GEOLOGICAL APPLICATIONS OF SYNCHROTRON RADIATION

摘要

Synchrotron-based, Earth sciences research carried out over the last 5 years is reviewed with special attention being given to X-ray absorption studies; X-ray diffraction and X-ray fluorescence microprobe applications are considered more briefly. A comprehensive bibliography is included. The main part of the paper summarizes recent work carried out at the Daresbury SRS. K-edge XAS studies of glasses as models for silicate melts provide information on the local structural environments of Si, Fe~(2+) and Fe~(3+). By analogy with synthetic "leucites" which contain Fe~(2+) and Fe~(3+) in tetrahedral framework sites, it seems that many model 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 Fe~(2+) in the tetrahedral T2 site, while Ti occupies the distorted Ml octahedral site. L-edge spectroscopy is used to identify the valencies and electronic structures of Mn and Fe in minerals and the Fe~(2+) :Fe~(3+) ratio in a natural spinel is determined. The polarized nature of the synchrotron beam is exploited in determining the Fe X-ray absorption anisotropy in single crystal tourmaline and epidote. XRD powder studies include Rietveld-refinement structure determination and compressibility studies. Synthetic "leucites" having the stoichiometry K_2MgSi_5O_(12) have distinctly different structures. The dry-synthesized form is cubic Ia3d with Si and Mg fully disordered on tetrahedral framework sites, while the hydrothermally-synthesized polymorph is monoclinic P2_1/c with Si and Mg fully disordered on, respectively, 10 and 2 tetrahedral sites. The reversible tetragonal to orthorhombic phase transition in gillespite (BaFeSi_4O_(10)) 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 interpreted in terms of ferroelastic and coelastic phenomena and the related order parameters analysed using Landau theory. The compressibility of MgCO_3, determined up to 20 GPa, has been combined with thermochemical data to obtain an "equation of state" for magnesite and it is found that magnesite is likely to be the main host for carbon in the Earth's lower mantle.