Xiande Xie

The Majorite-Pyrope + Magnesiowüstite Assemblage: Constraints on the History of Shock Veins in Chondrites

Shock veins in the Sixiangkou (L6) chondrite contain two high-pressure assemblages: (i) majorite-pyrope solid solution plus magnesiowüstite that crystallized at high pressures and temperatures from a shock-induced silicate melt of bulk Sixiangkou composition and (ii) ringwoodite plus low-calcium majorite that were produced by solid-state transformation of olivine and low-calcium pyroxene. The morphology and chemistry of the majorite-pyrope garnet and the size of the magnesiowüstite crystals indicate a longer duration at high pressure and temperature than predicted by impact scenarios. This pressure-temperature regime is constrained by the olivine-ringwoodite and orthopyroxene-majorite phase transformations, fusion of the meteorite constituents, and crystallization of majorite-pyrope solid solution plus magnesiowüstite from that melt under high pressure.

Arrangement models of alkylammonium cations in the interlayer of HDTMA~+ pillared montmorillonites

The orientation of HDTMA+ in the interlayer of organic pillared montmorillonites prepared at different concentrations of HDTMA+ have been studied using X-ray powder diffraction (XRD) and theoretical calculation. A series of arrangement models of HDTMA+ in the interlayer of montmorillonite have been proposed as lateral-monolayer (LM), lateral-bilayer (LB), pseudotrilayer (PT), paraffin-type-monolayer (PM), paraffin-type-bilayer (PB). With the increase of the concentration of HDTMA+, the arrangement model of HDTMA+ in the interlayer of montmorillonites changes as lateral-monolayer→lateral-bilayer→paraffin-type monolayer→pseudotrilayer→paraffin-type-bilayer and the packing density of HDTMA+ in the interlayer increases gradually. In the intermediate stages, different models may coexist.

NATURAL OCCURRENCE AND SYNTHESIS OF TWO NEW POSTSPINEL POLYMORPHS OF CHROMITE

A high-pressure polymorph of chromite, the first natural sample with the calcium ferrite structure, has been discovered in the shock veins of the Suizhou meteorite. Synchrotron x-ray diffraction analyses reveal an orthorhombic CaFe2O4-type (CF) structure. The unit-cell parameters are a = 8.954(7) Å, b = 2.986(2) Å, c = 9.891(7) Å, V = 264.5(4) Å3 (Z = 4) with space group Pnma. The new phase has a density of 5.62 g/cm3, which is 9.4% denser than chromite-spinel. We performed laser-heated diamond anvil cell experiments to establish that chromite-spinel transforms to CF at 12.5 GPa and then to the recently discovered CaTi2O4-type (CT) structure above 20 GPa. With the ubiquitous presence of chromite, the CF and CT phases may be among the important index minerals for natural transition sequence and pressure and temperature conditions in mantle rocks, shock-metamorphosed terrestrial rocks, and meteorites.

Location and migration of cations in Cu2+-adsorbed montmorillonite

Locations of Cu2+ ion in Cu(2+)-adsorbed montmorillonite have been studied by electron paramagnetic resonance (EPR), supplemented by X-ray diffraction (XRD) and differential thermal analysis (DTA). In the EPR spectra of Cu(2+)-adsorbed montmorillonite, three signals, corresponding to Cu2+ ion, have been simultaneously recorded. Some Cu2+ ions seemed to replace the original interlayer metal cations and some entered into the hexagonal cavities. A small fraction of Cu2+ ions penetrated into the octahedral vacancies. There were two ways for the adsorption of Cu2+ ion by montmorillonite--exchangeable and specific. On heating, the hydrated Cu2+ ion in the interlayer loses the coordinating water and then enters into the hexagonal cavities. When the heating temperature further increased, dehydroxylation occurs, which facilitates Cu2+ ion in the hexagonal cavities to penetrate into the octahedral vacancies.

Natural CaTi2O4-structured FeCr2O4 polymorph in the Suizhou meteorite and its significance in mantle mineralogy

Abstract The first natural occurrence of a high-pressure polymorph of chromite has been discovered in the shock-metamorphosed Suizhou meteorite. The composition of this high-pressure polymorph is identical to that of the precursor chromite. The Raman spectrum of this polymorph is distinct from that of chromite. Synchrotron X-ray diffraction analysis revealed that this polymorph has an orthorhombic CaTi2O4- type structure. The cell parameters are: a = 9.462(6) A, b = 9.562(9) A, c = 2.916(1) A, V = 263.8(4) A3 (Z=4), space group = Bbmm, and the density = 5.63 g/cm3 (the numbers in parentheses are standard deviations on the last significant digits). This polymorph is 11.5% denser than chromite. The P-T conditions for the phase transformation from chromite to the CaTi2O4-structured polymorph are estimated at 20–23 GPa and 1800 to 2000 °C, respectively. This dense CaTi2O4-structured FeCr2O4 phase could be a host phase for Cr, Al, Fe, Mg and Mn and other metallic elements in the deep Earth.

Tuite, γ-Ca 3 (PO 4 ) 2 : a new mineral from the Suizhou L6 chondrite

Tuite, γ-Ca 3 (PO 4 ) 2 , the high-pressure polymorph of whitlockite, was found in a shock melt vein of the Suizhou chondrite. It occurs as polycrystalline aggregates in association with coarse-grained aggregates of ringwoodite, majorite and NaAlSi 3 O 8 -hollandite. These high-pressure mineral assemblages are enclosed in the fine-grained matrix of the vein that consists of majorite-pyrope ss garnet, metal and troilite. This new mineral is trigonal, R 3 m,a = 5.258 A, c = 18.727 A. The tuite is colourless and transparent with vitreous lustre and white streak; it is uniaxial (+) with high refractive indices (ϵ = 1.706, ω = 1.701) and low birefringence (0.005). Microprobe analyses of tuite yield an empirical formula (Ca 2.51 Mg 0.29 ) 2.80 Na 0.28 (P 1.01 O 4 ) 2 . The simplified formula, Ca 3 (PO 4 ) 2 , is the same as whitlockite outside the veins. Stronger reflections on synchrotron radiation X-ray powder diffraction pattern of tuite are d = 2.628 (100), 2.891 (80), 1.945 (47), 1.730 (25) and 1.567(22). The figures in brackets are relative intensities. Tuite in the shock vein of this meteorite was transformed from whitlockite via a shock-produced solid-state reaction, while the pressure and temperature in the shock vein attained 23 GPa and 2000°C. This new mineral and the mineral name (tuite) have been approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.

NATURAL HIGH-PRESSURE POLYMORPH OF MERRILLITE IN THE SHOCK VEINS OF THE SUIZHOU METEORITE

Abstract A new high-pressure polymorph of merrillite with the structure of trigonal γ-Ca 3 (PO 4 ) 2 was found in the shock-produced veins of the Suizhou meteorite, where it coexists with ringwoodite, majorite, NaAlSi 3 O 8 -hollandite, and majorite-pyrope garnet. The crystallographic nature of this natural γ-Ca 3 (PO 4 ) 2 phase was characterized by Raman spectroscopy and X-ray diffraction, and all data compare favorably to the same data obtained from γ-Ca 3 (PO 4 ) 2 synthesized at 14 GPa and 1400°C. The cell parameters of this new high-pressure mineral are a = 5.258(1) angstroms and c = 18.727(3) angstroms, space group R-3m, and density = 3.447 (g/cm 3 ), where the number in parentheses are standard deviations in the last significant digits. The natural occurrence of the γ-Ca 3 (PO 4 ) 2 phase together with other high-pressure minerals constrains the pressure of the shock veins at about 23 GPa. The Suizhou meteorite provides the first naturally occurring example of γ-Ca 3 (PO 4 ) 2 polymorph.

Shock-related mineralogical features and P-T history of the Suizhou L6 chondrite

The Suizhou meteorite, classified as an L6 chondrite, contains weakly shocked olivine and pyroxene, but almost all the plagioclase in the meteorite was melted and transformed into maskelynite during shock metamorphism. Chromite was heavily fragmented and granulated, and many tiny chromite fragments were incorporated into the molten plagioclase as inclusions. Metal and troilite show no obvious intragranular textures, but many tiny rounded FeNi metal grains were deposited in the intersecting joints of planar fractures in olivine and pyroxene. A few very thin shock melt veins occur in the Suizhou meteorite, which contain abundant high-pressure phases, including coarse-grained ringwoodite, majorite, (Na,Ca)AlSi3O8-hollandite and fine-grained liquidus majorite-pyrope garnet. The shock features of this meteorite match shock stages 3 to 5, while the presence of ringwoodite in Suizhou veins is considered to appear at stage 6. It is estimated that the Suizhou meteorite experienced a shock pressure and shock temperature of up to 22 GPa and 1000°C, respectively. The shearing friction along veins raised the temperature within the veins. Shock-induced pressure and temperature in the shock veins attained 22 GPa and 1900°C. Therefore, the actual shock level of the Suizhou meteorite could correspond to stage 3–4. A longer duration of the shock pressure and temperature regime in the Suizhou meteorite plays an important role in the pervasive melting of plagioclase in the unmelted part of the meteorite, as well as in the formation of abundant high-pressure phases in the very thin shock-melt veins. It appears that maskelynite cannot be used as the sole criteria for evaluating the shock stage of shock-metamorphosed chondrites.

Records of magnetic properties in Quaternary loess and its paleoclimatic significance: a brief review

Due to the successive deposition, Quaternary loess–paleosol sequences potentially provide one of the best terrestrial records of paleoclimatic changes. Magnetostratigraphy and paleoclimate are two major aspects of magnetic investigation in northern China and other regions of the world. The classic loess sections across the Chinese Loess Plateau indicate that wind-blown loess deposition began close to the base of Matuyama (2.6 Ma). However, progress has also been made in extending the record below the loess into the Red Clay, and the aeolian record can now be carried back beyond 7 Ma. The surprising similarity between the initial susceptibility (IS) curve obtained from numerous loess–paleosol sequences and the oxygen isotope fluctuation from the deep-sea sediments provides impressive evidence of the global significance of the magnetic record. As a useful proxy in paleoclimatic studies, magnetic susceptibility (MS) has been extensively used to reconstruct the paleoclimate. However, the actual mechanism of MS enhancement of paleosols remains controversial. The important questions—why does IS fluctuate in loess sections, and what are the causes of the IS enhancement in paleosols?—have been matters of debate in the last 10 years. Currently, the viewpoint that pedogenesis plays an important role in the MS enhancement of paleosols has been generally accepted. Pedogenic magnetite is an important contributor to the MS enhancement. IS is not only simply controlled by the abundance of strong magnetic minerals, but also by their grain-size distributions. The increment of superparamagnetic (SP) grains (o0.1mm), which is induced by pedogenesis, could enhance the IS of paleosols. The formation of SP minerals has been debated as either biotic or abiotic. It is now possible to propose a quantitative model for the reconstruction of paleoclimate according to the paleoprecipitation deduced from proxy indicators, such as IS fluctuation, grain size, stable isotope composition, CaCO3 content, Rb/Sr, and ratio of citrate-bicarbonatedithionite-extractable Fe2O3 to total Fe2O3 (FeD/Fet). r 2002 Elsevier Science Ltd and INQUA. All rights reserved.

A comparative study of naturally and experimentally shocked chondrites

Samples of the Jilin H5 chondrite were experimentally shock-loaded at the peak pressures of 12, 27, 39, 53, 78, 83, 93, and 133 GPa. The aim of this study is to compare experimentally shock-induced phenomena with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced shock phenomena in Hand L-chondrites. Planar fractures, mosaicism, brecciation in olivine and pyroxene, as well as transformation of plagioclase into diaplectic glass were observed in the Jilin samples shocked at pressures lower than 53 GPa. Shock-induced chondritic melts were first obtained at P > 78 GPa and more than 60% of the whole-rock melting was achieved at P similar to 133 GPa, and that shook-induced silicate melt consists of quenched microcrystalline olivine and pyroxene, metal, troilite and vesicular glass. No high-pressure phases were observed in any of the experimentally shocked samples, neither in the deformed nor in the molten regions. Deformation features in Jilin samples shock-loaded below 53 GPa are comparable to those found in H- and L-chondrites. The mineral assemblages in the molten regions in the shocked Jilin samples are also comparable to those encountered in the heavily shocked Yanzhuang (H6) and some Antarctic H-chondrites, but differ considerably from those found in heavily shocked Sixiangkou and many other L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine, pyroxene, plagioclase and high-pressure liquidus phases, whereas shock melt veins in heavily shocked H-chondrites contain mainly low-pressure mineral assemblages. The differences in the mineral constituents of shock melt veins in L- and H-chondrites clearly indicate differences in the shock histories of these meteorites. While crystallization in the shook melt veins in L-chondrites took place at high pressures, crystallization in shock-induced melt in most H-chondrites took place after decompression. It is evident that the thickness and abundance of shock melt veins and size of melt regions is not necessarily a quantitative measure of the degree of shock. The duration of the high-pressure regime, the time of the cooling and the P-T regime during the crystallization path, and the post-shock temperatures are stringent parameters that control the evolution of the shock-induced melt. So, scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids