Ode to Arsenopyrite

This is from Cornwall life. Cornwall is associated with mining. Walk around and manifestations of mining are frequently met.

I saw, puzzled what it was and meant, and got explanations. This is about my journey of knowledge on one aspect of minerals, geology and mining.

To you I'm explaining first-off what it proved to be...

Arsenopyrite [Wikipedia] is the mineral iron arsenic sulfide (FeAsS).
This article goes on (quoting):
"When arsenopyrite is heated, it produces sulfur and arsenic vapor. With 46% arsenic content, arsenopyrite, along with orpiment, is a principal ore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, the mineral slowly converts into iron arsenates."

One of these things I was seeing; part of what got my curiousity.

The path of observation, enquiry and investigation...


My first encounter.
What is this iron'y yellow stuff?!
Also seeing some instances of it having stuck-together and formed big lumps - seemingly having accreted right there from fines tipped there... (not captured in these pictures).

Speaking solely about Cornwall which is all I have observed;
tin mines are surrounded by granules of hard white (quartz - yes) and similar-sized darker also hard material. Whereas Tywarnhale was my first encounter that copper mines are surrounded by very iron'y material.

Mental note made, but "parked".

Wheal Charlotte approach from landward

That "cement-like" line - what is it?!
It's not cement - that's obvious on close inspection.
It was as if whatever it was had accreted to this hard material under where the wheels of wagons/vehicles had rolled.
(again letting you in on the secret - that proved to be correct)

I asked around and had joined the local mining society by then.
It's thought the explanation is this - while the mine had been abandoned since about 1850(?), the area was used for an Army base in the Second World War (WW2) and it is thought a track was bulldozed. The arsenopyrite newly exposed to the air and compacted by tyres of trucks accreted to what is seen.
Caveat: this is the theory...

Wheal Charlotte tailings dump

This shows how, in about 170 years, the tailings remain barren of vegetation.
The arsenic content prevents the growth of plants?

West of Porthtowan, near Wheal Tye

These pictures capture the ability of arsenopyrite dumped as fines to accrete into a quite hard strong material.
This sulphide ore since depositing just less than 300million years was never exposed to oxygen until mined.
Comminuted (broken into fine material - historically by stamps (like a mechanised mortar-and-pestle)) to free the metallic mineral to be recovered, the discarded "tailings" material has a lot of surface area to react with oxygen. Which is does; one result seen here of agglomeration.

Again, what is seen is nothing growing on the arsenopyrite. The small size and large number of shafts nearby suggests "early" mining. Wary of writing my guess centuries since abandonment. But emphasising that absence of vegetation is for systematic reason. Presumably of the suphide'y arsenical conditions.

so to close the commentary

I have become more able to recognise various geologies now.

I do feel quite some awe coming upon this barren arsenical / arsenopyrite tailings dumps.

Couple of observations:
* people lead complete lives, little documented, putting food on their tables doing this mining, while begetting future generations
* the Royal Navy gained advantage sailing the world enforcing the British empire with wooden sailing ships lined with copper from mines like these - marine organisms cannot foul copper-sheated hulls, so the ships slipped faster through the water

I can't vouch for historical accuracy, but I see both things.
I feel put in contact with our forebearers.

(R. Smith, 04Apr2024 to 05Apr2024)