The Master of Darwin College introduced Sir John Meurig Thomas as one who had spent his academic life in Wales, Cambridge and London; and who at present was combining the rôles of being Master of Peterhouse with research at the Royal Institution (R.I.). As Sir John's lecture unfolded, it was interesting to note how many eminent scientists from Peterhouse and from the R.I. contributed to his story!
Sir John's lecture was given to a close-packed audience spanning several generations, from young students to elderly and distinguished professors, in the large lecture theatre opposite Newnham College; and it was illustrated with numerous colourful slides and amusing anecdotes about the people involved. His story started in 1912, with the young William Lawrence Bragg walking along the Cambridge 'Backs', pondering over the diffraction of X-rays which had just been discovered by Max von Laue and his team in Munich. On 11 November 1912, he read his paper to the Cambridge Philosophical Society which explained 'Laue spots' by the simple idea of reflections from planes of atoms. This is how the subject of X-ray crystallography began. And there were many Nobel prizes to be won on the way, including one shared in 1915 between WLB and his father W.H.Bragg, who was Professor at Leeds.
Other famous names mentioned included Linus Pauling (who, at the age of 21 had solved the structure of the mineral molybdenite by X-ray diffraction), John Desmond Bernal (known to his friends as "The Sage"), the Norwegian geochemist V.M. Goldschmidt who carefully listed ionic radii, and W.T.Astbury of Leeds, the first Professor of Biophysics, who studied the structures of fibres (such as silk and wool).
Sixty two years ago Max Perutz arrived at Peterhouse from Vienna, and was given rhodonite to work on - "flakes from a lump of slag" - and he learnt how to solve crystal structures too. John Kendrew (of Peterhouse) and Dorothy Crowfoot (later Hodgkin) also worked on minerals initially; and J.D.Bernal (aged 23) solved the structure of graphite at the R.I. Sir John showed a model of the graphite structure to the audience and, to indicate its scale, said that a piece of graphite the size of one's fingernail, if magnified by the same factor as the model, would stretch from Cambridge to Vladivostok!
It is extraordinary that the total number of minerals is a mere 3700, or so; compared with more than ten million different chemical compounds. There are even four million species of insects, of which 300,000 are beetles. This fact prompted J B S Haldane to remark that he didn't know much about the nature of God, except that He was "inordinately fond of beetles"!
The seventeenth-century Danish anatomist Steno (who later became a bishop) measured the angles of quartz crystals and showed that the angles between corresponding faces of different specimens were always the same. The Swedes Baron Kronsted and Berzelius classified minerals according to chemical composition. Ideas on packing and symmetry were advanced by Kepler, Harriot, Hooke, Huygens, Dalton, Federov, Schoenflies and Barlow: (the last named having been an estate agent in Islington!). Dana's classification is the definitive study of mineralogy. Another Petrean, Lord Kelvin, wrote in 1904 about the chirality of quartz: there were left and right handed varieties, with slightly more specimens of left than right. There are left and right handed versions of the 100 or so amino acids: the basis of life itself: and Nature appears to use about 20, all of which are left handed. It is conjectured that these amino acids might have been formed at the surface of left-handed quartz; or, as Leslie Orgel (also of Peterhouse) showed by experiment, on the surface of ice, of which there are eleven different varieties. The molecule of the drug Aspirin crystallises in 29 different forms, which prompted someone to remark that there were "twenty-nine different ways of getting a headache"!
Even ten years after the Braggs had demonstrated the 6-6 octahedral coordination of halite (NaCl), and the calcium ions in fluorite surrounded by eight fluorine ions, some chemists were still unable to accept the ionic structure of these compounds (as a letter to Nature by a Professor Armstrong amply made clear). Earlier, Arrhenius's thesis had been rejected by Uppsala University in which he had proposed that NaOH dissolved in water formed ions. Michael Faraday (of the Royal Institution) had shown that ions were present in molten substances; and the words he coined to describe them can be seen (when using a magnifying glass) on the present-day �20 note.
Even though the Braggs determined the ionic distances in alkali halides, they never became seriously interested in quantum mechanics. But Pauling did make sense of ionic radii and was able to group them as a periodic table in a remarkable study of alumino-silicates published (in J. Amer. Chem. Soc) in 1935.
Recurring themes throughout Sir John's lecture were the relationships between each individual crystal structure and the properties (chemical and physical) of the substance. Questions such as "Can one play with stoichiometry?" lead into ideas employed in modern materials science and catalysis.
The polymorphism of calcium carbonate was illustrated by the growth of snail shells of calcite/aragonite. Apparently, if a zoologist frightens a particular snail whilst it is laying an egg, it will grow yet another crystallographic form, namely vaterite! Starting with colourless corundum, if a few chromium ions replace aluminium ions (about 1 in 5000) then a beautiful ruby results. This should be green, but ruby is red. This apparent anomaly of structural mimicry was explained in a one-page paper in Nature, vol 179 p 1348 June 29, 1957 'Ion compression and the colour of ruby' by Leslie Orgel (Peterhouse).
Sir John then discussed various silicate structures based upon tetrahedra arranged in chains and sheets. Egyptian blue was a substance used to decorate the fine sculptured head of Queen Nefertiti, but the secret of its composition had been lost in pre-Roman times. Humphry Davy (of the Royal Institution) guessed that it was a flux of silicon dioxide with alabaster and some malachite, a hunch later (1978) proved correct by the diffraction pattern of a powder sample as small as a microgramme in weight.
Perovskites get their name from a Russian finance minister of the last century and their structures form the basis of most modern high-temperature superconductors. 25000 such structures have been determined in the past decade!
Some open-structure silicates can allow small atoms, such as hydrogen and helium, to pass through; and others can filter whole molecules (e.g. methane) in industrially important processes such as in the manufacture of petrol from oil. They have been given intriguing names, such as DAF-5 in honour of Davy and Faraday (of the Royal Institution), and have strikingly beautiful structures, some based upon truncated octahedra, which are visualised by computer drawings and models. Most of the state of Nevada is apparently made of mordenite, an example of an important catalyst in the manufacture of petrol from natural gas. The economy of New Zealand depends upon the use of such microporous inorganic catalysts. Zeolites are also used in detergents and water softening; and some can act as solid acid catalysts 1000 times as strong as wet mineral acids.
Sir John left us in no doubt that structural mineralogy is an enormously exciting subject with highly important economic applications worldwide. His Royal Institution team is now making novel zeolitic solids by using structure-directing template molecules. He concluded his brilliant lecture with the example of ZSM-5, a porous catalyst synthesised in America in the early 1970s but recently discovered naturally occuring by Italian geochemists in Antarctica. It has a structure based upon rings of 10, 6, 5, 5,5 and 6 members. Amazingly, the same pattern is depicted on the walls of a mosque built in Baku, Azerbaijan in 1086! He finally remarked, "There's nothing new under the Sun!"
Moreton Moore
Editor's Note This report is published as a contribution to National Minerals week, 22-29 June 1998
Page last updated 14 July 1998
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