MA23A-01
High-P/T Neutron Diffraction Study of Hydrous Minerals
Significant amounts of water are carried into the mantle via subduction of oceanic crust that contains hydrous minerals. Thus studying the structures and stability of hydrous minerals at elevated temperature and pressure conditions is important in determining the mode and fate of water in the Earth's interior. Since neutrons are more sensitive than X-rays to hydrogen and other light elements, in situ high-P/T neutron diffraction is a powerful tool for such studies. Using the high-P and/or high-T apparatuses coupled with time-of-flight neutron diffraction at LANSCE, we have investigated the structures and stability of several hydrous minerals, including portlandite, nickel hydroxide, brucite and jarosite. Rietveld analysis of the obtained data allowed determination of unit-cell parameters, atomic positions and atomic displacement parameters at various P/T conditions. The bulk moduli, thermal expansion coefficients and dehydroxylation P/T conditions of these compounds have also been determined. These studies thus provide important insights into the mechanisms of water storage and transport in the deep Earth.
MA23A-02
Neutron Scattering at Oak Ridge National Laboratory
Oak Ridge National Laboratory (ORNL) is home to two world-class neutron sources, the High Flux Isotope Reactor (HFIR) and the newly constructed Spallation Neutron Source (SNS). Suites of neutron scattering instruments at each facility span a broad spectrum of methodologies, including powder and single crystal diffraction, quasi-elastic scattering, inelastic scattering, reflectometry, and small angle scattering (SANS). For each of these methods, one or more instruments are available and designed to cover ranges of momentum and energy space that enable the study of many physical and chemical materials processes over different length scales. Both the HFIR and SNS user programs operate under a single management structure, http://neutrons.ornl.gov/. Currently, the geoscience community is grossly under-represented at these user facilities, yet the opportunities abound to do various in situ experiments with physically realistic samples that mimic geological conditions. ORNL has a long history in the development of neutron science, extending from the very beginnings of the field, pioneered by Clifford Shull during the period 1946-1955 at the historic Graphite Reactor, the first reactor designed and built for continuous operation. Clifford G. Shull was awarded the 1994 Nobel Prize in Physics with Canadian Bertram Brockhouse. This is the longest time after which the original work was completed that the Nobel Prize has been awarded. This research is supported by UT Battelle, LLC under Contract No. DE-AC05-00OR22725 for the U.S. Department of Energy, Office of Science.
MA23A-03
Natural Cold Working of Gold and Testing the Authenticity of Large Gold Crystals
The origin and nature of gold in placer deposits has been recently investigated in the context of possible supergene growth, although very little has been published about the crystalline nature of placer gold. We present here the first observations of natural cold working of gold crystal surfaces in alluvial deposits. Because of the malleability of gold and the mechanical forces involved in bedload transport in rivers, alluvial placer gold is usually highly deformed and found in nugget form. Natural single crystals of gold from some placer deposits may show distinct single crystal morphologies indicating minimal physical deformation. However, the authenticity of large (cm scale) gold crystals reportedly from placer deposits in Venezuela, Russia and elsewhere have been questioned. It has been a challenge to find definitive ways to verify the authenticity of gold crystals by nondestructive means. Broad beam X-ray diffraction analysis of crystals with specific oriented faces (hkl) yield diffraction patterns that include all peaks expected for a polycrystalline sample. Similar results from previous investigations have been interpreted to indicate that the crystals were cast and are not real. One of our samples however yields a single crystal diffraction pattern for one {111} face, with very weak non-{hhh} diffraction peaks, while exhibiting polycrystalline patterns on all other faces. From this observation we hypothesize that the polycrystalline X-ray diffraction pattern produced by most placer gold crystal faces is the result of cold working of the surface, which transforms the surface of the crystals into a polycrystalline mosaic. Cold working may be the result of mechanical distortion associated with stream transport. To test this hypothesis small naturally undeformed crystals were tumbled in an air tumbler among similar sized grains of quartz (to mimic stream transport). After only 24 hours of tumbling, the X-ray diffraction patterns changed from those of single crystals to complete polycrystalline patterns. This test supports the hypothesis that mechanical deformation associated with stream transport can cause the formation of a polycrystalline surface layer on gold. Thus, standard lab-source X-rays are not sufficient to determine the bulk crystallinity of gold samples because of their short depth of penetration. To test the bulk crystallinity neutron diffraction was used. Penetration depths of 1-1.5 cm were calculated for the gold crystals. The Venezuelan samples gave well defined single crystal neutron diffraction patterns while the reported Russian samples proved to be polycrystalline throughout and are interpreted to be casts. Future researchers using X-ray diffraction to study gold crystals need to be aware that production of a polycrystalline diffraction pattern does not necessarily mean that the entire sample is polycrystalline, and other methods of analysis may be necessary to evaluate the crystalline nature of the sample.