HR: 0830h
AN: V11D-0522    [PDF]
TI: Layer shift phase transition in kaolins by computer
AU: * Hyde, R A
EM: robert.hyde@yale.edu
AF: Yale University, Department of Phyiscs, New Haven, CT 06520-8120 United States
AU: Steinle-Neumann, G
EM: g.steinle-neumann@uni-bayreuth.de
AF: University Bayreuth, Bayerisches Geoinstitut, Bayreuth, 95440 Germany
AU: Dera, P
EM: p.dera@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory, Washington, DC 20015.1035 United States
AB: In contrast to our previous understanding of dehydration and decomposition of hydrous layered silicates under pressure recent experiments on a simple clay polytype, Al$_2$Si$_2$O$_5$(OH)$_4$ dickite, show a single crystal - single crystal phase transition. This phase transition is characterized by a shift of the layers with respect to one another, and could provide evidence for a general type of phase transition in the layered structure of hydrated silicate frameworks. Not many clay minerals form crystals large enough to permit single-crystal X-ray diffraction studies, therefore, we use ab-initio solid state physics methods to investigate the possibility of the layer-shift phase transition under compression. We have performed computations using ultra-soft pseudopotentials in a planewave method (VASP) to study the energetics and structure of dickite and the other two kaolin polytypes, kaolinite and nacrite, as well as their observed or predicted high pressure forms, over a considerable compression range. The static computations were performed without symmetry constraints, at constant volume, with relaxation of the shape of the unit-cell and positions of the atoms. We analyze in detail the energetics of the kaolin polytypes and their structural response to compression. With these calculations we predict the layer shift phase transition to be of general character in the kaolin polytypes, and indicate that this may indeed be a phase transition that is of general character in layered hydrous minerals. If such a transition occurs in other hydrated minerals of the ocean floor during subduction, the release of water into the overying mantle wedge may be delayed to greater depths.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting