Pallasites: The meteorite jewels in the crown
David Bryant (UK)
Perhaps unsurprisingly (as a professional dealer in space rocks), I find all meteorites equally fascinating and, in their own way, aesthetically appealing. However, I have to admit, the meteorites known as the Pallasites, with their beautiful structure of olivine fragments suspended in a nickel-iron matrix, are probably the most visually exciting, particularly to the non-specialist. In addition to their undoubted beauty and rarity, Pallasites offer us an intriguing glimpse into the interior of a planet that make them among the most scientifically important of all meteorite types.
The name Pallasite is derived from that of the German naturalist, Simon Peter Pallas. Pallas was one of those amazingly observant and gifted polymaths, who seem to have been a lot more abundant during the eighteenth century, as well as lending his name to a whole class of meteorite, an eagle, a warbler, two species of bat, a wild cat and dozens of other plants and animals.
In 1772, Pallas obtained a 680kg lump of metal that had been found near Kransnojarsk in Siberia. When it was examined in St Petersburg, it was identified as a new type of stony meteorite. In keeping with tradition, it was named after the location where it was found, but, uniquely, the whole class of meteorites was named for Pallas.
There is still some debate about the actual origin of Pallasites. Although some meteorologists contend that the stony-iron structure resulted from a collision between a nickel-iron asteroidal core and a chunk of mantle material (as is the case with mesosiderites), most now believe they originated from the core-mantle boundary layer of differentiated planets that were shattered during vast impacts with other bodies.
That the fifty or so known pallasites derive from a number of such collisions is demonstrated by their chemical and structural differences and by the variation in their ages – from over four and a half billion to just a few hundred million years. (Of course, this reflects the time of the impacts that released them into space.) Additionally, the stony material in mesosiderites is eucritic in nature, indicating an origin on the surface of a planetary body: the olivine crystals in pallasites are so pure that those from the 45 kg Marjalahti meteorite were adopted as the official standard for peridot, the gemological name for crystalline olivine. (Eucrites are meteorites, many of which originate from the surface of the asteroid 4 Vesta.)
Given that Pallasites are beautiful, scientifically important and fascinating relics of planetary collisions, you may be considering adding an example or two to your collection. However, a word of warning. The fate of all polished iron on Earth is to go rusty. Therefore, sliced and polished Pallasites need extreme care to prevent this occurring. The ideal situation would be to display them in a closed cabinet with a dehumidifier. Perhaps surprisingly, these items are not as expensive as you might imagine and can be located quite easily on the Internet. A cheaper solution is to keep your samples in sealed specimen boxes with a silica gel sachet or two. I use the indicating types that change colour to show when they need replacing. Increasingly, meteorite dealers will coat thin polished slices in transparent acrylic to protect them. So long as the pallasite has been thoroughly dehydrated and dried before applying the coat, this process stabilises the meteorite for years.
The two that are most likely to be found in a general meteorite collector’s cabinet are Brahin, which was discovered in 1810, near Minsk in Belarus, and Brenham, originally discovered in Kansas in 1882. Both of these are somewhat prone to deterioration, but display beautiful olivine crystals in a range of colours. In the case of Brenham slices in particular, the crystals are usually in combinations of transparent green, yellow and pale orange.
Undoubtedly the most beautiful (and stable) of all Pallasites are Imilac from Argentina and Esquel, from the driest desert on Earth – the Atacama in Chile. Unfortunately, both are prohibitively expensive, at around £70/gram. A decent slice of either would command a three-figure sum.
Fortunately, there are stable meteorites that are reasonably priced. These are Pallasovka, found in Russia in July, 1990, and Jepara, discovered during the building of a furniture warehouse in Indonesia in 2008. The town of Pallasovka is named after the ubiquitous Peter Pallas, so here we have a Pallasite from Pallas town. The crystals are usually large and brown-orange in colour.
Jepara is an ancient fall in which the nickel-iron matrix has gradually transformed into magnetite, schreibersite and nickel sulphide. Since no further oxidation is possible, Jepara is very stable. Very thin slices display a beautiful structure of greys, silvers and browns, with pale yellow olivines. The limited number of slices I have are further stabilised with opticon and seem to be a really good investment for the private collector.
Other ‘fossil’ Pallasites turn up from time to time. The best-known is Huckitta, found in the Northern Territory in Australia in 1924. Here, the oxidation process is virtually complete, such that the nickel iron has oxidised to silver-grey magnetite and haematite and even the olivine has usually degraded to dark grey crystals. Other similar Pallasites appear on the market from time to time, notably recent northwest African examples (NWA 4482 and NWA 6576). Although lacking the stunning impact of those referred to above, these ancient falls are still fascinating and attractive in their own way. And, of course, they are utterly stable.
No meteorite collection should be without an example of these astonishing and rare meteorites. Now is a good time to take the plunge, but prices will rise dramatically in the very near future as demand outstrips supply. Generally available Pallasites include:
About the Author
David Bryant has been fascinated by rocks from space since he was a small boy, in large part due to correspondence he enjoyed with Sir Patrick Moore back in the 1950. He is the UK’s only full-time dealer in meteorites and writes and lectures widely about the subject. His website can be found at: www.spacerocksuk.com.