Weathering in Antarctic H and CR chondrites: Quantitative analysis through Mössbauer spectroscopy     

Roger G. Burns  Thomas H. Burbine  Duncan S. Fisher  Richard P. Binzel 《Meteoritics & planetary science》1995,30(6):625-633

Abstract— Mössbauer spectroscopy is a very useful tool for identifying ferric iron weathering products in meteorites because of the capability to quantify the relative amounts of ferric iron in them. Mössbauer measurements were made of 33 Antarctic H chondrites (predominately H5) and two paired Antarctic CR chondrites. The primary goals of this study are to determine if Mössbauer spectroscopy can be used to determine which phases are weathering in Antarctic meteorites and if the relative amounts of ferric iron correlate with terrestrial age. Determining which minerals are weathering in ordinary chondrites appears very difficult due to variations in composition for different ordinary chondrites of the same meteorite class and possible problems in preparing homogeneous samples. The analysis of the two paired CR chondrites appears to indicate that metallic iron is predominately weathering to produce ferric iron for this class of meteorite. No correlation is seen between the relative amounts of ferric iron and terrestrial age for ordinary chondrites. One Antarctic H5 chondrite (ALHA77294) with a short ¹⁴C age of 135 ± 200 years from the dating of interior carbonate weathering products does have a relatively low amount of ferric iron, which is consistent with this meteorite being exposed on the surface for a relatively short time.  相似文献   

Chemical characteristics of migmatites: accessory phase distribution and evidence for fast melt segregation rates   总被引：8，自引：0，他引：8  

G. R. Watt  I. M. Burns  G. A. Graham 《Contributions to Mineralogy and Petrology》1996,125(1):100-111

 Equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration (armouring, slow dissolution). When segregation occurs by channelised migration along high-porosity pathways, melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates. Evidence for channelised flow and deformation-enhanced melt segregation into boudin necks, fractures and micro-shears at low melt fractions is present in the Moine Kirtomy Migmatitie Suite (KMS) in Sutherland, Scotland. Melt migration distances are on a metre to tens of metres scale. Concordant leucosomes in stromatic migmatities in the KMS have low Zr contents, low LREE (light rare-earth element) and H (heavy) REE contents and positive Eu anomalies. REE patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of LREE- and HREE-bearing accessory phases in water-undersaturated melts. Melting in the KMS, however, occurred at or near the wet granite solidus, leaving biotite as a residual phase. Detailed back-scattered electron imaging shows that REE-bearing accessory phases remained as residual phases, and were concentrated in the melanosome and at the melanosome-leucosome boundary. Irregularly shaped patches of diatexite contain a small proportion of excess Zr, consistent with entrainment of melanosome-schlieren enriched in zircon. These data indicate that deformation-enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time-scale less than that required to saturate the melt in Zr. Leucosomes were thus prevented from equilibrating with accessory phases before extraction. Received: 12 July 1995 / Accepted: 4 March 1996  相似文献   

A structural phase-transition in K(Mg1−xCux)F3 perovskite     

Peter C. Burns  Frank C. Hawthorne  Anne M. Hofmeister  Stephanie L. Moret 《Physics and Chemistry of Minerals》1996,23(3):141-150

A complete solid-solution series between cubic (Pm 3 m) KMgF₃ and tetragonal (I4/mcm) KCuF₃ was synthesized at 730–735 °C in an inert atmosphere. X-ray powder-diffraction at room temperature shows that the transition between the cubic and tetragonal perovskite structures in the series K (Mg_1?xCu_x) F₃ occurs at x ～ 0.6. Rietveld structure-refinements were done for selected compositions. In the cubic phase, all parameters are linear with composition up to the transition point. At the transition point, there is a strong discontinuity in the cell volume; this is strongly anisotropic with expansion along the a axes and contraction along the c axis due to a pronounced axial elongation of the (Mg, Cu) F₆ octahedron that increases with increasing Cu content. The phase transition is first-order, with a discontinuity of ≈2% in the symmetry-breaking strain at x_C. It is proposed that the phase transition in K (Mg, Cu) F₃ is due to the onset of the cooperative Jahn-Teller effect. Compositional relationships for lattice vibrations in this solid solution were established using thin-film infrared spectroscopy. A phase transition occurring above 60 mole % KCuF₃ is indicated by the appearance of one of the two modes expected for the tetragonal phase; the weaker mode is not resolved below 80 mole % KCuF₃. Modes common to both structures vary smoothly and continuously across the binary; however, frequencies do not depend linearly on composition, nor is mode-softening discernable. Two-mode behaviour is observed only for the bending motion of the cubic phase, because this peak alone has non-overlapping end-member components.  相似文献   

Turbulence statistics of a Kelvin–Helmholtz billow event observed in the night-time boundary layer during the Cooperative Atmosphere–Surface Exchange Study field program     

William Blumen  Robert Banta  Sean P. Burns  David C. Fritts  Rob Newsom  Gregory S. Poulos  Jielun Sun   《Dynamics of Atmospheres and Oceans》2001,34(2-4)

An apparent shear flow instability occurred in the stably stratified night-time boundary layer on 6 October 1999 over the Cooperative Atmosphere–Surface Exchange Study (CASES-99) site in southeast Kansas. This instability promoted a train of billows which appeared to be in different stages of evolution. Data were collected by sonic anemometers and a high-frequency thermocouple array distributed on a 60 m tower at the site, and a high resolution Doppler lidar (HRDL), situated close to the tower. Data from these instruments were used to analyze the characteristics of the instability and the billow event. The instability occurred in a layer characterized by a minimum Richardson number Ri0.13, and where an inflection in the background wind profile was also documented. The billows, which translated over the site for approximately 30 min, were approximately L320 m in length and, after billow evolution they were contained in a layer depth H30 m. Their maximum amplitude, determined by HRDL data, occurred at a height of 56 m. Billow overturns, responsible for mixing of heat and momentum, and high-frequency intermittent turbulence produce kurtosis values above the Gaussian value of 3, particularly in the lower part of the active layer.  相似文献   

Mössbauer spectra of kyanite,aquamarine, and cordierite showing intervalence charge transfer     

Kathleen M. Parkin  Bruce M. Loeffler  Roger G. Burns 《Physics and Chemistry of Minerals》1977,1(3):301-311

The blue colors of several minerals and gems, including aquamarine (beryl, Be₃Al₂Si₆O₁₈) and cordierite (Al₃(Mg, Fe)₂Si₅AlO₁₈), have been attributed to charge transfer (CT) between adjacent Fe²⁺ and Fe³⁺ cations, while Fe²⁺→Ti⁴⁺ CT has been proposed for blue kyanites (Al₂SiO₅). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe²⁺ and Fe³⁺ were detected with MS, indicating that Fe²⁺→Fe³⁺ CT, Fe²⁺→Ti⁴⁺ CT, and Fe²⁺ and Fe³⁺ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe²⁺ and Fe³⁺ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm^?1 in E∥c spectra to Fe²⁺→Fe³⁺ CT between Fe cations replacing Al³⁺ ions 4.6Å apart along c. A band at 17,500 cm^?1 in E⊥c spectra of cordierite is generally assigned to Fe²⁺ (oct)→Fe³⁺ (tet) CT between cations only 2.74 Å apart. However, no Fe³⁺ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm^?1 band and reported to contain Fe³⁺. A quadrupole doublet with parameters consistent with tetrahedral Fe³⁺ appears in 77K MS, but the Fe³⁺/Fe²⁺ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe³⁺/Fe²⁺ ratios for minerals exhibiting Fe²⁺→Fe³⁺ CT.  相似文献   

Ilvaite: A study of temperature dependent electron delocalization by the mössbauer effect     

D. A. Nolet  R. G. Burns 《Physics and Chemistry of Minerals》1979,4(3):221-234

The mixed valence iron silicate ilvaite, CaFe ₂ ²⁺ Fe³⁺Si₂O₈(OH), displays electron delocalization associated with Fe²⁺→Fe³⁺ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe²⁺(A), Fe²⁺(B), Fe³⁺(A), and Fe²⁺(A)→Fe³⁺(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints.  相似文献   

Mössbauer spectroscopy: An introduction for inorganic chemists and geochemists: G.M. Bancroft. Mc-Graw-Hill Book Co. (United Kingdom) Ltd., 1973, 252 pp., US $ 21.50     

Roger G. Burns 《Chemical Geology》1975,16(1):65-67

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A detailed magnetic study of Cobb Seamount; a correction     

Ronald T. Merrill  Robert E. Burns 《Earth and Planetary Science Letters》1974,22(4):427-428

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Surface and downhole shear wave seismic methods for thick soil site investigations     

J. A. Hunter  B. Benjumea  J. B. Harris  R. D. Miller  S. E. Pullan  R. A. Burns  R. L. Good 《Soil Dynamics and Earthquake Engineering》2002,22(9-12):931-941

Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.  相似文献   

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)     

Peter C. Burns  Michael E. Fleet 《Physics and Chemistry of Minerals》1990,17(2):108-116

The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi₃O₈) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaAlSi₃O₈]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi₃O₈, grown in a Na₂WO₄ flux, was ordered by hydrothermal annealing below 820° C and dry annealing above 820° C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890° C 970° C. The gallium content of the T ₁o site increases continuously with decreasing temperature. The gallium contents of the T ₁m and T ₂m sites decrease smoothly with increasing ordering while the gallium content of the T ₂o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913±3° C and 937±3° C. These abrupt changes correlate with changes in the gallium content of the T ₂o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGa_0.5Al_0.5Si₃O₈) occurs in the temperature range 765° C-850° C, at a temperature intermediate to the transformation in albite (50% order at about 680±20° C) and gallium albite.  相似文献