How to find semiconductors in Cornish mine waste

On the right is an image of a typical Cornish mine dump, this one is at Wheal Prosper, St Hilary. Its been cut through to provide access to a field, and you can clearly see how layers have been deposited - perhaps from different areas of the mine, or different depths, containing different minerals.

The art of collecting minerals from mine waste is known as "fossicking", scanning the surface workings and locating rocks that may contain crystals of interest. We have been running workshops for some time in Cornwall where we take local families out to find crystals. Often the people we do this with have family history in Cornish mining, with plenty of stories to tell and knowledge of the sites we are going to.

A very important resource is mindat.org which contains details of many thousands of mines in the area. I often provide photos and information to this site, you can see the specimens I have collected here.

What to look for

Most often in the case of searching for semiconductors, we are looking for metallic mineralisation - the most abundant metal is iron, which is often associated with rarer metals. We are looking for heavy, red or yellow iron stained rocks - often coated in goethite, which is one of the minerals that comprises rust.

Many of the mines here were working copper, which is also very abundant, mostly in the form of chalcopyrite. The copper oxidises and decays into green and blue minerals like malachite and chrysocolla, some of these secondary forms, such as chalcocite (or grey copper) are also great semiconductors themselves.

The other material that Cornwall has become associated with is tin, which was mined for centuries before copper became important. The principle ore of tin is cassiterite which can also be found in the waste dumps. It is extremely heavy (six times the mass of water) and is also a semiconductor.