East Wheal Towan exotic semiconductor field trip
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One of the tasks I've been funded to do is "Researching mine history and searching mine sites for semiconducting material", and for the first trip of this project I decided to head to the north coast to check out a few places:
These are all within a short walk of each other near the cliffs between Porthtowan and Chapel Porth beach.
And on the way back:
The reason for these choices are that there are many semiconducting minerals that are very commonly found on the mine waste dumps in Cornwall, but I've been wanting to expand the possibilities a little. There are two that I've tested in the past but haven't been able to find enough of to use much.
- cuprite a red transparent copper oxide - common is some copper mines but not others, I think this is to do with the temperature of the mineral formations. I've found well formed but tiny specimens before, but the Porthtowan area seems to have larger amounts of less well crystallised material - perfect for use as natural semiconductors.
- wolframite an ore of tungsten, which Nangiles mine was used for mining at one time. I've seen negative resistance in this mineral before, which has potential to build more interesting circuits.
East Wheal Towan
East Wheal Towan turned out to be quite abundant, I think because it's a minor site that not many people have been to (added to the fact it's surrounded in very prickly gorse I had to carry the dog over). Most of the minerals I found had very coarse crystallisation with nicely formed faces.
This is a specimen I found with arsenopyrite, galena and sphalerite all hosted inside a matrix of well formed quartz and green chlorite. None of these are rare, but it's satisfying to find all of them in one small sample - and given that previously there were no confirmed minerals listed on the mindat.org locality it means I could add them all in one go.
The sharply defined silver crystals are arsenopyrite, the darker clusters on the top left and lower middle are galena and the grey mass on the right above the vein of arsenopyrite is sphalerite.
I also found plenty of good material for some semiconductor curve traces, firstly a small piece of sharply defined milky quartz with bright chalcopyrite that had survived weathering in between the quartz faces. This is quite noisy, as is common with chalcopyrite and pretty leaky but with a defined corner where it start conducting well over 1V:
There was also a lot of arsenopyrite as usual, this was from a mass similar to the photo above, and it behaves pretty strangely - it is very tricky to find a junction, and when I did it was quite extreme. I've never seen arsenopyrite have such a sudden threshold as this:
There was also quite a lot of galena, I took some curve traces from a thin vein which has broken away so you can access the galena on multiple sides. Testing it with a multimeter it's electrically connected all the way through (registering a few Kohms across the piece). This provides another unusual curve - like an exponential, usually galena has clearer thresholds. I'm not sure if this is something to do with the thin section, or perhaps its interaction with arsenopyrite which is also present. The interaction of different semiconducting minerals that have grown together is another avenue of research I haven't managed to follow, perhaps looking closer at the Perikon diode may lead this way.
So this is plenty to make some music specific to East Wheal Towan, and it does seem to have some interesting peculiarities to explore.
Charlotte United Mine
I spotted where I think this must be from the footpath, a small cluster of trees often seems to mark the location of a mine shaft - often trees seem to like growing right on top of backfilled shafts, I'm not sure why. Unfortunately it would have meant another trip through even deeper prickly gorse - and it was much further away from the path, so I decided to give this one a miss and press on.
Great Wheal Charlotte
The trail to Great Wheal Charlotte is lined with what I would guess possibly are the remains of a tramway that would have been used to carry away the ore - densely packed heavily mineralised ironstained rocks sit where the tracks would have been. The remains of the engine house (the chimney has fallen away relatively recently) are easily visible from some distance, and the mine waste dumps are pretty extensive.
Although a much larger site, it's quite hard to find mineralised rocks - one indicator is the red colour of the area in the photograph, this is coming from the copper, cuprite in fact. There was also a lot of malachite here, bright green spots and veins in white quartz.