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Fact sheet


Arsenopyrite contains arsenic, iron and sulphur. Originally it was a troublesome impurity mixed in with the tin and copper ores. To remove the arsenic and sulphur, the mixed ores were roasted at the mine site. The unwanted arsenic was driven off into the atmosphere as toxic arsenic trioxide gas. However, in the early 19th century new uses for arsenic were identified in pigments and pesticides. In order to capture the now valuable arsenic, labyrinthine flues were attached to the mines’ furnaces to cool the escaping arsenic trioxide and force it to be deposited as a solid white powder that could be collected and sold. Arsenic became the most important by-product of the mining industry in Cornwall with over half the world’s arsenic being produced by the 1870s. 

This specimen of unusually coarse crystals of arsenopyrite, is from St Agnes and was acquired by the Royal Institution of Cornwall before 1881.

Chemical formula: FeAsS

Specimen no. TRURI: 801.1585
Location: St Agnes
Grid Reference: SW 72 50


Additional images
  • Arsenopyrite cluster 8 cm across
  • Arsenopyrite cluster 8 cm across
  • Arsenopyrite cluster 8 cm across
  • Arsenopyrite cluster 8 cm across
50.312996, -5.203099
About this collection

This Collection focuses on Cornwall and West Devon’s mineralogical and mining heritage.  The specimens it features are drawn from the collection of the Royal Institution of Cornwall (RIC) held at the Royal Cornwall Museum (RCM). 

This collaborative project involving the RCM, the Cornish Mining World Heritage Site and The Open University explores how access to the RIC’s mineral collection and the stories it can tell can be widened using digital technology.  It includes radioactive minerals from Cornwall that would otherwise be inaccessible to the public for health and safety reasons.

Sample details

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.
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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: