vsg - Minsoc '97
Michele C. Warren1,2 (M.C.Warren@ed.ac.uk), Graeme J. Ackland1 (G.J.Ackland@ed.ac.uk), Bijaya B. Karki1 (Bijaya.Karki@ed.ac.uk) & Stewart J. Clark1 (S.J.Clark@ed.ac.uk)
1 Dept. of Physics & Astronomy, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, U.K.
2 As from January 1997: Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, U.K.
Ab initio electronic structure calculations of MgSiO3 and CaSiO3 perovskites are presented, which aim to determine the relative stability and probable transition pathways between the various perovskite phases via a variety of approaches. The equilibrium structures and total energies of cubic and orthorhombic phases of MgSiO3 are found, and a tetragonal phase is proposed as a possible intermediate (Warren & Ackland, 1996). The forces on atoms in any configuration may be calculated from first principles, and thus the phonons of each of these three phases at principal wavevectors are found.
Comparison of the phonon eigenvectors with the relative distortions of the phases suggests a transition pathway from the cubic phase to the orthorhombic, via the proposed tetragonal phase, involving the freezing in of only six phonons. Two of these modes are the familiar octahedral rotation mode: the extent to which these act as rigid unit modes is discussed. Parametrisation of a simple model system suggested that a transition between the tetragonal and orthorhombic phases may be possible in the mantle. However, initial results from recent molecular dynamics simulations using first-principles forces, at zero pressure and range of temperatures, do not show a transition below the melting point.
Similar investigations into CaSiO3 are in progress, and initial results will be presented, in the light of recent theoretical predictions (Stixrude et al., 1996) suggesting that there are instabilities equivalent to the octahedral rotational modes active in MgSiO3.
Stixrude, L., Cohen, R.E., Yu, R. & Krakauer, H., Am. Mineral., (in press 1996).
Warren, M.C. & Ackland, G.J., Phys. Chem. Minerals, 23, 107-118 (1996).
Mark D. Welch (firstname.lastname@example.org)
Department of Mineralogy, The Natural History Museum, London.
PbO-related oxychlorides occur in Pb-Cu-Mn ore deposits that have undergone extensive alteration by hydrothermal (<200°C) brines. Their structures are composed of alternating Cl and PbO (litharge-like) layers. The unusual crystal chemical feature of these minerals is the diversity of elements that partially substitute for Pb, including B, Si, P, V, As, Sb, Mo, W, I and S. Electron diffraction (TEM) reveals a rich diversity of superstructures arising from Pb-X ordering. Due to the very high X-ray absorption coefficients of these minerals and the weakness of superlattice reflections, previous XRD studies have only succeeded in determining the tetragonal/pseudotetragonal subcell. EDPs obtained with the electron beam normal to the layering allow the 2D superstructure motifs to be derived with reference to the (more intense) sublattice reflections. An introduction to the novel crystal chemistry of this unusual mineral group will be given.
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