Finding your own semiconductors

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. Usually the most common metallic shiny mineral you'll find is pyrite or fool's gold, this is a semiconductor - particularly the badly decayed and not well crystallised forms of it.

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. These are very colourful but not useful for our purposes, however some of these secondary forms, such as chalcocite (or grey copper) and cuprite 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.

Another element that is very common is arsenic, which binds with sulphur to form arsenopyrite or more rarely oxygen to form löllingite. These are both great semiconductors.

Lead is fairly common also, in the form of galena and it's many secondaries. Galena is often found in small patches and veins, material that is courser and richer that can be separated to use with a cat's whisker can be more tricky to find.

Other slightly rarer semiconducting minerals include wolframite (tungesten ore) which has some interesting properties such as negative resistance.

It's worth mentioning that while quartz has its own particular place in electronics history (not least in the extraction of silicon) it is not a semiconductor, and not useful for our purposes here.