Obsidian - Sandy Braes
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

Obsidian - Sandy Braes

This obsidian glass sample comes from a small quarry in County Antrim, Northern Ireland. It is part of the Tardee volcanic complex, which in turn is part of the North Atlantic volcanic province that erupted in the Paleogene. This locality is one of the few examples of such acidic volcanic rocks in the Paleogene rocks of Ireland and contains a range of rhyolite and agglomerate rocks that were contemporaneous with local basalt eruptions. The rock chemistry implies that it formed by fractional crystallisation from a basaltic magma, although evidence from strontium isotopes indicates the magma was contaminated by local crustal rocks. The rocks are preserved as a glass rather than crystallising to a more coarse-grained rock by rapid cooling.

In thin section it is clear that the rock is not pure glass but a fine-grained matrix of glass and randomly oriented feldspar laths less than 50 microns long. The glass is isotropic (black in crossed polars), whereas the feldspars exhibit low order birefringence colours. Also visible are large phenocrysts of sanidine, a potassium-rich variety of feldspar found only in volcanic rocks.

Additional images
  • rhyolite - width 2.1 cm
  • rhyolite - width 8 cm
57.448484, -6.589739
Sandy Braes, Co. Antrim, N. Ireland
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.
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: