New South West England Update 05 April 2022

5 April 2022

THIS week we remain within the British Isles, focusing on two of its best known mining regions and consequently two areas which have produced prolific numbers of fine specimens.

Today we concentrate on South West England, principally the county of Cornwall with a smattering from Devon and Somerset. Referring to mindat, Cornwall can boast 770 mineral species with a recorded 1882 localities, a pretty good statistic for a county ranked 12th in terms of its area within England. In comparison, Devon records a still impressive 421 species followed by Somerset with 130.

The driving force behind the mineral wealth of Cornwall and Devon is the Cornubian batholith, a huge body of granite underlying Devon and Cornwall, then extending out beneath the Atlantic, finally manifesting at surface as the Isles of Scilly, then beyond for yet another 100 km (62 miles). Each granite mass marks an individual pluton rising up from the batholith and although all are composed of granite, the granite composition changes markedly in terms of chemistry, mineralogy, texture and structure. Across the mainland, five granite outcrops dominate, the largest being Dartmoor in Devon. Moving into Cornwall and then heading south west we encounter the Bodmin Moor granite followed by the St Austell, Carnmenellis and finally Land’s End. Many other smaller plutons crop out between the major bosses, the largest being the Tregonning-Godolphin granite west of Land’s End, which we sometimes feature with specimens from the Megiliggar Rocks. In this update we feature a specimen from Hingston Down quarry which works a very small pluton outcropping between Bodmin Moor and Dartmoor, and adjacent to Kit Hill, yet another tiny pluton.

Before we study some of today’s specimens, let’s mention two features of the Cornubian batholith often overlooked, beginning first with relative ages of the plutons. It is all too easily imagined the batholith and plutons were all emplaced in one fell swoop; not so the case. Radiometric dating has now established a progressive sequence of pluton development beginning with Hemerdon at around 290 Ma and ending with one zone of the Land’s end granite at around 272 Ma. And don’t be deceived from this that the granites become younger towards the south west. The ages of individual plutons follow in no geographical sequence. Another now recognised misconception is that large globules of molten granite slowly rise through the country rock simply by piecemeal stopping. The buoyant granite actually follows structural pathways such as faults and fractures, both pre-existing and through new fracture systems created by the dynamic stresses surrounding the granite melt. The end result of these infinitely complex emplacement mechanisms is a vast diversity in the resulting mineral deposits we adore so much.

A firm favourite amongst collectors of SW England minerals is Cassiterite, so let’s begin our review here as we have three. Two are from St Agnes, yet another small satellite pluton and one from the little known Roskear China Clay Pit at St. Dennis. Of our two St Agnes Cassiterites, one has no specific mine attributed whereas the other is from Wheal Kitty. The former displays well-formed and relatively large lustrous Cassiterite and Quartz crystals covering an iron stained thin plate of Quartz veinstone. The Cassiterite crystals measure up to 1.3 cm, are deep clove brown to black and once back-lit become a translucent rich clove. From my knowledge, the Roskear China Clay Pit is a somewhat enigmatic locality as I did not know of it and can find no reference to it. However, the specimen is clearly labelled and the alteration in the granite proves this to be from the St Austell district and not the Roskear section of South Crofty, east of Camborne. Some small yet well-developed black Cassiterite crystals together with translucent clove brown, naturally etched Cassiterite nestle in small vugs along the ridge-like crest of this small cabinet specimen.

The Calcite from the Levant Section of Geevor mine at Pendeen close to St. Just is really beautiful, forming delightfully aesthetic folded, sinusoidal sprays and rosette’s, sometimes named “Levant Roses”. A bed of Quartz veinstone is covered with groups of curved, buff-tan Calcite crystals covering Quartz crystals of up to 1.5 cm tall, all of a dark chocolate brown with a hint of maroon, probably a very dark form of Eisenkiesel. This specimen was mined between 1969 and 1986.

More unusual minerals include an Acanthite with Native Silver on Galena, Siderite and Quartz from Wheal Ludcott at St Ive; a Connellite from Wheal Gorland; Chalcoalumite, Ajoite and Brochantite on Native Copper and Cuprite from New East Wheal Russell at Tavistock Hamlets and a lovely Bayldonite from its type locality at Penberthy Croft mine, St. Hilary, close to Marazion.

Representing Somerset we offer both a beauty and a beast! The beauty is a Hydrocerussite with Crednerite and Malachite from Torr Works Quarry at Cranmore, just east of Shepton Mallet. This quarry is commonly referred to as Merehead by collectors. Blocky crystals of Hydrocerussite display a platy stepped pyramidal habit and measure up to around 7 mm, sitting in a Calcite lined vug. Adjacent to these are metallic black, thin platy crystals of rare Crednerite and acicular green Malachite. And what of the beast I hear you cry! Well, not wishing to be cruel, this rather interesting and unique specimen is certainly no contender for Most aesthetic specimen of 2022. From Axbridge Hill in the Mendip Hills, this Marcasite-Pyrite pseudomorph after Calcite forms a partly hollow crude cone after a scalenohedral crystal of Calcite and was collected from the spoil heap of an old ochre mine.

Rounding up with a quick scan over some other specimens I see a super Chalcocite from Geevor mine; an excellent large cabinet specimen of milky white Quartz crystals pseudomorphing after Siderite from Bedford Consols near to Tavistock; Fluorapatite with Quartz from Hingston Down quarry, Calstock and a beautiful Aragonite on Quartz from Botallack mine, St. Just.

There are more, but you can enjoy exploring the photos and links to see what other goodies await your perusal. Minerals from South West England are probably the most generally sought after of any region in the UK. Not only are they beautiful (save for Somerset pseudomorphs!), every locality carries a fascinating geological setting and alluring history. Enjoy your meander through South West England with the possibility of picking up a few tasty specimens en route.PT.


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Author: JH
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