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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Santa Cruz Electronic Theses and Dissertations (2017)

Subduction zones play a fundamental role in the cycling of minerals from the surface of the Earth into the deep Earth. Minerals that are subducted experience high-pressure and temperature as they are transported to greater and greater depths. The work presented here takes advantage of pressure as a thermodynamic variable in order to gain insight into the physics and chemistry of subduction zone related mineral phases.

The experiments conducted here use the diamond anvil cell at pressures up to ~65 GPa in conjunction with synchrotron based single-crystal x-ray diffraction as well as Raman, infrared, and luminescence spectroscopy. The work is divided into three major parts (1) water bearing minerals found in subduction zones such as lawsonite [CaAl2Si2O7(OH)2· H2O] and clinochlore [Mg5Al(AlSi3O10)(OH)8], (2) B and F bearing minerals found in subduction zones such as topaz [Al2SiO4(F,OH)2], and dravite variety tourmaline [Na(Mg3)Al6(Si6O18(BO3)3(OH)3(OH)] and (3) mineral phases that are closely related to subduction zone minerals but not likely found in abundance in subduction zones such as beryl [Be3Al2Si6O18], and spodumene [LiAlSi2O6].

The high-pressure crystal structure of lawsonite was solved for the first time using synchrotron based single-crystal x-ray diffraction. The H-bonding environment of the high-pressure monoclinic phase of lawsonite was also determined. High-pressure luminescence spectra of clinochlore up to ~25 GPa show that the electronic properties of this sheet silicate are complex under high-pressure conditions. High-pressure luminescence spectra of topaz up to ~55 GPa show that the deformation mechanism of topaz likely changes above ~38 GPa. A new high-pressure phase of tourmaline that is stable above ~15 GPa at room temperature was discovered and solved using synchrotron based single-crystal x-ray diffraction. Luminescence spectra suggest this phase is stable to at least ~65 GPa.

A new high-pressure phase of beryl that is stable above ~14 GPa at room temperature was discovered using luminescence and Raman spectroscopy. Experimentally our results confirm the previous theoretically predicted high-pressure soft-mode transition. Beryl is structurally related to the high-temperature phase of cordierite [(Mg,Fe)2Al3(AlSi5O18)], a subduction zone mineral phase that shows extensive high-pressure polymorphism. Spodumene is certainly not abundant in the Earth's crust and upper mantle, however understanding high-pressure phase transitions in C2/c pyroxenes is important for understanding the high-pressure behavior of mantle relevant pyroxenes (e.g. hedenbergite [CaFeSi2O6] and diopside [CaMgSi2O6]). Luminescence spectra suggest that only one transition at 3.2 GPa occurs under compression from room pressure to ~15 GPa which is in contrast to previous work that suggests a second transition near 7.7 GPa.

    Cover page: High-pressure studies of subduction zone related mineral phases
    • Article
    • Peer Reviewed
    LBL Publications (2019)
    The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) Å at 5.0 GPa. The π∙∙∙π contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energy calculations (PIXEL) indicate that the π∙∙∙π interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic ( P 1 ¯ ) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I···N bond increases in length to 2.928(10) Å and become less linear [-1 and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm-1. The driving force of the transition is shown to be relief of strain built-up in the π∙∙∙π interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa.
      Cover page: The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2021)
      It is qualitatively well known that kinetics related to nucleation and growth can shift apparent phase boundaries from their equilibrium value. In this work, we have measured this effect in Bi using time-resolved X-ray diffraction with unprecedented 0.25 ms time resolution, accurately determining phase transition pressures at compression rates spanning five orders of magnitude (10-2-103 GPa/s) using the dynamic diamond anvil cell. An over-pressurization of the Bi-III/Bi-V phase boundary is observed at fast compression rates for different sample types and stress states, and the largest over-pressurization that is observed is ΔP = 2.5 GPa. The work presented here paves the way for future studies of transition kinetics at previously inaccessible compression rates.
        Cover page: Compression-rate dependence of pressure-induced phase transitions in Bi.
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