Kinetics of intracrystalline cation redistribution in olivine and its implication

The rate of cation redistribution between M 1 and M 2 sites in olivine is theoretically studied on the basis of elementary processes of cationic migration. Cationic migration in olivine structure is assumed to be the superposition of a unit exchange of cations between closely spaced couple of sites. Such a process gives rise to both cation redistribution and also cationic interdiffusion in olivine crystal. The time constant of cationic equilibration in the redistribution reaction is related to the interdiffusion coefficient along b-axis, and its numerical value is given as a function of temperature and composition in Mg-Fe olivine. This time constant is very short, e.g., in the order of 10^?2～10^?4 s at 1000 ° C. The temperature dependence of cation distribution in Mg-Fe olivine could not be detected by heating and quenching experiments in some previous works, because of insufficient cooling rate. A skepticism is presented for the utility of cation distribution as a geothermometer or rate meter of cooling. Cation redistribution in olivine in the deep upper mantle is sufficiently fast to take place almost in phase with the seismic waves of long periods.     

High-pressure metallization and electronic-magnetic propertiesof hexagonal cubanite (CuFe2S3)

^?57Fe Mössbauer studies at room temperature and temperature-dependent resistance studies have been performed on a natural specimen of cubanite (CuFe₂S₃) in a diamond-anvil cell at pressures up to ～10 GPa. An insulator-metal phase transition occurs in the range 3.4–5.8 GPa coinciding with a previously observed structural transition from an orthorhombic to a hexagonal NiAs (B8) structure. The room temperature data shows that the metallization process concurs with a gradual transition from a magnetically ordered phase at low pressure to a nonmagnetic or paramagnetic phase at high-pressure. The change in magnetic behaviour at the structural transition may be attributed to a reduction of the Fe-S-Fe superexchange angle formed by edge-sharing octahedra occurring in the high-pressure phase. The non-magnetic or paramagnetic metallic phase at high pressure is retained upon decompression to ambient pressure-temperature conditions, indicative of substantial hysteresis associated with the pressure driven orthorhombic→hexagonal structural transition. The pressure evolution of both the ⁵⁷Fe Mössbauer hyperfine interaction parameters and resistance behaviour is consistent with the transition from mixed-valence character in the low pressure orthorhombic structure to that of extended-electron delocalization in the hexagonal phase at high-pressure.     

Cooling history of lunar Mg-suite gabbronorite 76255, troctolite 76535 and Stillwater pyroxenite SC-936: The record in exsolution and ordering in pyroxenes

We have determined cooling rates of orthopyroxene crystals from two Mg-suite lunar samples (gabbronorite 76255 and troctolite 76535) and one terrestrial sample (orthopyroxenite SC-936 from the Stillwater Complex), on the basis of their Fe–Mg ordering states. In addition, a cooling rate of 76255 was determined by modeling the formation of exsolution lamellae in pyroxenes. The M1–M2 site occupancies of the orthopyroxene crystals were determined by single crystal X-ray diffraction and the rate constant for the ordering reaction was used along with calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature to calculate cooling rates. The closure temperatures (T_C) of cation ordering are 525 °C for 76255, 500 °C for 76535 and 350 °C for SC-936 corresponding to cooling rates of 4 × 10⁻² °C/year at the closure temperature for the lunar samples and 10⁻⁶ °C/year for the Stillwater sample. A cooling rate for 76255, determined by simulating the exsolution process, is 1.7 × 10⁻² °C/year at a closure temperature for exsolution of 700 °C. The Fe–Mg ordering cooling rate determined for 76535 reflects a complex thermal history superimposed on the initial plutonic provenance established for this sample [McCallum, I.S., Schwartz, J.M., 2001. Lunar Mg suite: thermobarometry and petrogenesis of parental magmas. J. Geophys. Res. 106, 27969–27983]. The preservation of a crystallization age of 4.51 Ga and a metamorphic age of 4.25 Ga for 76535 is consistent with a model in which excavation of this sample from the lower lunar crust took place while the sample was at a temperature above the closure temperatures for the Sm–Nd, U–Pb and Ar–Ar isotopic systems. Temperatures in excess of the isotopic closure temperatures (i.e., >600 °C) in the lower lunar crust were maintained by heat diffusing from concentrations of U- and Th-rich KREEP material at the base of the crust. On the other hand, 76255 formed at a much shallower depth in the lunar crust (2 km) and was well below its isotopic closure temperatures at the time of excavation, most likely during the Serenitatis basin-forming impact event. Both lunar samples were reheated during transport to the surface and deposition in hot ejecta blankets. The reheating was short lived but apparently sufficient to redistribute Fe and Mg in M sites in orthopyroxenes. For the lunar samples, the cooling rates based on Fe–Mg ordering represent final stage cooling within an ejecta blanket.     

The transformation behaviour of metal-enriched chalcopyrite

The transformation behaviour of metal-enriched chalcopyrite has been studied dynamically by in situ experiments in a transmission electron microscope. In the ordering transformations which take place on cooling from the high temperature form a series of phases is described and their behaviour is related to the cooling rate. The observations indicate that the two stable forms are 1. the disordered form with space group F¯43m and 2. the low temperature tetragonal form with space group P¯42m. Alternative metastable phases are formed under conditions when the formation of the low temperature form is kinetically impeded. The significance of these observations to low temperature phase relations in the central part of the Cu-Fe-S system is discussed.     

On the phase transformation proto- to clino-/orthoenstatite: Neutron powder investigation

The phase transformation proto- to coexisting ortho- and clino-enstatite has been studied by in-situ neutron powder diffraction with samples from Bamble/ Norway. The evolution with temperature was followed in two subsequent cycles with different cooling rates. Refined structural parameters did not indicate any significant anomalies. The transformation behaviour is different for proto to clino (rapid) and the proto to ortho inversion (sluggish). The latter proceeds in two steps: ortho-enstatite starts to grow slowly below 1180 K on cooling, whereas the main transformation takes place below 800 K simultaneously with the proto-clino inversion. The athermal martensitic character is confirmed for the proto to clino inversion. The diffuse background in the powder diagrams is related to stacking faults which are irreversibly created during the transformation and affect the degree of order.Work supported by funds of the BMFT under 03-SC1LMU     

Cation ordering of orthopyroxenes from the Skaergaard Intrusion: implications for the subsolidus cooling rates and permeabilities

 We have determined the quenched cation ordering states of five orthopyroxene crystals collected from the marginal border group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to retrieve their closure temperatures (T _c) of Fe–Mg ordering and cooling rates. According to existing thermal models for the Skaergaard pluton, conductive cooling dominated the high and low temperature regimes, which were separated by an intermediate temperature regime in which the cooling was controlled primarily by convective fluid circulation. The cooling rates retrieved from the quenched cation ordering states of the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states, ∼340–400^° C, which fall at the transition from convective to the lower temperature conductive cooling. The cooling rates obtained from the cation ordering states of orthopyroxene vary from ∼1 to 270 K/ka. These results agree well with a thermal model calculated using an assumed average permeability of 10^-12 cm² for the pluton, but not completely with a model calculated on the basis of an average permeability of 10^-13 cm², although both values produced shifts of δ¹⁸O that are comparable to those observed in the pluton. Received: 27 February 1995/Accepted: 20 July 1995     

不同W氧化度钠钨青铜的结构特征与合成方法探究

钠钨青铜NaxWO3(0≤x≤1)因其特殊的物理化学性质受到学者的日益关注,它是一种孔道结构的非严格计量含钨化合物,Na+填充于由[WO6]八面体排列组合起来的二级结构空隙中。随着x值的增加,W的平均氧化度逐渐降低,NaxWO3依次发生单斜-正交-四方Ⅰ(空间群I4/nmm)-六方-四方Ⅱ(空间群P4/mbm)-立方晶体结构的转变,结构对称型逐渐升高(四方Ⅱ型除外)。本文从晶体化学角度系统研究了不同W平均氧化度钠钨青铜的结构特征,并通过固相合成法得到了四方和立方钠钨青铜,通过水热合成法得到了六方钠钨青铜。实验结果表明,在固相合成过程中,还原剂NaBH4的量越多,W还原的价态越低,实现钠钨青铜NaxWO3由四方结构向立方结构的转变;在相同还原剂量情况下,合成温度越高,得到的钠钨青铜结构对称型越高;合成立方钠钨青铜的温度一般为700~850℃,高于四方钠钨青铜的合成温度(600~700℃);反应需要适当的酸碱度环境,合成四方和六方钠钨青铜的pH值应控制在6左右,在水热合成过程中,六方结构的钠钨青铜pH值设在2附近较适宜。本文为钠钨青铜系列化合物及不同结构钠钨青铜的合成提供了参考。     

青金石中的结构有序化和调制及其与光性异常间的关系

青金石结构有无序和有序之别。无序青金石为光性均质体,但它的有序化及由此引起的结构调制,则有可能使青金石产生光性异常。使结构仍保持立方对称的有序化并不改变它光性上的均质性,而其他形式的有序化以及结构调制,如果它们是多方向的 且在空间取向上具有立方对称的特征,则晶体仍是光性均质体,否则便转变为光性非均质体 。此外,还提出了根据选区电子衍射花样对结构调制方向进行三维判识的可能性;报道了在所研究青金石中发现的沿［110］方向n=4的有公度调制的有序结构,以及所观察到的其他有序结构和无公度调制结构。     

Temperature dependence of intersite distribution of Mg and Fe in olivine and the associated change of lattice parameters

The cation distribution of natural and heated ferromagnesian olivine with chemical composition, Fo₆₇Fa₃₃, from metagabbro was examined by X-ray diffraction. Heating and quenching experiments were made by a newly devised apparatus which enables us to obtain very fast quenching speed in comparison with the usual technique. The distribution constants, K _D=(Fe⁺²/Mg) _M1/(Fe⁺²/Mg) _M2, of the natural samples were less than 1.07, and those of heat-treated samples were more than 1.15, indicating that cation ordering takes place with temperature. The distribution of Fe⁺² and Mg is nearly random at low temperatures, whereas Fe⁺² shows a slight but significant preference for a smaller M1 site at high temperatures. The change of the distribution constant was observed on specimens which were heated for a short period of time (6–1,060 s) and quenched within 10 ms. Thus the rate of the cation reordering reaction is a very fast process. The lattice parameters b and c decrease whereas a increases with the increase of distribution constant. The overall effect on unit cell volume is a decrease with the increasing distribution constant, suggesting the presence of significant pressure dependence of the cation distribution towards the ordering of Fe at M1 site in ferromagnesian olivine.     

K-Space symmetry rules and their application to ordering behaviour in non-stoichiometric (metal-enriched) chalcopyrite

Electron-optical study of the transformation behaviour in metalenriched chalcopyrite indicates that several metastable ordered variants occur in short term cooling experiments (Putnis and McConnell, 1976). The implications of this behaviour are discussed in relation to the theory of symmetry point ordering. This permits one to distinguish between three types of ordering transformation which differ markedly, both in the nature of the structural changes involved, and in their kinetics. A brief account of the theory of symmetry point ordering, based on a group-theoretical treatment by Landau and Lifshitz, is included. Analysis of the experimental results obtained on metal-enriched chalcopyrite indicate that metastable transformation behaviour in this system is associated with symmetry point ordering, and that partially ordered structures are produced.     

Mechanisms of exsolution in sodic pyroxenes

The free energy curves for simple binary solid solutions with limited miscibility or atomic ordering have been combined to predict the phase relations and exsolution mechanisms for a system in which both ordering and exsolution are possible. The nature of the ordering process affects which exsolution mechanisms may be used. If the ordering is second (or higher) order in character then continuous mechanisms predominate and a ‘conditional spinodal’ (Alien and Cahn, 1976) can be described which operates between ordered and disordered end members. For a first order case, the ordered phase can only precipitate a disordered phase by nucleation and growth. Microstructures in omphacites observed by transmission electron microscopy include exsolution lamellae and antiphase domains and the relations between them in selected specimens have been used to interpret the exsolution mechanisms which operated under geological conditions. It appears that most omphacites undergo cation ordering, and then remain homogeneous or exsolve a disordered pyroxene by spinodal decomposition. The predominance of continuous mechanisms has been used to indicate that the C2/c→P2/n transformation may be second (or higher) order in character. A possible phase diagram for jadeite-augite is presented. It is based on the idea that there should be limited miscibility between the disordered end members at low temperatures and that the cation ordering at intermediate compositions (omphacite) is superimposed on a solvus. It is adequate to explain many of the observed microstructures and fits with petrographic evidence of broad two phase fields between impure jadeite and omphacite and between omphacite and sodic augite. The effect of adding acmite is analogous to increasing temperature so that the phase relations for jadeite-acmite-augite can also be predicted.     

Kinetic control of ordering and exsolution in omphacite

Omphacites with a range of compositions from the blueschist rocks of Syros, Greece and from included blocks in serpentinite from the Motagua fault zone, Guatemala, have been examined by transmission electron microscopy. The complex microstructures observed are all on a fine scale and are composed of exsolution and antiphase features. At least three different assemblages have been identified: (a) a single ordered phase with antiphase domains, (b) two exsolved phases, one ordered and the other disordered and (c) two exsolved phases, both apparently ordered. Selected area electron diffraction patterns provide evidence for P2/n, P2/c, and P2 space groups in different parts of the ordered crystals.The microstructures cannot all be readily explained in terms of a simple equilibrium phase diagram and some are attributed to alternative and metastable transformations. It is suggested that under the low temperature conditions of blueschist metamorphism, omphacite crystallised metastably below its ordering temperature with imperfect cation order. Subsequent development by cation ordering and exsolution was controlled by kinetics, the fastest available transformation being used to give reductions in free energy. Ordering preceded exsolution and may have occurred in a series of steps as predicted by the Ostwald step rule. Slight differences in composition and initial state of order locally influenced which reaction pathway was embarked upon and many of the products may be metastable. Such behaviour is apparently sensitive to thermal history.     

Structural states of Mg-cordierite I: Order parameters from synchrotron X-ray and NMR data

The hexagonal to orthorhombic phase transition in synthetic Mg-cordierite has been studied by (i) measuring the spontaneous strain associated with the transition using Synchrotron X-ray powder diffraction and (ii) measuring the degree of Al, Si order in terms of the number of Al-O-Al bonds per formula unit using solid state NMR spectroscopy. This defines the two order parametersQ andQ _od respectively, and their relationship as a function of annealing temperature and time is used to define the structural states of cordierite during the ordering sequence. The formation of modulated hexagonal cordierite within which a high degree of Al, Si order can be attained, results in a strongly non-linear relationship betweenQ andQ _od .The transition from modulated to orthorhombic cordierite is strongly first-order under all temperature conditions studied and involves a large step inQ, whileQ _od changes continuously throughout the ordering sequence with no marked discontinuity at the phase transition. The lattice distortion, traditionally defined in cordierite by the Δ index provides no full information on the degree of Al, Si order in anhydrous Mg-cordierite, and both order parameters must be used to define its structural state. Transmission electron microscopy has been used to study the mechanism of the transformation from hexagonal to modulated to orthorhombic cordierite.     

Mechanisms of polytype stabilization during the wurtzite-sphalerite transition

Two mechanisms of solid-state formation of polytypes are suggested. The first is structural-kinetic by nature, the stabilization of short-period polytype (for example, 4H) being due to the fact that there are two possible structural types of nuclei in the wurtzite structure: when new phase growth takes place by means of two-dimensional nucleation, these short-period polytypes become kinetically stable although they do not correspond to the minimum of free energy, and appear as transitional phases. The second mechanism of polytype formation below the transition temperature is influenced by impurities which stabilize the hexagonal modification, in which case polytypes may arise as states that are stable with respect to a diffusionless structural wurtzite-sphalerite transformation and metastable relative to small fluctuations of impurity concentration.     

高温高压下煤超微构造的变形特征

本文通过不同温、压实验条件下,不同煤级煤的应力-应变曲线及超微变形构造特征分析,深入探讨了不同煤化程度煤的变形行为及其影响因素。阐明了高温高压和差异应力作用是促使煤结构由无序向有序方向转化,使大分子结构有序畴和定向性增大的重要因素。论述了温度和压力在不同的煤级和不同的实验条件下,具有不同的作用。在中煤级阶段,虽然围压的增大可在一定程度上提高煤的强度,但温度的影响更为重要。较高煤级在小应变阶段,温度起主导作用;而到了大变形阶段,围压的作用又逐渐上升到主导地位。在整个变形过程中,定向压力都是不容忽视的重要因素。     

Sector trilling in cordierite and equilibrium overstepping in metamorphism

The origin of sector trilling in cordierite is due to the hexagonal-orthorhombic Al, Si ordering transformation which under non-equilibrium conditions proceeds via a short-range ordered modulated structure. The growth of these distortion waves associated with progressive ordering produces a strain field which is minimized by a cyclic distribution of symmetrically equivalent modulations.Sector and complexly trilled cordierites in metamorphic rocks grew as the hexagonal polymorph with a considerable degree of Al, Si disorder. The enthalpy and entropy of disorder are evaluated from recent experimental work. The implication is that, in metamorphic rocks, substantial overstepping of stable equilibrium phase boundaries is required to nucleate hexagonal cordierite. Moreover, its composition coexisting with other phases will also be significantly different from that of the stable ordered form.     

Nucleation and growth rates of homogeneously condensing water vapor in argon from shock tube experiments

A special shock tube process combining a reflected expansion wave with a weak shock wave is analyzed and calibrated. The process is employed to transfer water vapor carried in argon into a known supersaturated state for a short period of time (0.5 ms). During that period steady state homogeneous nucleation takes place followed by condensational growth. Nucleation and growth rates are measured by a 90° Mie-light scattering technique in the temperature range 200–260 K. The results are compared with existing theoretical models.     

Mechanism of chalcopyrite formation from iron monosulphides in aqueous solutions (< 100°C, pH 2–4.5)

In low-temperature aqueous solutions (< 100°C, pH 2–4.5), chalcopyrite (CuFeS₂) does not form through direct precipitation from solution. The pathway is exclusively via precursor iron sulphides and dissolved Cu salts. The reaction of dissolved Cu (II) salts with natural hexagonal pyrrhotite (Fe_0.9S) is diffusion controlled. The initial stage has an apparent activation energy of 11.4 ± 1.8 kJ mol⁻¹ and the rate (in units of mol dm⁻³s⁻¹ cm⁻²) is independent of the solid reactant surface area. The reaction proceeds through a series of metastable Cu-Fe-sulphide intermediaries. These phases form a series of ephemeral layers penetrating into the pyrrhotite surface. The first phase formed has the stoichiometry Cu_0.1Fe_0.9S. No Fe is released into the solution during its formation and this, together with the extremely low apparent activation energy and the stoichiometry, suggest that it is formed by stuffing of electron holes in the pyrrhotite structure with Cu ions. The transformation from the hexagonal close-packed arrangement of the pyrrhotite structure to the essentially cubic packing in chalcopyrite proceeds through a series of intermediaries, approximating in composition to members of the cubanite group. The rate of formation of these phases is controlled by the coupled diffusion of Fe (II), Fe (III), Cu (I) and Cu (II) species through the surface reaction zone, although the process as a whole can be approximated by steady-state diffusion of total Cu into a semi-infinite medium. Experiments with metastable precursor iron monosulphide phases, including amorphous FeS and synthetic mackinawite indicate similar reaction pathways.

The results suggest that chalcopyrite formation in low-temperature natural systems may be significantly constrained by kinetic factors. Chalcopyrite is, at least, a diagenetic mineral since its formation requires the prior formation of iron sulphides. However, at ambient temperatures its formation is probably limited to very early diagenesis.     

Composition features of isocubanite and polymorphous modifications of CuFe2S3 compound

The results of studying isocubanite from sulfide ores of recent oceanic black smokers and sulfide mud of the Red Sea are compared with those of isocubanite from ores of the Noril’sk ore field and isocubanite synthesized in the course of experimental study of the Cu–Fe–S system. Isocubanite associated with chalcopyrite is enriched in Cu, whereas that associated with pyrrhotite or with pyrrhotite and haycockite is enriched in Fe. According to data in the literature, the CuFe₂S₃ compound has four polymorphous modifications: orthorhombic cubanite, tetragonal, hexagonal, and cubic isocubanite. Cubanite, and tetragonal and hexagonal modifications of the CuFe₂S₃ compound are high-pressure minerals. Therefore, they may be used as barometers.     

The ordering behaviour of reedmergnerite,NaBSi3O8

Reedmergnerite (NaBSi₃O₈) has been synthesised hydrothermally from gels containing 10 wt.% excess SiO₂. The degree of B, Si order increases with time at constant temperature and pressure. Complete order is achieved in 250 h at 400° C, P _fluid=1 kbar and in 700 h at 300° C, 1 kbar. Lower pressures and/ or low water contents greatly reduce the rate of ordering. The ordering behaviour of reedmergnerite is insensitive to the composition of the co-existing fluid and this contrasts with the behaviour of albite. It is suggested that ordering takes place by a process of solution and re-precipitation.