Dry Storeroom No. 1

by Richard Fortey

  Mineral structure at the molecular level was first investigated by the great X-ray crystallographers: Sir Lawrence Bragg and his successors. The X-rays sneak in between the lattice of atoms to produce characteristic arrays of diffraction patterns related to the way the atoms are stacked. These modern investigators built upon centuries of work by early mineralogists. One could argue that science itself grew up among the alembics of the alchemists; on the bench in front of these arcane wiseacres would have been elemental sulphur crystals, or minerals with ancient names like realgar or orpiment. Some of these minerals were derived from smoking fumaroles around active volcanoes such as Mount Vesuvius, the very distillations of the bowels of the Earth. Between the seventeenth and the twentieth centuries the chemical elements were teased out of their compounds one by one. There was a metaphorical dimension to the discovery of a deeper truth about matter, which was mirrored in the geological depths from which many of the minerals originated. Mineralogy came from this ancient tradition, and the modern science gradually shed the esoteric baggage of its forebears. One of the earliest no-nonsense science*18 books was Georgius Agricola’s De rerum metallica (1555), a practical guide to mineralogy and the arts of mining. It remained useful for several centuries. As knowledge of chemistry and the elements developed, the old furnaces of the alchemists were replaced by the blowpipes of the assayers, and then by the batteries of reagents—strong acids, solvents and poisonous cyanides—used by “wet” chemists, the men and women who use test tubes and titration to identify the composition of a mineral. Even today there is still a “wet lab” in the Natural History Museum used to identify certain of the lighter elements. But most of the routine work of assessing chemical composition of the majority of minerals is now entrusted to high-tech equipment: electron probe microanalysis and ion probes can work on tiny quantities of material, even a sample only five microns across, that is, five-thousandths of a millimetre, plucking out and sorting its atoms to an accuracy of picagrams (that is, a million millionth of a gram). There is something almost mystical about these kinds of figures, something that should inspire in the ordinary person a feeling not unlike the awe felt by an initiate wandering into the alchemist’s lair. But as the figures are derived from machines, faced with dials and plasma screens that are familiar from a hundred films featuring the scientist at work, somehow the achievement of such accuracy can be taken on the nod. It is remarkable how the remarkable has become unremarked.

  New minerals are still being discovered regularly, and part of the job of the Mineralogy Department is to describe them chemically and crystallographically—and only then to provide a new name. Names have to be approved, and there is a special commission of distinguished mineralogists to make sure that something claimed as new really is new. The International Mineralogical Association has a Commission on New Minerals, Nomenclature and Classification to vet the validity of new discoveries. A couple of dozen species might be approved in any normal month—nothing to impress a beetle enthusiast, of course, but still proving that there is much to discover within the Earth. Chris Stanley tells me that most of the new minerals he has described and named are not very exciting to look at, often no more than a dusting of tiny crystals. It is no wonder they were undetected by earlier mineralogists. He and his late colleague Alan Criddle have named nearly a hundred new mineral species over their careers. It is only thanks to the delicacy of the new technology that they can be characterized so accurately.

  Occasionally, Chris gets a surprise. He showed me part of a borehole core from Serbia, something substantial enough to toss from hand to hand. The borehole had been put down through a thickness of volcanic rocks. This particular piece of core consisted largely of a milky-coloured material which proved to be a completely new mineral. It contained a high proportion of the lightest metallic element of them all, lithium, which makes it a most surprising find. In 2007 it attracted press attention because its chemical formula matched that of Green Kryptonite, the only substance to which Superman was vulnerable. It will be called Jadarite, after the place of its discovery. It must be published with details of its chemical formula, crystal structure and atomic proportions before it can be considered valid. As for naming, this geographic formality is common among new minerals; a site where a mineral species has been discovered will have “-ite” tacked on to the end of it to give the mineral name—not very imaginative, perhaps, but easy to follow. Quite frequently, a mineral will be named after a distinguished scientist, as in Zinnwaldite (after Dr. Zinnwald),*19 so this is not unlike celebrating a botanist in a plant species name. Several hundred new species of minerals have been named in the Mineralogy Department over the last decade. More confusing is the fact that rock types are very often-ites as well, ranging from andesites to tholeiites, and sometimes these are named after localities, too (like Chassignite above). Such rock types are mostly collections of minerals en masse. Their definitions are much laxer than those of minerals, so there are several varieties of andesite, but they are nonetheless part of the common language of geologists, just as the names of species help communication between biologists. Nomenclature is important.

  The hand specimen of the mineral Jadarite, recently named: chemically it is sodium lithium boron silicate hydroxide; it achieved unusual prominence in the media because of its resemblance to “green kryptonite”—the only substance known to weaken Superman.

  Rock is the real stuff. This is proved by the voyage to the Moon: such an expedition must be validated by the collection of rock samples. All that trouble to acquire something that looks so ordinary—but you cannot argue with it, because it is solid as a rock. I recall the excitement when Moon rock appeared in the Museum in the 1970s—here was evidence you could really believe. We all looked at the small tube containing the sample with respect. It is no coincidence that Jesus’ most reliable disciple was called Peter, “the rock” on which the Church was built. The Natural History Museum keeps historical collections of rocks. Early expeditions risked nearly as much to bring back these unglamorous lumps as did the first explorers on the Moon; they, too, collected hard and most incontrovertible testimony to their boldness. Part of the Mineralogy Department is located in the basement and has been labelled “Miner Alley.” One side of the corridor has high glass cases which include splendid crystal specimens and old optical instruments, all burnished brass and elaborate screws used to raise and lower their universal stages. On the other side there is a long rank of old-fashioned cabinets containing rocks—historically significant rocks numbered in sequence and purchased with the privations or even death of forgotten field staff. Here is the sad booty of Captain Scott’s Antarctic expedition. The oldest collection is probably that made by Sir William Hamilton, envoy to Naples and cuckolded husband of Nelson’s Lady Hamilton—and also a pioneer archaeologist and writer of the magnificent Campi Phlegraei celebrating the wonders of the Bay of Naples. Here, too, is preserved the geological collection of the Matthew Flinders expedition to Australia in 1801, probably the first rocks ever brought back from that continent. These rock types can easily be re-collected again today, but then they were the first blobs on a geological map. Now I guess that these drawers are mostly opened by historians rather than by geologists. No doubt if Moon travel ever became routine the first Moon rocks would become historical curios in their turn. The Vesuvian rocks of the Monticelli collection are the exception: they are still consulted, because of the precision of the times, dates and places of their acquisition by their careful collector. Geologists who want to know how magma evolves during a volcanic eruption have here a unique database of past crises.

  Even the way that rocks are studied—the science of petrology—has changed repeatedly. When I was a student, the chief tool for studying rocks was the prepared thin section (like the one on colour plate 13). The rock was trimmed of a thin slice, which was mounted on a slide, and then ground still thinner until its component minerals could be studied under the microscope by shining a directed beam through them. The mosaic of min
erals so revealed had a chequered beauty, like a brilliantly coloured abstract painting, especially when viewed under conditions of polarized light. We were taught to identify minerals by their optical properties, as our teachers had themselves been taught by the Cambridge legends of the light microscope, Professors Tilley and Harker. There was a certain satisfaction in learning these mineralogical skills, and some intellectual satisfaction to be had in linking our identifications with the chemistry of the rocks themselves. Ph.D. students cut literally hundreds of thin sections to get their data on composition of magmas, or the temperature and pressure conditions to which a particular gneiss had been subjected when it was deep within the Earth’s crust. A modest number of thin sections continue to be sliced today for petrological microscope study. Opaque minerals, such as those in meteorite “irons,” are still studied from highly polished surfaces. But the sophistication and convenience of the analytical machines have transformed the study of rocks, so that thin sections now play a lesser part in most research. Old timers will grumble, as old timers will, that the youngsters “wouldn’t know an adamellite if it hit them in the face.” But scientific instruments change, just as research priorities change—though I cannot see an old electron probe making as attractive an exhibit as those beautiful essays in tooled brass and hand-ground lenses that line the walls of “Miner Alley.”

  Alex Ball and Terry Williams are in charge of the Kingdom of the Machines that now occupies the basement area under the Earth Science galleries, and they will take you from room to room with proprietorial pride. The first thing you notice is how clean everything is compared with the average cluttered Museum office. Record books are neatly filed away above wiped-down benches—the word “shipshape” comes to mind. The Natural History Museum is rather well off in the latest technology, although, as Alex remarks, you have to run hard just to stay up with the leaders. Many university departments have to make do with one analytical machine that has to be constantly reprogrammed, but the Museum can find a machine to suit the job in hand, which means going into one dedicated room or another. Before the basement was commissioned, the machines were dotted about the Museum in obscure places. The first scanning electron microscope (SEM) was up and running in the Museum (1965) even before I joined the staff, thanks to the efforts of Ron Hedley, who later became the Director. He recognized its importance as a tool to see fine structures more clearly than ever before, by using electron beams that could discriminate detail much more finely than the traditional light source. It is easy to forget the astonishment of being able to see for the first time the eye of a fly, or those “hairs on legs,” with such precision.

  Electron microscopy is located in the Mineralogy Department. Images provide unrivalled details, even of fossils. Here (above) A photograph of the Cenozoic bryozoan (Exochella jellyae Brown, 1952) can be compared with the drawing of the holotype of Brown made in 1952 (bottom).

  This tool transformed the study of some animal and plant groups. Paul Taylor, the doyen of the bryozoans, is always stressing the beauty of his tiny, water-dwelling and mat-forming colonial animals—and he is right. Before the SEM these animals were usually illustrated by drawings, which varied greatly in quality. Now the exquisite and delicate patterns made by the colonies and the ornament of the little boxes in which each individual of the colony lived are both scientifically accurate and a delight to the eye thanks to the electron microscope. The scanning microscope was soon complemented by the transmission electron microscope, an instrument that has transformed our understanding of the organelles inside living cells and the way those cells collaborate to make tissues. I used the scanning electron microscope quite early in my Museum career. I always felt like a real scientist when I sauntered off to the machine. I soon had images of trilobite larvae a millimetre long blown up to the size of a small lobster, displayed on the green plasma screen attached to the microscope. The process of photography was slow because specimens had to be coated in a fine layer of gold to allow the electrons to “take” later machines allowed photography of uncoated specimens, and these are still in use today.

  So machine was added to machine, one by one, each with its own acronym, each doing a somewhat different job. For example, many mineralogists were not interested in getting good pictures so much as in analysing elemental composition. Much of the serious research money in the Museum came to be spent on this hardware. The latest version of the electron microprobe cost half a million pounds, but it saves much staff time because it is automated to assay for four elements simultaneously, and gradually works through an elemental “shopping list.” Mineral samples are analysed from polished surfaces of rock slices, or they are sometimes powdered, or put into solution, depending on the technique involved. Machines have to be continually updated as new levels of accuracy of measurement are achieved—so, for example, they can now routinely focus on minute areas just a thousandth of a millimetre across if required. The mind soon reels when confronted by the variety of “kit” in the Kingdom of the Machines, but I was happy to see my old machine still sitting in one of the rooms.*20

  Some mineralogical investigation is closer to industry than most of the research that goes on in the Natural History Museum. There is money to be made from knowing about how ores or gemstones form in nature. Many valuable metal ores are associated with special rocks known as volcanogenic massive sulphide (VMS) deposits—and, as with SNC meteorites, it might be better to stick with the acronym for reasons of brevity. These are interesting and unusual rocks because most of them originated in ocean basins where cold seawater meets hot fluids from deep within the Earth—yielding sulphide deposits rich in zinc and copper. The metals are dissolved in water as hot as 380 degrees centigrade, and when the metalliferous liquor hits cool sea temperatures the solubility of the valuable ions falls dramatically. The resulting massive metal sulphides can be thick enough to support large quarries. The sites where these strange deposits form today typically lie along the mid-ocean ridges, which is where the lithospheric plates of which the world is made are slowly, slowly moving apart. Heat rises from the interior of the Earth along the ridges, bringing treasure—not just zinc and copper, but silver and gold as well. Sulphurous fluids belch out of vents known as “black smokers” which track the ridges on the ocean floor—and build up dark chimneys of iron pyrites out of sight of the sunlit world above. In the same hidden world thrives a whole ecosystem of bizarre animals, whose economy is based upon the sulphurous exhalations of the smokers rather than upon sunlight. There are shrimps that cultivate sulphur bacteria, or scrape them from the walls of the chimneys. There are vestimentiferan worms that house bacteria in their guts and form thickets of tubes. There are giant clams. It is an extraordinary world, and one that can be visited only in special diving craft like Alvin, the U.S. Navy’s deep submergence “submarine” that can withstand the enormous pressures at depth. As far as mining is concerned, it might as well be on the Moon. Many of the “fossil” VMS deposits are associated with former volcanic island arcs, like those around the Pacific Rim today. They are preserved only in accessible locations on the continents because of the inexorable movement of the tectonic plates around the Earth. Pieces of ancient ocean floor finish up incorporated into the continents, and then they are available to miners. Geologists recognize an appropriate tectonic setting, typically where ancient sea floors have been subducted away, and this makes for excellent prospecting.

  VMS deposits are important sources of industrial and precious metals. Gold may be the legendary lure for the adventurer, but more commonplace metals like copper may prove to be more relevant to the future of the world. It is a curious thought that the electrical wiring in our homes might have started out on an ocean floor millions of years ago. Richard Herrington is the Museum mineralogist and metallurgist with a special interest in VMS formation. He is full of enthusiasm for heavy dark rocks made mostly of massive iron pyrites—iron sulphide, otherwise known as “fool’s gold”—not one of nature’s most elegant productions to my eye. Bu
t his work with Crispin Little of Leeds University and Russian colleagues has cast a brilliant light on ancient oceans, showing that the community of life around “black smokers” has been established on Earth for hundreds of millions of years. These scientists have been working in the copper/zinc mining districts in the Urals, especially around that city whose name leaves little to the imagination, Magnitogorsk. Its famous ice hockey team is the best in Russia and leaves even less to the imagination—they are called Metallurg Magnitogorsk. A glance at a map of the world will show how the Ural mountain chain snakes across the centre of Russia from Novaya Zemlya in the north, by way of some of the new republics, all the way down to Kazakhstan in the south. It is a huge wrinkle on the surface of the globe. This shape alone would suggest to a modern geologist that the Urals represents the aftermath of a vanished ocean—for linear mountain chains like the present Himalaya are thrown up in slow convulsions of the Earth’s crust when continents collide. Just the place for VMSs. The eastern and western halves of Russia came together several hundred million years ago at the end of the Palaeozoic Era. Prior to that they lay on separate plates, and an ocean with offshore islands lay between them—one that was consumed by a subduction zone on its western side. This eventual welding of the two continents together was a long-drawn-out process, and several island arcs found themselves plastered on to the nascent Uralic chain many millions of years before the continents themselves collided—they were the advance skirmishes before the main onslaught.