No abstract

Orientation distributions of garnet and omphacite in eclogite from the ultra-high pressure Dabie Shan belt in east-central China were determined from neutron diffraction data by the Rietveld method. Diffraction spectra were recorded in 16 sample orientations with seven detectors, with a kappa-geometry texture goniometer at the time-of-¯ight (TOF) neutron facility at the Intense Pulsed Neutron Source (IPNS). The textures of the two minerals were extracted simultaneously from 16 Â 7 = 112 diffraction spectra, covering a large portion of the pole ®gure. The texture analysis was performed both with the Williams± Imhof±Matthies±Vinel (WIMV) method and the harmonic method, implemented in the program package MAUD. The incomplete pole-®gure coverage introduced arti®cial oscillations in the case of the harmonic method. The discrete WIMV method produced better results, which illustrate a more or less random orientation distribution for cubic garnet. Apparently elongated grains turned out to be layers of randomly oriented crystals. Monoclinic omphacite displays a sharp texture, with [001] parallel to the lineation direction. The texture data obtained by neutron diffraction were veri®ed with EBSP (electron backscatter pattern) measurements.

Two ancient Greek coins were studied by time-of-flight (TOF) neutron diffraction to investigate their authenticity. Diffraction spectra were analyzed with the Rietveld programs GSAS and MAUD for phase proportions and texture. Both samples revealed very similar textures, consistent with pressing between dies. Also, both samples had similar phase proportions around 85% of silver and 15% of copper. The coin analysis revealed limitations of currently used processing methods for TOF neutron diffraction data. Despite the advantages of neutron diffraction for non-destructive bulk analyses, great care must be paid to the procedures to avoid artifacts. The conventional irregular and sparse coverage used at HIPD-LANSCE is problematic if absorption effects exist and the harmonic texture expansion is extended beyond l max = 6. Direct texture methods, such as EWIMV, are less affected by coverage limitations and a new method to deal with absorption is introduced. C 2004 Kluwer Academic Publishers