An Ancient Cornish Specimen of "Cubical Copper Pyrites" from Dolcoath

Old mineral specimens can acquire fascinating veneers of history in addition to their scientific and aesthetic interest.

Shown here is one such example; not beautiful on the face of it, but full of interest, nonetheless.



An ancient mineral specimen from the Dalcoath Mine, Cornwall, about 7 cm across. Large chalcopyrite crystals, to 35 mm across, grow on a bed of quartz crystals, each to 3 cm long. Two generations of the iron carbonate siderite later partially encrusted both quartz and chalcopyrite, the last generation being a "dusting" of microcrystals.



On the evidence of its oldest attached label, the specimen was probably mined around two centuries ago. How can we tell? The oldest of several labels describes the specimen as follows:


"Black Oxide of Copper 

coating Cubical Copper Pyrites and 

Carbte of Iron on Quartz

Dalcoath Mine

Cornwall"


The old handwritten label accompanying the specimen illustrated above.


"Black oxide of copper" is a synonym for tenorite (though in this case we may be looking at a sulphide), copper pyrites is chalcopyrite and carbonate of iron is siderite.

The word "cubical" seems very out of place here, but could make perfect sense when we consider that chalcopyrite was thought to crystallise in the Cubic crystal system until its true tetragonal nature was recognized by W. Haidinger in 1822 (On the Crystallizations of Copper Pyrites. Memoirs of the Wernerian Natural History Society Edinburgh. Volume IV, p.1).


The first page of Haidinger's 1822 redescription of the crystallography of chalcopyrite.



Sowerby (1804, British Mineralogy, Taylor & Co., London) is a wonderful source of illustrations of British minerals collected in the late 18th and early 19th centuries. He notes (pg. 159) that British chalcopyrites "appear to be always inclined to tarnish, very often assuming a coat, either of the colour of blued steel, or blueish black; and it often has the green patina, or oxide of copper, on the surface, which count Bournon speaks of in his description of yellow copper; Phi. Trans. for 1801."



An illustration from Sowerby (1804) showing similar Cornish black-coated near-tetrahedra of chalcopyrite on quartz (TAB. LXXVIII, see pg 161).



The spelling of "Dalcoath" seems to precede that of the modern Dolcoath, but (I found, on searching old newspaper articles) was used as late as the 1920s. There's a lovely story about Dalcoath in the "Mining Journal" from 1847 as follows:

"The Three Conjurors.—At the bottom shaft of the Dalcoath mine (one the deepest known), professors Airey, Whewell, and Sedgwick were desirous to try some experiments where they could swing the pendulum at a great depth, and also at a considerable elevation, and had their apparatus lowered to the bottom of the shaft, and, after spending a portion of two or three days there, it was packed up in an iron bucket or kibble, with shavings, into which a spark from miner's candle must have fallen, for, the midway up the shaft, the shavings took fire, the rope was burned, the bucket fell to the bottom, and the apparatus was destroyed. The miners declared that nothing could have burned the rope but the devil, and that the three professors must be magicians. The day after, there was a violent storm, and these miners were overhead saying they were quite sure it was owing to those men that had been underground ; and one said it must be the little 'un (Professor Airey), because he see'd him standing with his back against 'the churchyard !"


So, sometimes a mineral specimen becomes a historical artefact in addition to its scientific interest.

If anybody recognises the handwriting on this label or is familiar with other early specimens from Dolcoath showing this paragenesis, please do let me know.






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