Bassett and Grylls exotic semiconductor field trip

The second search for exotic semiconductors was at Bassett and Grylls mine in Wendron. This is a very different location to the Porthtowan trip as it was a tin, rather than copper mine. It is well overgrown but still with quite of a lot of visible infrastructure from ore processing activities that went on here.

This location was initiated by a discovery that hematite I'd found from here previously seemed to have a low resistance in contrast to hematite from elsewhere, so I wanted to see if I could find some more usable quantities for sound making. It's also a site rich in pale brown cassiterite. I also wanted to see if there was any magnetite present here, as I'd noticed magnetic material in the stream previously when searching previously for alluvial cassiterite.

Any remaining waste piles at this site have either been flattened or are completely overgrown. Due to this and the infrastructure present I only look at loose rocks on the bare surface and on the paths across the site. There is a lot of industrial process waste (slag) in the ground here, a lot of coal dust too - which coats everything and makes it look like there are crystals everywhere.

Rocks collected in this trip (there was another being cleaned in the ultrasonic bath when I took the photo). The black veins consist of tourmaline - which often hosts more minerals. The red rock on the left side in the middle is a piece of slag with a big bubble in and a patch green copper secondary minerals visible.

Using a magnet I found an extremely magnetic, seemingly crystalline dark rock - which I was pretty happy about as it resembled magnetite. Later on testing it, it even provided weak but detectable semiconducting properties with extremely small pyramidal crystals on an exposed area. However when I tried to split it to expose more, I discovered a malleable metallic substance - which means it almost certainly originates from the smelting waste found here. It's quite possible that the crystals are in fact magnetite, just artificially created material - but that is outside of the scope of this project (we are only interested in 370 million year old semiconductors).

As is sometimes the case, it wasn't until a few days later when I started looking more closely at what I'd collected that I started to find more interesting things.

The blue shiny bubbles in these cavities are botryoidal (or spherical) goethite. The cavities (or vugs) are mostly rectangular in the surrounding quartz, and will have been made by pyrite crystals that have decomposed into the goethite, which is essentially rust! I'm not sure what's causing the iridescence, but I think there is some chalcopyrite present which may be the source of some copper secondary minerals that might be the reason.

Splitting a rock that had bands of tourmaline and quartz I had a lucky find - the break was across a vein of specularite, much richer than the rock I found before but less well crystallised.

Although most of the crystals are very small - it's a sparkly rock, and there are some larger plates present. The field of view of this image is about 1mm, but the structure of the specularite is quite visible:

The resistance across most parts of this vein is in the order of 500K to 1M Ohm, and it seems to be in-between in terms of the hematite I've found previously from Tywarnhayle mine, which is not shiny at all and has no electrical conduction. Some areas do not conduct enough to get a trace, but some of the more reflective areas are - just about.

Compared to the previous example I found at this location:

It seems it may have a similar curve shape (which is unusual compared to other minerals) but it has a much higher resistance. The bottom one is half the sensitivity (20mV) of the top graph (10mV) which is the maximum my oscilloscope will do. I used silver cat's whiskers as they are more conductive, but I might experiment with some other materials. I was lucky though to find the mineral I was looking for on this trip, even though it took a few days to find by going through the few rocks I picked up.