Amorphous-amorphous phase transitions in multicomponent minerals at high pressures |
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It was carried out the experimental and theoretical
research of dynamic and static mechanisms of the amorphization in
order to receive the general physical picture of microscopic processes
underlying of the amorphization in a number of the frame minerals at
applying of external pressure. It were determined the conditions of
appearance of two different on topology of amorphous phases in process
amorphization the same mineral. It was found that if the rate of
compression is properly chosen to make the dynamics of zeolite
amorphization much faster than that of a conventional glass of the same
composition, i.e., τ amorph < τ glass,
then a LDA (low-density amorphous) glass
will be obtained, which may have physical properties different from any
obtainable by the usual routes, even by the slowest cooling from the melt.
The calculation of the lattice dynamics shows the following picture of
amorphization pressure: the random displacements in the amorphous phase
are associated with the incipient instability of at least one nearly flat
phonon branch. These random displacements are large enough to eliminate
the diffraction picture through the reduction of the Debye-Waller factor,
but not so large as to destroy the crystal structure topology. The LDA
structure is rearranged at further increase in pressure to a denser
disordered HDA (high-density amorphous) phase. The HDA phase is a usual
"thermal" glass whose atomic structure is not topologically equivalent to
any crystalline structure. The phase transition between LDA and HDA
zeolites was detected using a high pressure Raman spectroscopy. It is
found that this transition is apparently of the first order and occurs
with a silicon coordination rise, i.e., with change of topology. It is
considered, that amorphization caused of the pressure will be reversible
if crystal topology is kept in an amorphous phase. This picture, being
physically probable, should be still confirmed. In this connection it was
investigated the reversibility of the mineral amorphization at compression
in penetrating and not penetrating environments, in particular, at
compression in pure water. Incorporation of water into synthetic olivine
in the forsterite-water-graphite system at high pressures and temperatures
has been investigated by micro-IR spectroscopy and micro-Raman
spectroscopy. Then it was carried out the experimental research of the
synthetic zeolite NaA in pure water at high pressures by synchrotron X-ray
powder method. It was carried out the analysis of structure by Rietveld
refinement in order to receive the information on filling of the water
sites in α - and β - cage in synthetic zeolite NaA. Subsequently the reverse
Monte Carlo and the Rietveld modelling methods will be incorporated for
the local structural configurations modelling in HDA and LDA phases.
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