SOME UNUSUAL POLISHED SLABS

October 26, 2020

Peter Rosewarne

Polished slabs and spheres don’t usually figure highly in my wish-list of mineral specimens but, over the years, some colourful and interesting ones have caught my eye and have been added to the Rosey Collection. This short article highlights some of what I hope you will agree are both unusual, interesting and colourful polished slabs from various localities around the world. The slabs very briefly described and illustrated herein are Sonora Sunrise and Laguna Agate from Mexico, Schalenblende from Poland, variscite from the USA, eclogite from South Africa, Condor Agate from Argentina, and prehnite and Sunset Jasper from Australia. Two more common minerals are included at the end for interest.

This article concentrates on the aesthetics of each slab, illustrated by photographs, with details of locality but only minimal information is given on mineralogy and no dimensions. Snippets of information on source areas and minerals have been gleaned from the Internet and books but not in sufficient detail to warrant a list of references.


Sonora Sunrise

This rock is also known as Sonora Sunset but for reasons that will become apparent when you read about the last ‘unusual’ slab listed in this article, I’ve used Sunrise here. It also fits with this being the opening description in this article. This is a very colourful slab of green chrysocolla separated from a central area of orange-red cuprite by a zone of black manganese oxide. It comes from the famous Milpillas Mine (famous for inter alia world-class specimens of azurite, malachite, brochantite and cuprite) in Sonora, Mexico. I acquired this specimen from Douglass Minerals, attracted by its unique colour combination.


Laguna Agate

Laguna Agate is apparently one of the most prized agates in the world owing to its tight banding and bright colours. It is found almost due south of El Paso in the state of Chihuahua, Mexico and is mined in a mountainous area associated with andesitic lavas. This piece was acquired from Hummingbird Minerals and lives up to its reputation, I think?


Schalenblende

This is probably one of the strangest looking examples of sphalerite that you’ll ever see. It’s mainly a botryoidal (colloform, reniform – take your pick) combination of sphalerite (brown and yellow – the latter possibly wurtzite), marcasite and galena (metallic grey). It’s from the Olkusz Mine east of Katowice in Poland. Schalenblende is thought to have originated from rapid crystallisation of a sulfide gel and the name is derived from the German for shale ore.


Variscite

Variscite is a light green/turquoise-coloured hydrated aluminium phosphate mineral formed by deposition from phosphate‑bearing water reacting with aluminium-rich rocks. It normally forms as nodules and a notable occurrence is Fairfield in Utah, USA. This specimen comes from the Little Green Monster Variscite Mine in that area and was acquired from Dakota Matrix Minerals. The light-coloured mineral is a calcium aluminium phosphate called crandallite and this may be the first and last time it ever gets a mention in MinChat!


Eclogite Figure 5 
The slab in Figure 5 was acquired from Richard Harrison as part of a trade for a very nice N’Chwaning rhodochrosite. It’s from a nodule in kimberlite at the Rovic Diamond Mine, now closed, near Boshof in the Free State. I’ve also got a rough piece and a polished sphere of this material but what sets this slab apart from your ‘normal’ eclogite is the presence of accessory kyanite within the primary omphacite pyroxene and pyrope garnet.

Figure 6
This can be seen as small blue patches in Figure 6. This eclogite is rather dull in comparison with some examples with bright green omphacite and red garnets, as shown in Figure 7.

Figure 7
This specimen was acquired from Jo Wicht, in a trade for a nice yellow Moroccan fluorite with red quartz, but its provenance is unknown and it may even be leaning more towards a garnet peridotite as there appear to be two green minerals present, one showing dark fracture or cleavage lines more typical of olivine. But who cares; it’s beautiful! Eclogites deserve a whole article to themselves, but not here.


Condor Agate
This is probably the least aesthetic of the pieces described herein but the name has a ‘ring’ to it. This apparently popular agate was discovered in 1993 at San Rafael, Mendoza Province in the Patagonia region of Argentina. It is mined from shallow surface excavations. This one has a quartz centre. Acquired from the Cal Neva Mineral Company.


Prehnite

I bought this piece in Perth, Australia in 2015 attracted both by the colour and the difference in habit compared to the more familiar material from Namibia. It originates from Wave Hill in the Northern Territories, the source of the finest gem-grade prehnite in the world. It occurs in basalt and varies from green to yellow and goes under the trade name of SunJade®.


Sunset Jasper

A fitting name to end this part of the article. This rock was also bought in Perth at the same time as the prehnite and I’m still kicking myself for not rather or additionally buying a slab of Pilbara tiger’s eye which has a lovely reddish hue that doesn’t seem to be present in the South African tiger’s eye. Anyhow, not much to say about this one apart from that it originates from the Hamersley iron ore mining area in the Pilbara Region of Western Australia and brings down the curtain on the first part of this brief article on unusual polished slabs.

However, two additional small polished slabs of rhodochrosite and malachite are included below for their aesthetics to round-off this article, as they are not unusual minerals. The only unusual aspect is that, in my experience, most rhodochrosite and malachite specimens are either raw mineral specimens or polished carvings/ornaments, eggs or spheres, so perhaps they qualify as unusual polished slabs?


Rhodochrosite (Figures 11 and 12)

Figure 11 is a section through a stalactite/stalagmite (my ‘A’ level geology teacher taught us to remember that tites come down and mites go up) from the famous Capillitas Mine in Argentina. It was acquired from John Betts Fine Minerals. This mine is located at an altitude of about 3 000 m and has been worked since Inca times and hence the earlier name given to the mineral, Inca Rose. Unlike the crystalline occurrences at e.g. N’Chwaning (scalenohedrons) in South Africa and the Sweethome Mine (rhombohedrons) in the USA, Capillitas rhodochrosite is almost entirely formed as stalactites and stalagmites and massive deposits (e.g. Figure 12, from UC Minerals). The site was originally mined for sulfides but these have been worked-out. 

Malachite
This one isn’t labelled but I recall winning it in an online auction and that it is from the DRC and is also from a stalactite/stalagmite but that’s about all I can say about it. And that really does bring down the final curtain on this article. PR
 

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Lesley Andrews

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I do give myself complications.

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Peter Rosewarne

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Duncan Miller

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August 23, 2020

by Peter Rosewarne

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Few people know that ...


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July 27, 2020
Duncan Miller



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July 27, 2020

Lesley Andrews

Peter Carl Fabergé was born in St Petersburg in 1846, and eventually became Goldsmith to the Imperial Court of Russia, and a supplier of wonderful artworks worldwide. He assumed charge of the workshop his father had established in St Petersburg when he was only twenty-four years old, and presented the first Imperial Easter Egg to the Russian royal family in 1884. The workshop survived until 1918, by which time Russia had succumbed to revolution and the royal family was no mor...


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