GEOLOGICAL APPLICATIONS OF SYNCHROTRON-RADIATION

Synchrotron-based, Earth sciences research carried out over the last 5 years is reviewed with special attention being given to X-ray absorpt ion studies; X-ray diffraction and X-ray fluorescence microprobe appli cations are considered more briefly. A comprehensive bibliography is i ncluded. The main part of the paper summarizes recent work carried out at the Daresbury SRS. K-edge XAS studies of glasses as models for sil icate melts provide information on the local structural environments o f Si, Fe2+ and Fe3+. By analogy with synthetic ''leucites'' which cont ain Fe2+ and Fe3+ in tetrahedral framework sites, it seems that many m odel glasses also contain both oxidation states of Fe in the network, rather than as network modifiers. The structural sites occupied by the minor elements Mn, Zn and Ti in staurolite have been identified using XAFS; Mn and Zn substitute for Fe2+ in the tetrahedral T2 site, while Ti occupies the distorted M2 octahedral site. L-edge spectroscopy is used to identify the valencies and electronic structures of Mn and Fe in minerals and the Fe2+:Fe3+ ratio in a natural spinel is determined. The polarized nature of the synchrotron beam is exploited in determin ing the Fe X-ray absorption anisotropy in single crystal tourmaline an d epidote. XRD powder studies include Rietveld-refinement structure de termination and compressibility studies. Synthetic ''leucites'' having the stoichiometry K2MgSi5O12 have distinctly different structures. Th e dry-synthesized form is cubic Ia3d with Si and Mg fully disordered o n tetrahedral framework sites, while the hydrothermally-synthesized po lymorph is monoclinic P2(1)/c with Si and Mg fully disordered on, resp ectively, 10 and 2 tetrahedral sites. The reversible tetragonal to ort horhombic phase transition in gillespite (BaFeSi4O10) has been studied in a diamond anvil cell using ED detection and found to occur at 1.2 +/- 0.1 GPa. The anomalous compressibility observed has been interpret ed in terms of ferroelastic and coelastic phenomena and the related or der parameters analysed using Landau theory. The compressibility of Mg CO3, determined up to 20 GPa, has been combined with thermochemical da ta to obtain an ''equation of state'' far magnesite and it is found th at magnesite is likely to be the main host for carbon in the Earth's l ower mantle.