Geochemistry of the vanishingly tiny

The British press has been awash with speculation that the Prince of Wales is worried about nanotechnology and the slim possibility that the next big threat after Osama and SARS might be minute, self-replicating robots that invade our bodily orifices.  It stemmed from the Prince of Wales’ having asked experts for a briefing, and that may well have been just HRH’s curiosity about a changing world.  There is rarely an issue of the weekly science journals without news of some discovery of phenomena that occur in nanotubes and minuscule cavities; the world at scales less than a micrometre is beginning to seem strange.  Rocks are full of pore spaces and inter-grain boundaries with the dimensions on which new wings of the other sciences are emerging.  So it is no surprise to learn that there will soon be “nanogeochemistry” (Wang, Y. et al. 2003.  Nanogeochemistry: geochemical reactions and mass transfers in nanopores.  Geology, v. 31, p. 387-390).  The use of natural and artificial zeolites as ionic filters has been around for a long time, so this is a branch with a new name, rather than a fundamental breakthrough.  But zeolites are profitable, and only now has “blue-skies” research turned up the magnification.

Typical nanopores and pathways are grain boundaries in crystalline rocks, cleavage planes in phyllosilicates and clay minerals, and pores in fine-grained sediments, such as diatomite and kaolin, and minerals that have been precipitated as amorphous masses rather than discrete crystals, a good example being the iron oxy-hydroxides in soils.    To see these structures requires advanced transmission electron microscopy, and even with them the features are somewhat indistinct.  Nanopores can make up to 40% of a material’s porosity, and having such minute radii they contribute as much as 90% of the internal surface area that is exposed to chemical reactions.  Artificial materials that show nanoporosity have internal surface areas as high as hundreds of square metres per gram. Clearly, such materials in nature must play a major, but largely uncharted role in geochemical change.  Among the oddities discovered by Wang and colleagues at the Sandia National Laboratories and the University of New Mexico, are inclusions of native copper in weathered clay minerals and equally small particles of gold along microfractures in mylonites.  Their experiments with artificial simulants of natural fine-grained materials focussed on two simple phenomena: the electrical charge on small surfaces in relation to acidty; and their ability to absorb trace elements.  The paper is highly technical, but the conclusions are surprising .  Nanopores develop unusually high surface-charge densities that should affect their ability to adsorb ions, and also exert controls on reactions that might seem unlikely in macro-scale simulations of geological conditions.  Indeed, finely porous materials enrich trace elements by an order of magnitude compared with isolated small particles, and encourage precipitation or solution of different compounds when that would be unexpected in more open systems.  As well as bearing on burial of toxic and radioactive wastes, and on mineralising processes, nano-scale processes are probably central to the whole process of weathering.  Interestingly, such small scales exclude even the tiniest bacteria, so that the geochemical processes seem unlikely to impinge on life.  However, spaces in rocks comprise a nested series of dimensions, and changing conditions may well flush material from one scale to another.  In particular, bacteria of various kinds can control pH at the micro-scale, thereby creating the ambient conditions for nano-scale geochemistry.

Potassium in the core

It might seem impossible for planetary cores dominated by iron-nickel alloys to contain any source of heat generation.  The main three elements (uranium, thorium and potassium) with long-lived radioactive isotopes and sufficient abundance to produce substantial heat energy are all highly concentrated in the Earth’s crust.  That is because they are incompatible with the minerals in mantle rocks, and so readily enter magmas that contribute to continental growth.  However, the only natural materials that bear any resemblance to geoscientists’ notions of core materials, metallic meteorites, contain abundant sulphur.  Theoretically, potassium can enter sulphide minerals.  So, since as long ago as the 1970s there has been debate about whether motion in the core was driven entirely by residual heat from Earth’s accretion and the formation of the core, or that it contained its own heat source in the form of 40K.  If the first was true, then the self-exciting dynamo responsible for the Earth’s magnetic field has been running down over geological time, because heat is transferred across the core-mantle boundary, eventually to reach the surface by convection.  The existence of a solid inner core might result from such cooling, though its formation would release latent heat of crystallization and prolong inner motion.  However, some calculations suggest that core motion and so geomagnetism ought to have vanished long ago, through loss of core heat to the surface.  Substantial potassium in the core would demand considerable revision of ideas about the bulk evolution of the Earth, and other rocky planets.  Experiments to prove that iron-sulphur alloys can contain abundant potassium have had a chequered history.  Research at the University of Minnesota and the Carnegie Institute of Washington has discovered why there were such ambiguous results (Murthy, V.R. et al, 2003.  Experimental evidence that potassium is a substantial radioactive heat source in planetary cores.  Nature, v. 423, p. 163-165).  The problem was in the preparation of samples for analysis.  Rama Murthy and colleagues found that the oils used in polishing samples for electron-microprobe analysis actually leach potassium from the sulphides in them, nearly all disappearing in a few days of contact.  With great care, they repeated experiments on mixtures of metallic iron, iron sulphide and potassium bearing glass held at high temperature under pressures between 5 and 10 % of those experienced in the core.  Their results show that potassium can indeed enter core materials with high sulphur contents.  The higher the temperature the more gets in, and their most extreme run saw almost 4 % K in the quenched sulphide.  Plan are afoot to discover if uranium and thorium might also be in core materials.

Incidentally, in the week that the film The Matrix: Reloaded was premiered in the USA, a proposal to send a probe to the core-mantle boundary also appeared (Stephenson, D.J. 2003.  Mission to Earth’s core – a modest proposal.  Nature, v. 423, p. 239). David Stephenson, of the California Institute of Technology, builds on the notion of the “China Syndrome”, in which meltdown of the core of a nuclear reactor would lead to superdense molten uranium melting its way through the mantle.  In his proposal, ruggedised instruments in a capsule the size of a grapefruit would make the journey, along with about 10 million tons of molten iron, by propagating a large crack started by a 10 Mt nuclear explosion.  Data is to be transmitted by modulated acoustic signals in the kHz range.  The article helps to demonstrate the delays in publication, even in a prestigious weekly journal; it should have appeared 6 weeks earlier….

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