Two mica granite - Rosemanowes
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

Two mica granite - Rosemanowes

This two mica granite sample comes from a British Geological Survey borehole that was drilled at the Rosemanowes quarry borehole, Cornwall as part of the 'Hot Dry Rock' project that ran from 1977 to 1980. The borehole penetrated the Carnmenellis Granite to over one kilometre depth. The site was chosen because the granite in the area has the highest heat flow in England (120 milliwatts per square metre). The borehole did not develop into an operational geothermal exploitation scheme but the research laid the foundations for similar projects elsewhere.

In thin section the rock contains 100–200 micron diameter orthoclase (which appears mottled and altered), rarer plagioclase grains, quartz, and both biotite and muscovite are common. Biotite is distinguished by its brown pleochroism and black pleochroic haloes caused by radiation damage from zircon grains that sit at the core of each halo. The high heat flow recorded in the granite is the result of natural radiation present in these small grains producing heat throughout the granite.

Additional images
  • granite - width 3 cm
50.1681, -5.1717
Rosemanowes quarry borehole, Cornwall
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: