Journal of Conference Abstracts

The Atomic Structure of Disordered Silicates

Martin T. Dove (martin@minp.esc.cam.ac.uk)

Department of Earth Sciences, University of Cambridge, Downing Street Cambridge, CB2 3EQ.

Bragg diffraction contains information about the average structure, whereas diffuse scattering contains information about distortions of this average structure at a local level. In this talk I will describe recent work where Bragg and diffuse neutron scattering from polycrystalline samples are used together to determine the local structure in disordered silicates. The experimental data were obtained on the LAD diffractometer at ISIS, which gave a strong signal up to Q = 50 Å^­1. Fourier transform of the data produces the pair distribution functions, which directly gives bond lengths and short-range atomic distances.

I will first present results for the alpha and beta phases of cristobalite, SiO₂. The average structure of the beta phase gives short linear Si­O­Si bonds. It is generally agreed that it is likely that the actual structure is disordered to allow the Si­O­Si bond angle to be nearer 150°, but there are several interpretations of the nature of this disorder. The experimental data confirm the presence of considerable disorder, and show that models supposing the beta phase is composed of domains with the structure of the alpha phase cannot be right. The second set of results will be from samples of dehydrated analcime, NaAlSi₂O₈, that have been annealed for different periods of time at 900°C. On prolonged heating the structure transforms to an amorphous phase. The data show the way the local structure changes as the long-range order disappears.

Dove, M.T., Keen, D.A., Hannon, A.C., Swainson, I.P., Phys. Chem. Min. (submitted) (1996).

Can Dihedral Angle Measurements be Used to Determine Textural Equilibrium?

M. T. Elliott (mikee@liv.ac.uk) & M. J. Cheadle (mjc44@liv.ac.uk)

Jane Herdman Labs., Department of Earth Sciences, University of Liverpool, Brownlow Hill, Liverpool L69 3BX.

The identification of textural equilibrium in igneous and metamorphic rocks can provide important information about the physical conditions and processes that affected the rocks during their formation. In particular, fluid distribution and hence permeability in a crystallizing rock is governed by the degree of equilibrium attained. The current practice for assessing textural equilibrium is to measure the dihedral angle; this is the angle between the tangents to the grain boundaries where three grains meet or where two grains and a fluid meet. These measurements are then compared with theoretically predicted measurements for a totally equilibrated texture. If the measurements match, then the texture is said to be equilibrated. However, if there is a mis-match little can be deduced about the degree of equilibrium because the results for a completely un-equilibrated texture are unknown. Indeed it is possible that geometric effects due to crystal shape and nuclei distribution may allow the apparent dihedral angle distribution in an un-equilibrated rock to mimic that of an equilibrated rock.

To investigate the above problem, apparent dihedral angle curves for un-equilibrated textures were generated using 3-d numerical simulations of crystallized rocks which permit no textural equilibration. These simple simulations of growing, interacting crystals allow control over all the major factors involved in the textural evolution of a rock, such as nuclei distribution, nucleation rate and crystal morphology. The simulations show that un-equilibrated textures can produce results which are similar to reported results that have previously been interpreted as equilibrated textures. They also show that the measurement of the median dihedral angle for a rock is a totally inadequate method for assessing textural equilibrium and consequently predictions of permeability based on median dihedral angle measurements are fundamentally flawed.