Average structure of an An (sub 48) plagioclase from the Hogarth Ranges

FitzGerald, John D., Parise, John B., & Mackinnon, Ian D.R. (1986) Average structure of an An (sub 48) plagioclase from the Hogarth Ranges. American Mineralogist, 71(11-12), pp. 1399-1408.

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The average structure (CI) of a volcanic plagioclase megacryst with composition Ano, from the Hogarth Ranges, Australia, has been determined using three-dimensional, singlecrystal neutron and X-ray diffraction data. Least squaresr efinements, incorporating anisotropic thermal motion of all atoms and an extinction correction, resulted in weighted R factors (based on intensities) of 0.076 and 0.056, respectively, for the neutron and X-ray data. Very weak e reflections could be detected in long-exposure X-ray and electron diffraction photographs of this crystal, but the refined average structure is believed to be unaffected by the presence of such a weak superstructure. The ratio of the scattering power of Na to that of Ca is different for X ray and neutron radiation, and this radiation-dependence of scattering power has been used to determine the distribution of Na and Ca over a split-atom M site (two sites designated M' and M") in this Ano, plagioclase. Relative peak-height ratios M'/M", revealed in difference Fourier sections calculated from neutron and X-ray data, formed the basis for the cation-distribution analysis. As neutron and X-ray data sets were directly compared in this analysis, it was important that systematic bias between refined neutron and X-ray positional parameters could be demonstrated to be absent. In summary, with an M-site model constrained only by the electron-microprobedetermined bulk composition of the crystal, the following values were obtained for the M-site occupanciesN: ar, : 0.29(7),N ar. : 0.23(7),C ar, : 0.15(4),a nd Car" : 0.33(4). These results indicate that restrictive assumptions about M sites, on which previous plagioclase refinements have been based, are not applicable to this Ano, and possibly not to the entire compositional range. T-site ordering determined by (T-O) bond-length variation-t,o : 0.51(l), trm = t2o = t2m = 0.32(l)-is weak, as might be expectedf rom the volcanic origin of this megacryst.

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ID Code: 55664
Item Type: Journal Article
Refereed: Yes
Additional Information: GeoRef, Copyright 2009, American Geological Institute.
Hogarth Ranges
northern New South Wales
Keywords: crystal structure, experimental studies, feldspar group, framework silicates, megacrysts, mineralogy, plagioclase, refinement, statistical analysis, X-ray data
ISSN: 0003-004X
Subjects: Australian and New Zealand Standard Research Classification > EARTH SCIENCES (040000) > GEOLOGY (040300) > Mineralogy and Crystallography (040306)
Divisions: Current > Institutes > Institute for Future Environments
Deposited On: 21 Jan 2013 00:42
Last Modified: 21 Jan 2013 00:42

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