Dehydration dynamics of Ba-phillipsite: an in situ synchrotron powder diffraction study

 The stepwise dehydration process of the Ba-exchanged form of the zeolite phillipsite was studied by in situ synchrotron X-ray powder diffraction. A series of structure refinements were performed using the Rietveld method on powder diffraction data measured in the interval between 332 and 712 K. At 482 K, more than half of the water molecules were lost. The continuous water loss causes the Ba cations to migrate inside the zeolite channels in order to achieve a stable coordination with the framework oxygens. The dehydration process was completed at 663 K, where a new, completely dehydrated stable phase was detected. The temperature range of stability of this phase was more than 100 K, thanks to the stable coordination of the Ba cations with the framework oxygens. This phase is the first example of completely dehydrated zeolite containing divalent (barium) cations. Received: 8 January 2001 / Accepted: 1 November 2001     

On the elastic behaviour of zeolite mordenite: a synchrotron powder diffraction study

The high-pressure elastic behaviour of a synthetic zeolite mordenite, Na₆Al_6.02Si_42.02O₉₆·19H₂O [a=18.131(2), b=20.507(2), c=7.5221(5) Å, space group Cmc2₁], has been investigated by means of in situ synchrotron X-ray powder diffraction up to 5.68 GPa. No phase transition has been observed within the pressure range investigated. Axial and volume bulk moduli have been calculated using a truncated second-order Birch–Murnaghan equation-of-state (II-BM-EoS). The refined elastic parameters are: V ₀=2801(11) ų, K _T0= 41(2) GPa for the unit-cell volume; a ₀=18.138(32) Å, K _T0(a)=70(8) GPa for the a-axis; b ₀=20.517(35) Å, K _T0(b)=29(2) GPa for the b-axis and c ₀=7.531(5) Å, K _T0(c)=38(1) GPa for the c-axis [K _T0(a): K _T0(b): K _T0(c)=2.41:1.00:1.31]. Axial and volume Eulerian finite strain versus “normalized stress” plots (fe–Fe plot) show an almost linear trend and the weighted linear regression through the data points yields the following intercept values: Fe(0)=39(4) GPa for V; Fe _a (0)=65(18) GPa for a; Fe _b (0)=28(3) GPa for b; Fe _c (0)=38(2) GPa for c. The magnitudes of the principal Lagrangian unit-strain coefficients, between 0.47 GPa (the lowest HP-data point) and each measured P>0.47 GPa, were calculated. The unit-strain ellipsoid is oriented with ε₁ || b, ε₂ || c, ε₃ || a and |ε₁|> |ε₂|> |ε₃|. Between 0.47 and 5.68 GPa the relationship between the unit-strain coefficient is ε₁: ε₂: ε₃=2.16:1.81:1.00. The reasons of the elastic anisotropy are discussed.An erratum to this article can be found at     

Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction

Thermal behaviour and kinetics of dehydration of gypsum in air have been investigated using in situ real-time laboratory parallel-beam X-ray powder diffraction data evaluated by the Rietveld method. Thermal expansion has been analysed from 298 to 373 K. The high-temperature limits for the cell edges and for the cell volume, calculated using the Einstein equation, are 4.29 × 10⁻⁶, 4.94 × 10⁻⁵, 2.97 × 10⁻⁵, and 8.21 × 10⁻⁵. Thermal expansion of gypsum is strongly anisotropic being larger along the b axis mainly due to the weakening of hydrogen bond. Dehydration of gypsum has been investigated in isothermal conditions within the 348–403 K range with a temperature increase of 5 K. Dehydration proceeds through the CaSO₄·2H₂O → CaSO₄·0.5H₂O → γ-CaSO₄ steps. Experimental data have been fitted with the Avrami equation to calculate the empirical activation energy of the process. No change in transformation mechanism has been observed within the analysed temperature range and the corresponding E _a is 109(12) kJ/mol.     

In situ X-ray diffraction study of an aluminous phase in MORB under lower mantle conditions

In situ X-ray diffraction study was conducted to identify the crystal structure of the “Al-phase”, which was previously reported to form in basaltic compositions at pressures and temperatures of the uppermost part of the lower mantle. Le Bail whole-pattern fitting method was adopted to investigate the structure of the Al-phase under high pressure and temperature as well as ambient conditions. Observed patterns were satisfactorily fitted using the “hexagonal phase” with space group P6₃/m (plus minor amount of garnet) under both of these conditions. On the other hand, the calcium ferrite structure model proposed in some earlier studies based on quench experiments yielded profile-fitting results at significantly lower confidence levels, particularly at simultaneous high pressure and high temperature conditions, suggesting that this phase may not form in oceanic crust materials subducted in the uppermost lower mantle. The difference in densities of hexagonal and calcium ferrite phases, however, is only ~1% under pressures and temperatures of the uppermost part of the lower mantle conditions, which yields a negligible effect on the bulk density of the subducted oceanic crust.     

High temperature structural and thermoelastic behaviour of mantle orthopyroxene: an in situ neutron powder diffraction study

The temperature induced structural evolution and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula ^M2(Mg_0.856Ca_0.025Fe²⁺ _0.119) ^M1(Mg_0.957Fe²⁺ _0.011Fe³⁺ _0.016Cr_0.011Al_0.005)Al_0.032Si_1.968O₆, from a suite of high pressure ultramafic nodules of mantle origin, have been investigated by in-situ neutron powder diffraction at several temperatures starting from 1,200°C down to 150°C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, α = V ⁻¹(∂V/∂T) _P0, varies linearly with T. The axial thermal expansion coefficients, α_j = l _j⁻¹(∂l _j/∂T)_P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (ɛ//a, ɛ//b, ɛ//c), increase continuously with T. However, the orientation of the unit-strain ellipsoid appears to change with T. With decreasing T, the values of the unit-strain coefficients along the b and c axes tend to converge. The orientation at ΔT = 1,080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TA _n OA_3n and TB _n OB_3n , are kinked differently. At room-T, the TB _n OB_3n chain is more strongly kinked by about 23° than the TA _n OA_3n chain. With increasing T, the difference decreases by 3° for the TB _n OB_3n chain. The intersite cation exchange reaction between M1 and M2 (Mg²⁺ and Fe²⁺) shows a slight residual order at 1,200°C followed by reordering with decreasing temperature although seemingly not with a definite progressive trend. At the lowest temperature reached (150°C), reordering has occurred with the same value of partitioning coefficient K _D as that before heating. The absence of the expected phase transition is most likely due to the presence of minor amounts of Fe³⁺, Al, Ca and Cr which must play a crucial role on the thermoelastic behaviour and phase stability fields in natural Opx, with consequent important petrologic and geological implications.     

本期文章导读

正117高温高压下元素配分的原位实验与计算模拟研究进展作者:黄圣轩,巫翔*,秦善导读撰文:本刊编辑部高温高压下元素配分行为研究揭示了地球内部物质迁移、富集、分异的动力学过程,有助于解决元素成矿、超临界流体性质、地球及行星演化等基础地质问题,近年来成为地球化学研究热点之一。同步辐射微聚焦X射线荧光光谱结合金刚石压腔技术原位测试方法逐渐取     

In situ study of the $$ R\overline{3} c \to R\overline{3} m $$ orientational disorder in calcite

The temperature dependences of the crystal structure and intensities of the (113) and (211) reflections in calcite, CaCO₃, were studied using Rietveld structure refinements based on synchrotron powder X-ray diffraction data. Calcite transforms from to at about T _c = 1,240 K. A CO₃ group occupies, statistically, two positions with equal frequency in the disordered phase, but with unequal frequency in the partially ordered phase. One position for the CO₃ group is rotated by 180° with respect to the other. The unequal occupancy of the two orientations in the partially ordered phase is obtained directly from the occupancy factor, x, for the O1 site and gives rise to the order parameter, S = 2x − 1. The a cell parameter shows a negative thermal expansion at low T, followed by a plateau region at higher T, then a steeper contraction towards T _c, where the CO₃ groups disorder in a rapid process. Using a modified Bragg–Williams model, fits were obtained for the order parameter S, and for the intensities of the (113) and (211) reflections. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The determination ofin situ rock thermal properties and the simulation of climate in an underground mine

Summary This paper describes the results from a continuing research project started in 1984, which is investigating improved methods for predicting the underground climate in hot mines. The Homestake Gold Mine in South Dakota was the site for testing methods, based on radial heat flow theory, to measurein situ values of the thermal conductivity and diffusivity of rock. The methods used proved to be quite within the capability of mine staff to carry out for themselves. The results were acceptable and verified thatin situ thermal rock properties can differ considerably from laboratory determined values, in this case by a factor of approximately two.A mine climate simulation program, CLIMSIM, was applied to case studies from the Homestake mine using the measured thermal rock parameters. The results showed good correlation between the measured and simulated values.     

田黄的X射线衍射研究

用X射线衍射分析的方法测定了12个田黄样品,其中,青田石1个,寿山石9个,昌化田黄石2个。确定青田石由叶蜡石(58%)、绢云母(36.1%)和高岭石(5.9%)组成;昌化田黄石由迪开石组成;寿山石分别由迪开石、高岭石、叶蜡石及绢云母组成。     

High-T phase transition of synthetic ANaB(LiMg)CMg5Si8O22(OH)2 amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study

The synthetic amphibole Na_0.95(Li_0.95Mg_1.05)Mg₅Si₈O₂₂(OH)₂ was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P2₁ /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm⁻¹, respectively, and two minor bands at 3,667 and 3,687 cm⁻¹. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site (^A□_0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T _c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P2₁ /m ⇔ C2/m phase transition at 255°C for synthetic amphibole ^ANa_0.8^B(Na_0.8Mg_1.2)^CMg₅Si₈O₂₂(OH)₂ indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T _c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.     

Determination of cation distribution in (Co,Ni,Zn)2SiO4 olivine by synchrotron X-ray diffraction

 The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal diffraction method. The crystal data are (Co_0.377Ni_0.396Zn_0.227)₂SiO₄, M _r  = 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm³, Z = 4, D _x  = 4.864 g cm⁻³, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)_M1/(A/B)_M2] = −0.272 (IR _A -IR _B ) + 3.65 (EN _A −EN _B ), where IR (pm) and EN are ionic radius and electronegativity, respectively. Received: 8 April 1999 / Revised, accepted: 7 September 1999     

Structural changes and oxidation of ferroan phlogopite with increasing temperature: in situ neutron powder diffraction and Fourier transform infrared spectroscopy

The thermal response of the natural ferroan phlogopite-1M, K₂(Mg_4.46Fe_0.83Al_{0. 34}Ti_0.22)(Si_5.51Al_{2. 49})O₂₀[OH_3.59F_0.41] from Quebec, Canada, was studied with an in situ neutron powder diffraction. The in situ temperature conditions were set up at ?263, 25, 100°C and thereafter at a 100°C intervals up to 900°C. The crystal structure was refined by the Rietveld method (R _p=2.35–2.78%, R _wp=3.01–3.52%). The orientation of the O–H vector of the sample was determined by the refinement of the diffraction pattern. With increasing temperature, the angle of the OH bond to the (001) plane decreased from 87.3 to 72.5°. At room temperature, a = 5.13 Å, b = 9.20 Å, c = 10.21 Å, β = 100.06° and V(volume) = 491.69 ų. The expansion rate of the unit cell dimensions varied discontinuously with a break at 500°C. The shape of the M-octahedron underwent some significant changes such as flattening at 500°C. At temperatures above 500°C, the octahedral thickness and mean distance was decreased, while the octahedral flattening angle increased. Those results were attributed to the Fe oxidation and dehydroxylation processes. The dehydroxylation mechanism of the ferroan phlogopite was studied by the Fourier transform infrared spectroscopy (FTIR) after heated at temperatures ranging from 25 to 800°C with an electric furnace in a vacuum. In the OH stretching region, the intensity of the OH band associated with Fe²⁺(N _B-band) begun to decrease outstandingly at 500°C. The changes of the IR spectra confirmed that dehydroxylation was closely related to the oxidation in the vacuum of the ferrous iron in the M-octahedron. The decrease in the angle of the OH bond to the (001) plane, with increasing temperature, might be related to the imbalance of charge in the M-octahedra due to Fe oxidation.     

Determination of the phase boundary between ilmenite and perovskite in MgSiO3 by in situ X-ray diffraction and quench experiments

Determination of the phase boundary between ilmenite and perovskite structures in MgSiO₃ has been made at pressures between 18 and 24 GPa and temperatures up to 2000 °C by in situ X-ray diffraction measurements using synchrotron radiation and quench experiments. It was difficult to precisely define the phase boundary by the present in situ X-ray observations, because the grain growth of ilmenite hindered the estimation of relative abundances of these phases. Moreover, the slow reaction kinetics between these two phases made it difficult to determine the phase boundary by changing pressure and temperature conditions during in situ X-ray diffraction measurements. Nevertheless, the phase boundary was well constrained by quench method with a pressure calibration based on the spinel-postspinel boundary of Mg₂SiO₄ determined by in situ X-ray experiments. This yielded the ilmenite-perovskite phase boundary of P (GPa) = 25.0 (±0.2) – 0.003 T (°C) for a temperature range of 1200–1800 °C, which is generally consistent with the results of the present in situ X-ray diffraction measurements within the uncertainty of ∼±0.5 GPa. The phase boundary thus determined between ilmenite and perovskite phases in MgSiO₃ is slightly (∼0.5 GPa) lower than that of the spinel-postspinel transformation in Mg₂SiO₄. Received: 19 May 1999 / Accepted: 21 March 2000     

A diametral deformation method forin situ stress and rock property measurement

Summary Failure deformation of a borehole boundary in response to hydraulic loading via a plastic cylinder is directly related to both the initial stress state and the material properties of the ground. These relationships are utilized for determination ofin situ stress states and material properties of complex geological formations, including fractured hard rock and soft ductile media, by directly measuring diametral deformation of the borehole in relation to the applied pressure. This new method was first developed by means of finite element simulation models, and later confirmed by laboratory model studies. This method was then field tested in an outcropping of homogeneous sandstone and later applied to three underground geological formations: highly stratified shale, rock salt, and fractured green schist. Some of the results of these field applications are presented here to illustrate the validity and usefulness of this method, especially for complex ground where the conventional methods of overcoring and hydrofracturing may be ineffective. The instrumentation of this method involves a borehole probe which utilizes electrically powered hydraulic pumping and computerized data acquisition for rapid measurement and on-site analysis. The significance of the new method is its ability to obtain input data ofin situ stress states and material properties for finite element analysis of earth structures in complex ground.     

Thermal behaviour of β-anhydrite CaSO<Subscript>4</Subscript> to 1,263 K

The thermal behaviour of β-anhydrite CaSO₄ has been investigated to 1,263 K in-situ real-time using laboratory parallel-beam X-ray powder diffraction data. The cell parameters expanded anisotropically, the c axis being the “softest”. This behaviour is due to the deformation of the CaO₈ polyhedron. In fact the two longest, independent, Ca–O bond distances show a significant component along the z direction.     

Pressure–volume–temperature equation of state of andradite and grossular, by high-pressure and -temperature powder diffraction

 The thermoelastic parameters of natural andradite and grossular have been investigated by high-pressure and -temperature synchrotron X-ray powder diffraction, at ESRF, on the ID30 beamline. The P–V–T data have been fitted by Birch-Murnaghan-like EOSs, using both the approximated and the general form. We have obtained for andradite K ₀=158.0(±1.5) GPa, (dK/dT )₀=−0.020(3) GPa K⁻¹ and α₀=31.6(2) 10⁻⁶ K⁻¹, and for grossular K ₀=168.2(±1.7) GPa, (dK/dT)₀=−0.016(3) GPa K⁻¹ and α₀=27.8(2) 10⁻⁶ K⁻¹. Comparisons between the present issues and thermoelastic properties of garnets earlier determined are carried out. Received: 7 July 2000 / Accepted: 20 October 2000     

Kinetic study of the dehydroxylation of chrysotile asbestos with temperature by in situ XRPD

 The kinetics of the dehydroxylation of chrysotile was followed in situ at high temperature using real-time conventional and synchrotron powder diffraction (XRPD). This is the first time kinetics parameters have been calculated for the dehydroxylation of chrysotile. The value of the order of the reaction mechanism calculated using the Avrami model indicates that the rate-limiting step of the reaction is a one-dimensional diffusion with an instantaneous nucleation or a deceleratory rate of nucleation of the reaction product. Hence, the rate-limiting step is the one-dimensional diffusion of the water molecules formed in the interlayer region by direct condensation of two hydrogen atoms and an oxygen atom. The calculated apparent activation energy of the reaction in the temperature range 620–750 °C is 184 kJ mol⁻¹. The diffusion path is along the axis of the fibrils forming the fibers. The amorphous or short-range ordered dehydroxylate of chrysotile is extremely unstable because forsterite readily nucleates in the Mg-rich regions. Moreover, it is less stable than the dehydroxylate of kaolinite, the so-called metakaolinite, which forms mullite at about 950 °C. This difference is interpreted in terms of the different nature of the two ions Mg²⁺ and Al³⁺ and their function as glass modifier and glass-forming ion, respectively. Received: 10 April 2002 / Accepted: 7 January 2003 Acknowledgements This work is part of a COFIN project (04 Scienze della Terra, NR 17, 2000) supported by MURST. Dr Dapiaggi is kindly acknowledged for help during the data collection at the Dipartimento di Scienze della Terra, University of Milan.     

X-ray single-crystal study of spinels: in situ heating

 Two MgAl₂O₄ stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized, and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts, the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample, which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range, only partially reversible order–disorder processes occurred in the time span used for the experiments. Received: 16 July 2001 / Accepted: 8 January 2002