East Wheal Towan exotic semiconductor field trip
More actions
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 a bit further away than the mines in the Gwennap area that I'm mainly been exploring and so are likely to have different mineralogy, and thus perhaps different sounding semiconducting mine waste minerals:
These are all within a short walk of each other near the cliffs between Porthtowan and Chapel Porth beach.
East Wheal Towan
East Wheal Towan turned out to be quite abundant in minerals, 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 our dog over). Most of the minerals I found had very coarse crystallisation with nicely formed (euhedral) faces.
This is a rock I found containing 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 silver arsenopyrite is sphalerite.
I also found plenty of good material here 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 running through the rock. 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 yet, perhaps looking closer at the Perikon diode may lead to some experiments around that.
This is plenty to make some music specific to East Wheal Towan in the future, and it does seem to have some interesting peculiarities to explore.
Charlotte United Mine
I spotted where I think this mine must have been from the footpath, a small cluster of trees often seems to mark the location of a mine shaft - small trees seem to like growing right on top of backfilled shafts. 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 look like they could have supported the tracks, but it might just be the remains of a road. The remains of the engine house (only one wall remains) is a good signpost, and the mine waste dumps are pretty extensive here.
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. This indicates a lot of carbonates in the rock, and was entirely absent in East Wheal Towan, about half a kilometre away.
Other than a piece of arsenopyrite from the track way and some glittery pyrite not usable as a semiconductor as it was too disseminated in the matrix, I found it difficult to find much - the one thing I did find however was quite something.
This is native copper, and it's quite rare to find in it's elemental form - this is the first time I've found any. It was in a dark druzy tourmaline rock just under the surface at the base of the dump shown in the photograph above. It's comprised of a thin layer that has solidified in a fracture of the host rock, and has nearly all converted into red cuprite and green malachite. This is the largest fragment, but in other chunks of this rock the copper has lifted away from the surface and you can bend it.
I had hopes that I would be able to get a curve trace from the cuprite coating the copper, but unfortunately it seems impossible. I'm not entirely clear why, but if the cuprite is not connected together electrically than it would make sense that this would be difficult as I have to use two cat's whiskers to connect to the small fragments. The copper itself conducts very happily of course, I include a curve trace so at least there is one on this site of a normal conductor for reference.