New Tsumeb Update 30 November 2021

30 November 2021

FOR both of this week’s updates we return to one of our old favourite localities, Tsumeb. Crystal Classics has always specialised in Tsumeb minerals, including our specimen recovery project at the mine in the early 2000s. Tsumeb always makes it in to any top ten list of mineral localities around the world and many would place it at the top. Personal preference aside, there is no doubting Tsumeb is one of the most complexly mineralised spots on Earth, where 44 of the 92 known elements have been concentrated.

The Tsumeb ore body is described as a near-vertical pipe descending to at least 2 km with economic mineralisation down to 1.8 km (5,905 ft). The pipe tended to by ovalized over its entire length, varying in diameter from 20 m (65 ft) to 180 m (590 ft) in diameter. The structure was formed through various geological processes, still many of which are not fully described. The stratigraphy is that mainly of carbonate rocks which, as we are all well aware, can be subject to dissolution whereby sinkholes and cave systems form. Such processes appear to have initially formed the deep pipe which was subsequently infilled with granular sand-like rock debris, possibly nothing more than eroded surface material which gradually back-filled the subsurface cave system. With this structure now in place, it was a matter of time and chance for brines rich in concentrated metals should enter this permeable and highly porous structure, with space enough for the metals to precipitate and gradually consolidate into the ore body we are now so familiar with. Read on, and we will look at some of today’s mineral specimens which formed here around 530 million years ago during the Cambrian Period.

We will start with a specimen which is most likely to be the earliest collected in this selection. This miniature displays deep rich azure blue Linarite crystals nestling in a matrix of intermixed Brochantite, Quartz and Cerussite. The Linarite occurs in several vugs around the specimen with the finest group of crystals on the top face. Linarite formed in the near-surface sections of the Tsumeb ore body, suggesting this is an old specimen mined between about 1900 and 1915.

All bar two specimens are miniatures and although none are rare species, they are all fine examples. Take the lead arsenate Mimetite. Although Mimetite was found throughout most of the mine, larger sized crystals such as this were comparatively rare. Measuring 3.8 cm tall, the largest Mimetite crystal in this tight cluster piggybacks five distinctly similar yet smaller crystals around one half of the prism body. All crystals are translucent, with bases and some terminations of delicate lemon yellow, while the majority of prism bodies are coloured jet black through finely disseminated inclusions of probable Galena. Another great example of its species is the Tarnowitzite, a lead-rich variety of Aragonite. Lustrous white crystals measuring to nearly 1 cm form a radiating hedgehog-like cluster at one end of the specimen. This is rather unusually associated with micro-crystalline blue Azurite on a vuggy Dolomitic matrix mixed with massive granular Tarnowitzite.

It would appear whatever mineral is present at Tsumeb, it occurs in its best or near to best form in the world. Strangely, this is not the case for Galena as nicely crystallised specimens are few and far between, despite Galena being one of its principal ores over almost 100 years of production. This large thumbnail has a matrix of massive Galena on top of which is an excellent Galena crystal displaying a complexly modified cuboctahedral prism measuring up to 0.9 cm. This and other smaller Galena crystals are surrounded by micro-crystals of Quartz and small dark Smithsonite crystals containing Galena inclusions.

Some really eye-catching specimens include a pale pastel, minty apple green Smithsonite with a remarkably pearlescent lustre; a glassy rib of transparent rhombic Calcite crystals tinted red by the presence of iron and an Azurite with Bayldonite and Malachite pseudomorphing after Azurite. The latter forms beautiful combinations of colours and textures and amongst the lime green Bayldonite is a small hexagonal pseudomorph of Bayldonite after Mimetite.

One mineral not known for its beauty at Tsumeb is the iron oxide-hydroxide, Goethite and hence is not always present in Tsumeb collections. But please don’t skip this bit as this example certainly bucks the trend! Superb, slightly distorted spheres to 1.4 cm diameter of semi-lustrous black Goethite form a grape-like cluster: sounding more enticing yet? These are on a mixed matrix of rusty Limonite and 1 mm rhombohedral Calcite crystals. And where some of the spheres are broken, they reveal a shell of radiating Goethite micro-crystals with a hollow interior lined with transparent glassy Calcite crystals.

Other species to look through include Beudantite, Gartrellite, Zincolivenite, Hydrocerussite, Wulfenite, Rosasite and a terrific Cuprite. For nostalgic purposes, my last mention today is of a miniature Mottramite. Dark green (almost black) to pistachio green Mottramite, forms plumes of foliated crystals in frond-like clusters. This choice example reminds me of the Tsumeb Mottramites which were often seen at mineral shows during the mid to late 1980s, the likes of which I have scarcely seen since. The lighter pistachio green plumes surround the central core of dark crystals.

No matter how many Tsumeb specimens you may have, there is always one more the like of which appears to have eluded you, so strike while the iron is hot! If you have been considering on starting a Tsumeb collection, it is never too late to begin and today’s selection provides many a fine example, with many at readily affordable prices. Enjoy exploring at what Tsumeb has to offer! PT

                                         

If you'd like to see a video of any of the specimens listed above then please contact us at orders@crystalclassics.co.uk and we'll be happy to assist you. We endeavour to respond to all enquires within 24 hours during weekdays, however at the weekend it might take us a little longer to respond.

 

CRYSTAL CLASSICS WINTER OPEN DAY

Crystal Classics are delighted to announce that our 2021 Winter Open Day will be taking place on Saturday, December 11 from 10am to 5pm.

To book your place please email Debbie at debbie@crystalclassics.co.uk or by phone 01935 862 673 to register your interest.

We have many new collections to showcase as well as thousands of superb specimens in our showroom which are set out in beautiful display cases and drawered cabinets beneath oak beams bedecked with antique crystal models and other mining and mineralogical ephemera.

Complimentary food will also be provided by The Village Cafe which is located next door to our offices.

As is standard, we will be doing a bumper British Update on Saturday morning to coincide with the Open Day, so for that week only, there will be no update as usual on Tuesday or Friday.

It promises to be a great day and we look forward to welcoming you to our beautiful showroom in East Coker, Somerset.

 

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MINERAL WISH LIST

We also offer an additional service that enables you, as our customer, to contact us via orders@crystalclassics.co.uk with your mineral wish list to enhance your current collection, but are unable to find the right specimens on our website. We have 1000's of specimens in our general stock that do not appear online.

Author: JH
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