Great Flat Lode exotic semiconductor field trip

The Great Flat Lode is the name given to a huge vein of ore that extends for several miles underneath the area of Carnkie, south east of Camborne. Unlike most lodes in Cornwall that are more or less vertical, this one is more parallel with the surface and only increases in depth slowly. This is also the of modern mining, as recent developments at South Crofty mine (one of the last to close, in the early 90s) hope to reach a potential parallel tin lode some distance underneath the great flat lode in the next few years.

I didn't have any particular semiconducting minerals I was searching for on this trip, it was more about learning about the nature of the rock formation here. The geology is quite different to the mines further to the east I'm more used to - some of the mines here extend deeper into the earth, and the temperature and chemistry resulted in different assemblages of minerals to form. A big difference is that most of the mineralisation seem to be formed in granite rather than the killas (judging from the rock I found on the dumps). Also from these observations there seems to be much less arsenopyrite and pyrite, less iron in general (and where it does occur more likely as hematite). Miners and balmaidens moving around Cornwall must have been very aware of these differences, it's quite disorientating.

One theory I want to test is do these differences result in different using semiconductors from here?

There are also more radioactive minerals present, so I took a bit more notice of the Geiger counter. There also seem to be less recent and still exposed mine dumps here, so I wasn't sure which sites would be overgrown or even accessible, let alone if I could find anything.

South Condurrow Mine

I started to the west of the great flat lode, at Condurrow (Marshall's shaft). This site is very overgrown, but I found a path in to the engine house and a small network of paths leading on, one heading to a small area of exposed rocks falling out of the side of a dump as it weathers away.

Cassiterite

It took a bit of time here to find anything, and I didn't find much in quantity - but it more than made up for it in quality. I found a large rock that was very hard to split - comprised of fine "blue peach" tourmaline mixed with quartz. At first I didn't think it was worth the effort, but I realised it was shot through with veins of cassiterite (tin ore). There is something quite special with cassiterite in the way it reflects light, and it shows very well in this rock. It's semi transparent brown but reflecting the light in different directions makes it metallic and opaque.The photo below shows both appearances - everything except a bit of white quartz on the lower left is the same mineral. The finer crystals are the small ones, formed in vugs (cavities) in the vein, but there are good crystal forms with sharp edges up to 2-3mm in size all through the rock.

There was a large quantity of cassiterite in this rock, and I have a lot of chips of it that are a convenient size to use with cat's whiskers. This material seems to be a slightly better semiconductor than cassiterite I've found previously from further east, but it's tricky to use for two reasons, firstly it's disseminated in the matrix such that the crystals are not connected together electrically, so you have to position connections on a small area. Secondly some of it seems to be covered with some kind of insulating coating that, if you scratch it away it works better.

Two curve traces taken of cassiterite from South Condurrow, the one on the left is taken from a small crystal which is lower resistance and the one on the right is a larger crystal which has higher resistance but shows less leakage in the negative left half.

Chalcocite

This chalcocite was found in a copper green/blue stained rock lying on the surface. The smaller green crystals are brochancite, a fairly common copper secondary mineral (although only the second time I've found any). I'm always pleased to find masses of chalcoctite as it's one of the better and more consistent semiconducting minerals in my experience. This is the third location I've found usable material (after Penstruthal Mine and Wheal Prosper).

As with chalcocite I've tested before, this specimen shows undetectable leakage (which is very unusual for a naturally grown mineral) and an almost instantaneous 'switch on' voltage, which is actually a bit variable - this plot overlays 5 or 6 scans and it starts conducting anywhere from 1 to 1.5V. I've also noticed an effect chalcocite has similar to a Shockley diode, in that it stays 'turned on' until a much lower voltage is reached - I tried using a sine wave instead of a sawtooth wave to show this in the following curve trace.

What's happening here is that the crystal blocks electrical current until quite a high voltage is reached - anywhere up to the maximum of 2.5V which shows as the horizontal line on the left of the trace. Then once turned on, the diagonal part above it is the current dropping back to zero as the voltage drops back down.

East Wheal Grenville

The second site was only a short distance away, at one point part of the same amalgamated mine. However after a walk around the site it appeared to be entirely overgrown with the only surface rocks visible in the tracks here and there. It's possible there may have been some in under the tall bracken - but difficult to tell at this time of year.