Coupled (H, M) substitutions in forsterite     

Feiwu Zhang  Kate Wright 《Geochimica et cosmochimica acta》2010,74(20):5958-5965

Coupled substitutions involving hydrogen plus trivalent elements (Al, Eu, Fe, Ga, Gd, Lu, Mn, Nd, Pu, Sc, Y and Yb) in forsterite (Mg₂SiO₄) are studied using atomistic simulation methods. Incorporation of hydrogen is energetically favourable when included in the forsterite lattice as hydroxyl groups (OH⁻) at O3 sites while the trivalent cations replace either magnesium or silicon. Our calculations show a strong dependence on the ionic radius of the impurity species and some variation with pressure. There are also significant structural distortions around the impurity defects. At low pressure (0 GPa), the smaller trivalent cations, (e.g. Al, Fe, Mn and Ga) substitute into forsterite by replacing Si as: . The larger trivalent cations (e.g. Eu, Gd, Lu, Nd, Pu, Y and Yb) however, replace Mg at the M2 site coupled with an Mg1 vacancy as described by . At 12 GPa, the large cations are more stable at Mg1 relative to Mg2, but both are predicted to be less stable than configurations associated with Si vacancies. The trivalent ionic radius has a significant effect on the H incorporation mechanism, however, the high formation energy of Si vacancies suggests that the presence of H in forsterite could inhibit incorporation of these elements, particularly at high pressure.  相似文献   

Discussion of ‘brackish water faunas from the St Maughans Formation: the old red sandstone section at Ammons Hill,Hereford and Worcester,UK’ by W.J. Barclay,P.A. Rathbone,D.E. White and J.B. Richardson     

V. P. Wright  S. B. Marriott 《Geological Journal》1996,31(1):89-91

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Fabrics,facies control and diagenesis of lacustrine ooids and associated grains from the Upper Triassic,southwest England     

P. G. Milroy  V. P. Wright 《Geological Journal》2002,37(1):35-53

Petrographic analysis of ooids from the Upper Triassic (Mercia Mudstone Group) of southwest England provides an opportunity to assess in detail the origins, transport pathways and diagenesis of an ancient oolite. The Clevedon Oolite is dolomitized and contains a variety of dissolved ooids (oomoulds) and associated grains. The oomoulds occur in well‐sorted, planar and cross‐stratified grainstones, packstones, sandstones and conglomerates associated with shoreface, intershoal, foreshore, beachrock and littoral strandplain deposits. The ooids grew in suspension in the shoreface zone and developed a radial aragonite microstructure. The ooids grew to 0.80 mm in diameter, after which they fractured or ceased growing. Broken grains deposited on or near mobile shoals were rapidly recoated, while other grains, deposited in less agitated, intershoal and lower foreshore areas, were micritized or microbially bound into grapestone aggregates. Locally peloids, intraclasts, quartz grains and micritized grains from intershoal areas supplied nuclei for ooids on nearby shoals. Grains deposited in foreshore areas were rapidly cemented into beachrock and reworked into conglomerates. Soon after deposition, the ooids were subjected to widespread aragonite dissolution followed by dolomitization. The lack of pre‐dolomitization calcite, together with the abundance of early (pre‐compaction) dolospar cements and fabric‐selective dolomitization of micritic fabrics, suggest aragonite dissolution by dolomitizing fluids. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献