Garnet-biotite augen gneiss
Collection:
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Microscope
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Fact sheet

Garnet-biotite augen gneiss

Darwin wrote: “Mica, quartz and feldspar (porphyritic with large crystals of do.) arranged in plane; containing garnets and joining to 467. (It is a true gneiss, a most beautiful rock).”

The rock is a garnet-biotite augen gneiss. Some individual augen have cores of plagioclase feldspar mantled by quartz. Other augen are entirely quartz.

Additional images
  • Garnet-biotite augen gneiss
  • Garnet-biotite augen gneiss
  • Garnet-biotite augen gneiss
  • Garnet-biotite augen gneiss
  • Garnet-biotite augen gneiss
Map
-22.908333, -43.196389
Description:
Rio de Janeiro, Brazil
Precision:
Moderate
About this collection

This collection was a collaboration between The Open University and the Sedgwick Museum, created in 2009 to celebrate the bicentennial of the birth of Charles Darwin.

The Sedgwick Museum opened a new gallery, 'Darwin the Geologist', and created a museum-based virtual microscope to showcase rocks he collected during the Voyage of the Beagle. We enjoy this collection because Darwin did not always pick up a representative sample from the islands he visited; it's often the unusual rocks that caught his eye. Just explore the collection and you'll see what we mean.

Darwin collected some 2000 rock specimens on the voyage, under difficult conditions exacerbated by 'perpetual' seasickness. These specimens were gifted to the Sedgwick Museum after Darwin's death by his executors. Thin sections were made from these hand specimens in the early years of the Twentieth Century by Alfred Harker, a pioneer of thin section petrology. He curated a collection of 42,000 rock specimens, as well as numerous thin sections, from the 1880s up to his death in 1939: these specimens form the core of what is now known as the Harker Collection. Harker published 'Notes on the rocks of the "Beagle" Collection' in the Geological Magazine in 1907. He also made a handwritten copy of the catalogue (omitting the sedimentary rocks!); you can view the Contents list and a rationale for the catalogue on the Darwin Online site.

For those who have the opportunity, a visit to the museum is strongly recommended. See the Sedgwick Museum website for more information.

Sample details

Type
metamorphic
Category
augen gneiss
Rock-forming mineral
garnet
biotite
quartz
plagioclase
feldspar
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: