New British Update 01 March 2022

1 March 2022

TODAY, March 1st, we mark Saint David’s Day and extend a particular warm welcome to all our Welsh friends.

Saint David is the patron Saint of Wales and this Feast Day commemorates his death on March 1, 589. He was canonised in the early 12th century by Pope Callixtus II and many will be wearing the traditional Welsh emblems of the daffodil or leak with enormous pride. To honour this we feature three fine specimens from Wales among today’s British selection, two of which are from their type localities in the county of Gwynedd.

Brookite is one of the five polymorphs of titanium dioxide and its type locality is at Twll Maen Grisial near to Prenteg in Gwynedd. Polymorphs have the same chemical formula but form differing crystal structures due to varying depositional conditions. Brookite’s polymorphs are Anatase and Rutile and the less well known species Akaogiite and Riesite. North Wales is rich in titanium minerals, evoking fond memories of many a collecting trip back in the 1980s and 90s. Collecting at many sites is now banned as they are classed as Sites of Special Scientific Interest (SSSIs).

Our next Welsh type locality is on the slopes of the highest mountain in Wales, Snowdon, or in Welsh, Yr Wyddfa. Snowdon’s summit at 1,085 m (3,560 ft) overlooks the stunning scenery of the surrounding Snowdonia National Park. Snowden itself is composed of Ordovician volcanics and on its slopes by Llyn Llydaw lake is the Britannia mine, also known as Snowdon mine. This old copper mine dates to at least 1800 and is type locality to Lanthanite-(Ce), a rare cerium-lanthanum-neodymium carbonate. The specimen we have is a truly beautiful example, aesthetically enhanced with a partial coating of sea-green Malachite.

Last but by no means least of our Welsh triumvirate is a Marcasite from the famous Parc mine at Llanrwst in the Gwydyr Forest area, east of Capel Curig. No doubt many of you will own or know of the excellent seven volume work by Bennett and Vernon, Mines of the Gwydyr Forest, with Parc mine featuring large in Part 3. The Marcasite forms perfectly developed crystals of brassy metallic gold over a gently mounded dome of Calcite, over which several small clusters of milky white Calcite crystals are scattered.

Out of Wales we head south along the M5 and A30 into Cornwall, where we have a selection of Cassiterites plus the odd rarer Cornish locality. The Cassiterites come from Cligga mine near to Perranporth; Trevaunance mine at St Agnes and the old Wheal Bounty section of Wheal Pendarves at Killivose, south of Camborne. I find two of these just as fascinating for their matrix as for their Cassiterite. That from Cligga is a great example of tin mineralisation in a heavily greisenised matrix, rich in Muscovite mica and altered Feldspar. Cligga Head is famous for its surface exposure of greisen veins, a process in which hot fluids (gases or liquids) expelled from the granite melt cause alteration in the already solidifying surrounding granite. Such mineralisation and deposits are termed pneumatolytic.

The Cassiterite from Trevaunance mine at St. Agnes shows an excellent cross-section of a mineralised vein structure, typical of many Cornish mines. Detailed illustrations of such veins may be seen in H.G. Dines’ classic work The Metalliferous Mining Region of South-West England. This particular specimen is composed of sub-parallel veins of Cassiterite, Quartz, Feldspar and bluish-black tourmalinised Quartz know by Cornish miners as Peach, which widen at one point in a boudinage (sausage-like) structure.

Other Cornish goodies include a fine small miniature Siderite from the seldom encountered Bucklers mine at St Austell, a very old mine which was worked primarily for its tin and copper. Although now a discredited mineral, Andrewsite remains a Cornish icon and famously featured as the front cover photo on Embrey and Symes Minerals of Cornwall and Devon (1987). Andrewsite is now determined as a mixture of the iron phosphates Hentschelite and Rockbridgeite with the presence of minor Chalcosiderite. On this specimen it forms a micro-botryoidal, black to very dark green velvety surface adjacent to emerald green Malachite.

Venturing out of Cornwall, we may only have one specimen from the Southern Pennine Orefield in Derbyshire, but what a little beauty it is; a Galena with Sphalerite and Quartz from Millclose mine at South Darley, a little to the northwest of Matlock. Metallic grey, cubic Galena crystals to 1.2 cm display intriguing developments of the cuboctahedral habit. Larger crystals are dominated by the cube, with tiny bevelled corners representing the octahedron. Alighted epitaxially to many of the larger prism’s corners are cuboctahedral Galena crystals with the emphasis on the octahedron, the opposite to their larger counterparts. This is a remarkable miniature and it is little wonder the late Richard (Dick) Barstow had it in his collection, whose white label accompanies the specimen.

Just some of the other specimens not already mentioned are a Pyromorphite from Balliway Rigg in the Caldbeck Fells; a warm mauve Fluorite from Boltsburn mine; apple green Fluorite from West Pasture mine in Weardale and an impressive cabinet specimen of lenticular Calcite overgrowing complex Fluorite epimorphs from Cambokeels mine.

British minerals are always of tremendous interest to so many of our followers and we hope this remains the case today. We are delighted to include the three Welsh specimens and amongst the 21 maybe one or more are waiting to fill important gaps in your collection. On Friday we extend the old Entente Cordiale as we cross the English Channel with a selection of 21 magnificent specimens from France. PT


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Author: JH
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