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Dr Jan Zalasiewicz of the Department of Geology.

The mineral zoo

Jan Zalasiewicz of the Paleobiology Group in the Department of Geology has produced his latest column in the Palaeontological Association Newsletter

It’s a distinct memory from quite the better part of half a century back, from those comic-book adventures that reached those parts that the worthy and more improving style of fiction didn’t. A memory that plumbed the depths, quite literally. Now what was there? A hero and heroine, certainly, glass spheres around their heads, exploring Atlantis, encountering the Atlantean mer-people (mostly the bad guys, I seem to recall), and undergoing the standard (but oh so absorbing) comicstrip rollercoaster of perilous adventure. Did they have a plucky dog, as was customary, to warn them of lurking mer-villains, barks emerging in bubbles from its own canine diver’s helmet? No matter - among all the near scrapes and nick-of-time escapes, one of the plotline McGuffins bit deep, and lodged in the memory after most else faded. Orichalcum.

Now here was a comic-book writer who had done some homework. Orichalcum was certainly part of the Atlantis legend. It was the fabled lost metal, written about by Plato, some 9000 years (so he said) after Atlantis – if it ever existed – sank in to the ocean. Almost as prized as gold, it supposedly clad the inner walls of Poseidon’s temple on Atlantis. What was it? It apparently shone with a ‘red light’, and so has been variously interpreted as amber, or as a copper-gold alloy (the Romans termed it ‘aurichalcum’), or - by more sceptical commentators - as simply something invented by Plato’s fertile imagination. And as for Atlanteans being the bad hats… well, according to Plato, Atlantis was the antithesis, the polar opposite, of the ‘perfect society’ of ancient Athens. This society, as outlined in Plato’s Republic, though, has itself had its critics: Karl Popper, for instance, thought it essentially totalitarian. Now there’s a moral dilemma to ponder on while following the breathless adventures of our heroes (and their dog) amid the drowned temples.

Orichalcum was part of a menagerie of mythical metals and minerals, that seem to capture the imagination more effectively than do copper and iron and feldspar, in the same way that rocs and griffons and the mighty kraken, not to forget Nessiteras rhombopteryx (1), will always fascinate more than can elephant and giraffe and gnu. There was the philosopher’s stone, for instance, that tempted otherwise sane men into alchemy from the time of the Greeks to… well, almost to the time that Henri Becquerel and his uranium-fogged photographic plate pointed the way towards the real transmutation of the elements.

The philosopher’s stone was unusual for the calibre of the seekers after it. Isaac Newton, for example – though the search was not always for the base motive of turning ordinary metal into gold, but because finding it would bring enlightenment, and perhaps also immortality. And there was Mozart too, with his dabbling in Freemasonry (then a society with strong alchemical leanings) – and with his opera of the same name. Well, not quite his, to be honest – or not, directly, much more than five minute’s worth. Der Stein der Weisen is one of the few things he did in committee, with five local composers who may not have been up among the immortals, but who could for sure turn out a good tune. Long overlooked, then revived with fanfare in 1996 (when Mozart’s role in it was confirmed), this opera – or perhaps musical entertainment – foreshadows the Magic Flute, with Lubano and Lubanara directly ancestral to Papageno and Papagena, and the wonderfully named Nadir as forerunner of Tamino. It’s charming and fleet-footedly good-humoured stuff, with Mozart’s contribution fitting in nicely, rather than towering above the rest. The Stone itself, by the way, exits early in the libretto, carried off by an eagle, and re-appears, eagle-borne again, at the end (a carrier pigeon might have done the job just as well, but with rocks of such aristocracy, one has to maintain standards, don’t y’know).

Orichalcum and the philosopher’s stone aside, the Earth is a good place to hunt for new, diverse and exotic mineral species – the best in the Solar System, indeed. For the living planet is also the increasingly mineral-rich planet – and the two phenomena go hand in hand. This is the thesis advanced recently by the mineralogist Robert Hazen and his colleagues. Thus, the inorganic world has followed (rather than led) the ever more complex, evolving biosphere, and an ever-greater range and diversity of mineral species has been generated throughout Earth history. Moreover, Hazen and company quite explicitly discuss aspects of Darwin’s dangerous idea in this context. Even given the universality of Darwinian evolution to whatever and wherever life might have arisen in the cosmos, might it really apply, in any sense, to quartz and feldspar, to sapphire and emerald?

Let’s set out their case. They start at the beginning of any kind of mineral existence, in the dust of interstellar clouds, where elements, originally forged in supernova explosions, have condensed into the earliest minerals. Rare relict grains of such stardust have been found, after painstaking searches, in meteorites (and are identified by isotope patterns that are quite outrageously not of the Earth, or for that matter of anywhere in the Solar System). As regards diversity, they’re a pretty dull lot – just enough to populate a very, very small collector’s cabinet: about a dozen mineral species all told. Mind, there is diamond in there, and a few other familiar forms: graphite, magnesian olivine, rutile and corundum, plus some less familiar, such as moissanite, a silicon carbide, and hibonite, a calcium aluminium oxide.

Take these as a starting point, and cook them up in nebula of a sun that is just beginning to fire up. More minerals appear in those flash-melted droplets known as chondrules. They include augite and magnetite and calcium feldspar, which any undergraduate student should recognise, and quite a few that would surely stump them: the iron/nickel phases kamacite, taenite, troilite, for instance. At this point, there are about 60 recognisable minerals.

Then clump these into meteorites and then planetesimals, and mix in the effects of the water/ice that is also whirling around the newborn sun. It is the beginning of what one might call weathering, or more precisely aqueous alteration. Hydroxides are formed, and sulphates, and carbonates, and chlorite and talc and other phyllosilicates (this is the start of the long story of Mud). As the planetesimals collide, and grow bigger, new phases appear. Some are related to impact shock, and some to differentiation as the planetesimals begin to melt, and separate out into ‘core’ and ‘mantle/crust’, each with their own minerals. There are now some 250 minerals all told.

The biggest planetesimals grow into planets, albeit still with violent histories of bombardment and collision. Such history has wiped out direct memories-in-rock, at least on Earth, where the first half-billion years of the Hadean is to all intents and purposes tempora incognita. Nevertheless, one can surmise the mineralogical fallout as plate tectonics started, as continents were seeded and grew, as magma bodies slowly crystallized and separated out. If the planetary body is essentially devoid of water (like our Moon), the number of minerals can rise to perhaps some 350.

Add water, though, (not least to allow, by hydrous lubrication, the kick-starting of the plate tectonics engine, and hence the seeding and growth of terrestrial continents), and yet more minerals can be conjured out of a promising new planet. There are those associated with granite bodies, say, when those last water-enriched dregs of magma create networks of pegmatite veins. In this particular type of mineralogical Aladdin’s Cave, about 550 minerals alone have been recognised, some found nowhere else, as complex compounds of lithium and boron and caesium and tantalum crystallized out. These magma bodies then, as heat engines, drive water along fractures through the crust, leaving trails of further minerals, of ores of copper and zinc and lead, of molybdenum and uranium.

So far, so good. Here we have an increasingly complex physico-chemical system. It is evolution in one of the several senses of the word (that is, change through time), but nothing, really, to further philosophize about, especially in the pages of a pamphlet dedicated to those of the fossilish persuasion. But then life turned up, and the world changed, and so did its minerals.

Life, of course, had been primed by those minerals, and by some of them in particular. It is much easier to string amino-acids together, for instance, when clay minerals are around as a handy scaffolding. But once a fully functioning microbe had appeared, and multiplied, it, or rather a countless they, then began to take the world around it, and to transform it.

Not immediately, according to Hazen & co., are at least not by so very much (speaking purely in terms of new mineral production), in the first billion years or so of their existence. The early microbes lived in an anoxic world, and their products were not for the most part novelties per se, being cherts and various iron minerals, and - once their own remains had been metamorphosed – graphite (which had drifted in interstellar dust for billions of years before the Earth was formed at all). Increases in mineral complexity likely did come about, though, if often by roundabout means – the carbonate minerals associated with stromatolites, say, being altered an intruding magma to produce dozens of new skarn minerals.

But it was the neat trick that some of the microbes eventually invented, the production en masse (and eventual taming) of that chemical dynamite, free oxygen, via photosynthesis that, Hazen et al. argue, drove perhaps the greatest revolution in new mineral production on Earth.

About two and a half billion years ago, the world split into two. There was the world without free oxygen, which was (and remains) most of it, of course – pretty well all of its interior – and (in those times) most of the ocean depths. And then there appeared the oxygenated – and hence highly oxidising – world of the land surface and of the shallow seas. In this chemically schizoid world, the numbers of new minerals climbed dramatically. In the seas and the marine strata that formed in them, in Banded Iron Formations and in carbonates and mudstones, there appeared pyrolusite and rhodochrosite, minnesotaite and ferri-annite, turquise and malachite, and dozens – indeed many hundreds – more. In that transition, the numbers of mineral types probably roughly doubled, from the 1500 or so likely present some 2.5 billion years ago in what were hydrous, but essentially anoxic times. Today, for comparison, some 4300 minerals are recognised, of which some half are oxidised and hydrated mineral species that mostly have their roots in the phenomenon of photosynthesis. Most minerals, hence, can be said to be a product of biology: not directly as in being a component of a shell or bone, but indirectly, in reflecting and reacting to a world transformed by the action of living organisms.

Nothing much happened, after that, for a further billion years. The Proterozoic was pretty dull mineralogically, with not much in the way of innovation: much as it was (and perhaps because it was) a time when life also found it tough to develop and diversify. That in turn may have been because those oceans, still largely anoxic at depth, scavenged and buried (into sulphides) many of the elements essential for life, such as iron and phosphorus and molybdenum, as Anbar and Knoll proposed in 2002.

The next step came with those impossibly Hollywoodesque events, of Snowball Earth and the Cambrian explosion (still too far-fetched, both, for any sensible scriptwriter to entertain). By whatever cascade of environmental feedbacks these were triggered (and one might write fill a good-sized library with models and scenarios and hypotheses on these topics) they brought in the time when animals and plants, having learnt the difficult trick of being multicellular, made minerals on a planetary scale.

Biomineralization, then, became commonplace, and brought in some further novelties, albeit not on the scale of the Great Photosynthesis Event. There’s quite an emporium of mineral stuff secreted in living tissues, some a little surprising, such as the copper mineral atacamite in the jaws of one species of bloodworm. And each one of us human animals – to take one familiar and domestic example anatomized, as it were, by Yoder (2002) - secretes no fewer then 26 minerals, including (as well as the hydroxyapatite and whitlockite of bones and teeth) calcite and aragonite, quartz and gypsum, anatase and magnetite and periclase and (if the company is not excessively polite) urea. The biomineralization brought with it, of course, not just new minerals, but – even, say, in a humble whelk shell - wonderfully new and intricate ways of growing crystals and interleaving them with organic matter, a micro-engineering that is the envy – and perplexity - of human materials scientists (Rubner, 2003).

It’s quite a story, this, of the ever-increasing mineral diversity of a planet that has incubated life. There are sundry practical considerations as a consequence. Don’t, for instance, put the family savings and your shirt on shares in Interplanetary Prospecting Enterprises Inc. You’d lose it all, shirt included, because the place where minerals have been segregated and concentrated and emplaced, time and time again, is not in some outpost of the asteroid belt: it is here, at home, under our feet.

And here, at home, there are implications as regards the co-evolution of the living and non-living, which Hazen et al. discuss. Living organisms, as they note, at one level, provide environments, both surrounding them and within their tissues, that are geochemically distinctive and different from the wider surroundings. Thus, they amplify and accentuate a wider (and effectively unidirectional and irreversible) trend that they recognise in the mineral kingdom through time, one of ever-increasing complexity and diversity.

This doesn’t mean that the mineral species (unlike the living ones) are subject to change through natural selection. They don’t show heritable mutations as such, for example, or behave competitively. But their evolution (in the sense of change through time) has, on Earth, been driven in large measure by the evolution (through natural selection) of living organisms. And Hazen and colleagues even suggest that, in this, they show a kind of large-scale punctuated equilibrium – for instance the jump in mineral diversity after the Great Oxygenation Event.

Though, unlike living species, mineral species do not, for the most part, become extinct (in the sense of ceasing to form), even though one might imagine that the Earth has changed so much that some mineral-forming environments might have disappeared. To this, the response was that, although one can think of cosmic environments no longer replicated on Earth (those that formed some highly chemically reduced meteorite minerals, say), any major type of Earthly surface environment still tends to have persisted as a mineral-forming domain somewhere. Thus, despite the spread of oxygen, anoxic environments still remain, in abundance (just below our feet, for example). The Earth (as well as its minerals) has simply become - and then stayed - more various.

Venus, though, is quoted as being different. That planet is now searingly hot and very, very dry, while early in its history it might well have been simply warm and humid and (perhaps!) beginning to incubate life. In that runaway greenhouse transition, it lost its water, and any life that may have emerged, and a host of minerals also disappeared: those that had been hydrated. They would have reverted to anhydrous forms, and so a wave of mineral extinctions would have taken place on that unfortunate planet.

Not all planets undergo such fates, and hence there are further implications, for instance for those patient and optimistic scientists who search for other life beyond our own solar system. Far-off planets, if they have life, will also have a mineral spectral signature that is not so much characteristic of such-and-such a mineral (for who knows what a paratrilobite from around Alpha Centauri might make its paracarapace of), as distinctive through the exobiologically enhanced variety of its minerals, whatever those minerals might be.

By this token, Isaac Asimov’s splendid concept of the silicony is, alas, impossible. Appearing in one of his Asimov’s Mysteries short stories, siliconies were small life-forms that were not carbon- but were silicon-based (silicon being quadrivalent, like carbon, and the only other element able to form truly long-chain compounds). The siliconies lived on asteroids, absorbed energy from uranium ores, were intelligent (as ‘talking stones’) and could technically be described as cute. It’s a lovely idea, but asteroids with their modest total of some 250 minerals are just not places to grow complex life-forms – nor ones to generate concentrated uranium ores.

Exceptions might be found under some highly specific conditions, of course. Aficionados of the Superman saga will know that Krypton, the home planet of the super-powered protagonist, was reduced, on its destruction, to fragments of kryptonite. This suggests that it might have been, uniquely, a life-sustaining but still low-mineral-diversity planet. It was perhaps not entirely monomineralic, as real devotees will know, for in addition to the standard green variety, there has been red kryptonite (the ur-variety), gold kryptonite, blue kryptonite, x-kryptonite and anti-kryptonite, and yet others besides.

It’s all sheer tripe, of course, among the purest and most gloriously unadulterated nonsense (accept no imitations!) to have been created to feed the adolescent mind. Not, though, that that has stopped speculation on its mineral affinities in some quite unexpected quarters. One new mineral discovered by a Natural History Museum mineralogist was hailed as ‘the real kryptonite’ because its composition – sodium lithium boron silicate hydroxide, no less – was spookily close to the composition of kryptonite as outlined in the film Superman Returns (though the new mineral was in reality, alas, snow-white and as benign as you please). An alternative interpretation that was, by contrast, satisfyingly green, luminous and dangerous was concocted by University of Leicester chemists for the 60th birthday of the Superman fable: radioactive krypton difluoride, a nasty enough oxidant to sap anyone’s powers, let alone Superman’s. It’s the real M‘Coy? Perhaps not. Yet other authorities have suggested kryptonite to be a mixture of plutonium, tantalum, xenon, promethium, dialium (2), mercury and ‘unknown’. Tsk! – it’s as confusing a taxonomic mess as orichalcum.

And yet, reluctantly casting all hokum aside, the ever-diversifying catalogue of Earthly minerals, as bound up with the evolution and diversification of organic life, gives considerable pause for thought. Not least, because now a further stage seems to be upon us, a new punctuation event in terrestrial mineral evolution. It’s something that Hazen & colleagues refer to only in passing, in noting the artificial production of completely new types of garnet (one with yttrium and aluminium, manufactured as a faux-diamond).

This represents just one of many possible examples of what must, surely, be the greatest mineral diversification event since oxygen flooded the Earth’s surface two and a half billion years ago. How many synthetic minerals – those not found in any natural surroundings on Earth – have humans produced? There seem to be dozens of new garnet types alone (for gems, and for lasers too). There is synroc and other synthetic zeolites, made to try to hold radioactive waste. There is borazon, a compound of carbon, boron and nitrogen, that is famously harder than diamond. And somewhere in the flash-heated concoctions that form bricks and tiles, there are surely novel minerals too. Then, there are hundreds – or thousands? - of mineral chemists in hundreds of laboratories worldwide, putting together countless combinations of elements in different conditions of temperature and pressure and ambient chemical environment, to see what emerges, for all manner of uses, actual and potential. In this they are simply creating novel chemical micro-environments, much as that copper-secreting bloodworm does within its tissues – but doing so extra-corporeally (and with creative intent). I’m not aware of any catalogue of the entirely novel additions to Earthly mineralogy that have arisen in this way (the electronic web has proved highly opaque in this respect), but the total must run into thousands, if not many thousands – and likely is being added to daily.

One might plead that minerals so produced are not natural. Well, we are natural, a product of natural selection within a primate lineage. And so our products must also be natural, in the same way that a nest relates to a bird, or a web to a spider, rather than being ‘artificial’ (3). There again, one might protest that some of the new minerals are present in tiny amounts. Well, some of the natural (that is, non-humanly produced) minerals are utter rarities, while some of the human-manufactured ones – like borazon, say, as an abrasive – are now produced by the ton.

Tons only? Well, in total, humans have now exceeded that by some way. In fact we have now collectively more or less doubled the amount of metals being cycled at the Earth’s surface (Rauch & Paczyna 2009), with some popular species – copper, for instance, substantially exceeding that. A lot of this is naturally (4) in novel mineral combinations that in turn have their own specific effect on the biosphere. Take those fine mineral dusts fallen from the atmosphere, that fertilize the surface waters of the open oceans, particularly with iron. There is, though, iron and iron. Desert dust has lots of iron, but almost all as highly insoluble iron oxides and hydroxides – and hence of not much use to the poor iron-starved plankton. Glacially-ground rock flour has about ten times as much of its iron available form as a nutrient – but that’s still only a couple of percent. Industrial fly ash, now, has a staggering 70% of its iron in easily available form, mainly as sulphates (Schroth et al. 2009). Those serious people in certain multinational board rooms would, I’m sure, would be intrigued as to just how influential they are amongst the marine plankton that now form a large part of their clientele.

Natural phenomenon or not, human creativity with the inorganic world likely represents a purely temporary – indeed, fleeting – upsurge of mineral diversity, rather than the kind of effectively irreversible thresholds of the geological past highlighted by Hazen and his colleagues. For these synthetic minerals will only be made as long as humans make them. This not only means that if (or rather when, as some of my colleagues would insist) our own species becomes extinct, the new minerals will become extinct with us. Only a few fossil minerals might be left behind us in a thin Human Stratum, as distinctive and exotic as the shocked quartz and buckminsterfullerines left by meteorite impacts. Even within our sojourn, as technology advances at its dizzying and now quite unEarthly speed, we make and then discard different minerals, from generation to generation. It’s just another example of just how singular our own species is, and how distinctive an effect we are having on the world.

Alternative futures might be imagined, though. The imminent demise of the human species is not, quite, a foregone conclusion, although there may be future ambiguity in the meaning of the word ‘human’. Give it – give us – a few more generations, of sufficient stability to allow Moore’s Law to unfold, of that doubling in computer power every two years (holding steady, still, after four decades, with no sign of slowing). Who knows, then, in what form silicon intelligence might then fuse with human flesh – or which of these elements will gain the upper hand? From there, mineral evolution just may become mineral revolution, as sentient mineral comes to beget further mineral (selectively, naturally): the stone, then, may truly become the philosopher.

Now there’s a dystopia (a concept that is something else, apparently, to thank Plato’s Republic for) to wax dismal about. Heigh-ho. Perhaps the evolving polymineralic future will have its compensations, though. Our great-great-great-great-grandchildren – or perhaps grand*beings* – can relax in their condominiums lined with the latest shades of orichalcum (re-invention becoming all the rage), and contentedly muse upon the dreadful untidiness of the primitive, carbon-based past. So long as they remember, just before powering down, to let the silicony out for the night.

Jan Zalasiewicz 10/6/09

References

Anbar, A.D. and Knoll, A.H. 2002. Proterozoic ocean chemistry and evolution. A bioorganic bridge? Science, 297, 1137-1142.

Hazen, R.M., Papineau, D., Bleeker, W., Downs, R.T., Ferry, J.M., McCoy, T.J., Sverjensky, D.A. and Yang, H. 2008. Mineral evolution. American Mineralogist, 93, 1693-1720.

Rubner, M. 2003. Synthetic sea shell. Nature, 423, 925-6.

Schroth, A.W., Crusius, J., Sholkovitz, E.R. & Bostick, B.C. 2009. Iron solubility driven by speciation in dust sources to the ocean. Nature Geoscience, 2, 337-340.

Yoder, Jr., H.S. 2002. Geology: significant component of new multidisciplinary sciences. Proceedings of the American Philosophical Society, 146, 37-55.

Footnotes:

1. n.b. type specimen still missing.
2. n.b. Dialium is a genus of legume in the Fabaceae family.
3. One might argue that the very word ‘artificial’ is in itself… artificial.
4. Or unnaturally, if you wish.

This article first appeared in the Palaeontological Association Newsletter