width 10 cm
  • coarse variety - width 10 cm
  • microgranite - width 2.7 cm
55.2539, -5.1161
Ailsa Craig, Firth of Clyde, 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: