Monday 9 December 2013

There's Bismuthinite in Them Thar Hills - Filiform Crystals, Cambokeels, Weardale

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One day, when Cambokeels mine, Weardale was still in operation in the 1980's, my Dad & I spent an hour or so on the spoil-heaps, before going up to Heights mine. We found some nice chalcopyrite crystals and light green massive fluorite which was light sensitive and lost its colour as you watched. I remember picking up a 5 x 4 x 2 cm drusy quartz (1mm points) which was pretty, in a sparkly sort of way, but not of any great interest. This specimen languished in my collection, unappreciated, until a week ago.

A friend had mentioned that his niece was interested in minerals, so I began sorting through my rocks for a small collection to give her. I'd recently bought a stereo microscope and had a quick look through the specimens I'd put together. The Cambokeels drusy quartz looked quite spectacular under the scope, but the underlying quartz matrix proved much more interesting. Fine grained, almost chalcedony-like in parts, with cavities a few millimeters across having minute, but beautiful clear quartz prisms, some doubly terminated. Then I noticed the presence of filiform needles of what I assumed was millerite, a mineral I was acquainted with, from its rare occurrence in the Namurian shale ironstone nodules at Coldberry Gutter, Teesdale. However the crystals' appearance and paragenesis was different - in that they were a steel grey/blue, as opposed to brassy millerite, and they were included inside the quartz. Millerite in ironstone nodules was the last mineral to crystallise - you can open a nodule and a breeze can blow the crystals away, I lost my best millerite spray some time ago, collecting when the Gutter was acting as a wind tunnel! The very fine needles in the Cambo specimens had formed before the quartz, were present running through the massive quartz, individual clear quartz prisms and free standing to a couple of millimeters into tiny cavities.

Referring to the Cambokeels Mine page on Mindat, I found no mention of millerite, but bismuthinite is noted (no photos though), and then I found this fascinating article in Mineralogical Magazine -

Bismuth-bearing assemblages from the Northern Pennine Orefield, by R.A.Ixer, B.Young & C.J.Stanley. Mineralogical Magazine, April 1996, Vol 60, pp. 317-324. Online here.

The descriptions of the bismuthinite crystals, and associated unidentified bismuth mineral globules, was exactly as I saw in the specimen I'd been about to give away! -

"Cambokeels Mine free standing acicular crystals up to 1 mm long grow out from the surfaces of euhedral quartz crystals lining vugs in quartz-rich veinstone." 

Soon I'd reduced this specimen into fragments, and I now have some lovely micromounts of a mineral I'd never before collected, and hadn't even realised was present in the specimen, or indeed Weardale!

I replaced the pretty drusy quartz gift (which I'd bashed to bits) with a better looking specimen (to the naked eye) from Heights, so my mate's niece didn't lose out. It was a win - win situation, especially after I sorted through the dust and tiny fragments from my reduction frenzy, as I found a perfect chalcopyrite octahedral crystal and some doubly terminated quartz floater assemblies (one with an included bismuthinite crystal) of great beauty. 

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Unrelated to this post, below is an example of
eclectic science esoterica 

3 microsecond exposure photograph, taken milliseconds after the detonation of  a 10+ kiloton nuclear fission bomb. The spikes protruding from the shock front of the fireball are guy ropes (which stabilised the tower) vapourising due to the light. The effect is known as the "rope trick."

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