Porphyritic basalt - Dunbar
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Fact sheet

Porphyritic basalt - Dunbar

This altered porphyritic basalt sample is from The Battery, a ruined gun emplacement built towards the end of the 1700s during the American War of Independence, at the entrance to Old Harbour at Dunbar. It became disused by the middle 1800s and was subsequently re-used as a hospital. The basalt is Carboniferous in age with 1-2 mm sized plagioclase phenocrysts and a fine grained groundmass.

The thin section shows that plagioclase phenocrysts are mostly altered to sericite, and groundmass contains altered grains of finer-grained plagioclase, altered biotite and altered glass. Another mafic mineral (originally augite or amphibole) is also present in the rock, but this is now totally replaced by green chlorite. The thin section also exhibits common flattened vesicles containing small euhedral quartz grains often at the margin, and a filling of carbonate cement.

Additional images
  • close up of porphyritic basalt
  • Hand specimen of porphyritic basalt
  • close up of porphyritic basalt
  • close up of porphyritic basalt
56.0056, -2.5115
The Old Harbour at Dunbar
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: