Phase Transitions In Minerals Research Articles

Recent advances in the study of mantle phase transitions

We review recent advances in the study of phase transitions of minerals in the Earth's mantle, including unsolved problems regarding these processes. The phase boundaries of (Mg,Fe) 2SiO 4 in the mantle transition zone are modified under the wet conditions by changes in thermodynamic properties of its polymorphs because of differences in the water solubilities of these minerals. The shift of phase boundaries has significant implications for the topography of the 410 km and 660 km seismic discontinuities. The post-perovskite phase with CaIrO 3 structure exists at pressures above 110 GPa and at high temperature. The effects of Al and Fe on phase transition are under debate. Several post-stishovite phases have been reported but equilibrium boundaries and existence in the real lower mantle are also still debatable. Spin transition occurs in magnesiowustite, perovskite, and post-perovskite phases at high pressures of the lower mantle and core. The pressure interval in the spin transition can be large at high temperature and so the transition might not cause a discrete change in physical properties under real Earth conditions.

A REVIEW ON PROCESS-RELATED CHARACTERISTICS OF PYRRHOTITE

In this review, the physical and chemical properties of the pyrrhotite group minerals are discussed, particularly their mineralogical behavior during mineral processing and phase transitions. Pyrrhotites are often associated with nickel ores and gold deposits. Consideration of the leaching processes of pyrrhotites focuses on the recovery of Ni and Au. The mechanisms of acid leaching are presented from physical, chemical, electrochemical, and crystallographic viewpoints. Other pyrrhotite processing techniques, including oxidation and flotation, are also included in this article.

On the relative importance of mineral phase transitions and viscosity stratification in controlling the sinking rates of detached slab remnants

[1] We employ a two-dimensional control-volume model of convection in a cylindrical shell to study sinking rates of cold parcels of material in the Earth's mantle. Our model uses periodic boundary conditions, at 0 and 360 degrees, to eliminate side-wall effects and includes mineral phase transitions at 410 km and 660 km depths and a depth-dependent viscosity. We compare the rates of sinking of cold material with and without phase boundaries and with and without viscosity increases with depth. When the viscosity increase is sufficiently large we find the sinking rate is relatively insensitive to the presence or absence of phase boundaries; the effects of viscosity dominate. If recent estimates of large viscosity increases in the lower mantle are correct, our model could account for mid-mantle slab remnants of Jurassic age independent of the existence of phase transition boundaries in the mantle.

Deep origin of the Hawaiian tilted plume conduit derived from receiver functions

We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3-D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection.

Extraction and determination of elemental selenium in sediments—A comparative study

This paper proposes a new technique to extract elemental Se from soil and sediment samples. In this study, we have identified that the purchased red elemental selenium standard (PF-Se) was impure and rather consisted of a mixture of CS 2 soluble amorphous elemental Se ( ca. 10%, w/w), water soluble oxidized Se ( ca. 15–17%, w/w) and, CS 2 insoluble red monoclinic elemental Se. In more recent studies, a slow oxidation and a mineral phase transition of this sample was also observed. The solubility of the amorphous elemental Se in CS 2 was at least 0.64 mg L −1. The black elemental Se purchased from Sigma–Aldrich had a much lower solubility in CS 2 (7.2 μg mL −1) compared to that given in the literature. Any selenium compounds with electrical charge and polar nature is insoluble in CS 2. In a sodium sulphite solution, PF-Se was completely dissolved thus giving a clear indication of the lack of selectivity in that extraction system. Other comparative studies also demonstrated that over extraction did occur with the Na 2SO 3 method. Compared to Na 2SO 3, CS 2 extraction of elemental Se is not only much simpler, straightforward and with higher analytical precision, but also much more selective and accurate. With HG-AFS, the detection limit can reach as low as 1.0 ng g −1 in sediment sample owing to a low reagent blank of CS 2 solvent.

Possible seismic signature of the α–β quartz transition in the lithosphere of Southern Tuscany (Italy)

This paper intends to throw some doubt on the consolidated practice of interpreting seismic data without considering mineral phase transitions. Reference is made to Southern Tuscany, where seismic surveys revealed the presence, within the crystalline basement of a prominent high-amplitude reflector exhibiting local "bright spot" features, known as the K-horizon. According to some researchers, the K-horizon is a layer of fractured rocks hosting high-pressure fluids, but its ultimate origin is unclear. On the basis of the extrapolation of measured temperatures in the Larderello and Amiata geothermal fields and gas geotemperatures from the San Pompeo-2 deep well, we propose that the K-horizon might be largely governed by the α–β transition of quartz and by the very high volumetric thermal expansion of quartz, a main constituent of local crystalline rocks, with respect to other minerals. The effects of these processes are probably suppressed almost everywhere by the abundance of minerals other than quartz and by the occurrence of partial melting, but might be evident in relatively quartz-rich rocks and in regions of very high geothermal gradient, such as Southern Tuscany. As a consequence of these processes, a layer of fractured rocks roughly following the α–β quartz transition temperature would develop and locally host high-pressure fluids, thus justifying in part the high reflectivity of the K-horizon. This hypothesis concords with previous interpretations, although the ruling factors in this case would be the α–β transition and the peculiar physical characteristics of quartz compared to other minerals. In our interpretation, the K-horizon is expected to fade away in rocks where quartz is scarce or absent, where microfractures are not produced by the phase transition. The local disappearance of the K-horizon could, therefore, depend on lithology, whereas its reflectivity would depend on the quantity and type of fluids trapped in the porous layer micro-fractured by the volumetric expansion of quartz.

Double-crossed again

An idea that a mineral phase transition may occur not once, but twice, close to the core–mantle boundary has been tested with seismic data. The resulting picture of the deep Earth is sure to provoke debate. A new model of the deep Earth allows geophysicists to measure variations in Earth's temperature near the boundary between the rocky mantle and core. Using a recently discovered phase change in the lowest part of the Earth's mantle, where atoms rearrange themselves in crystals under extreme pressure and temperature, the model also explains odd features of this part of the mantle, such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting. Combined with observations, the results imply very large variations in temperature, providing further evidence for circulation of material throughout the Earth's mantle.

Effect of mineral phase transitions on sedimentary basin subsidence and uplift

Metamorphic phase transitions influence rock density, which is a major parameter affecting lithosphere dynamics and basin subsidence. To assess the importance of these effects, we have computed realistic density models for a range of crustal and mantle mineralogies from thermodynamic data by free-energy minimization. These density distributions are incorporated into one- and two-dimensional kinematic models of basin subsidence. The results demonstrate that, compared to models in which density is solely temperature dependent, phase transitions have the effect of increasing post-rift subsidence while decreasing syn-rift subsidence. Discrepancies between our model results and those obtained with the conventional uniform stretching models can be up to 95% for reasonable parameter choices. The models also predict up to 1 km of syn-rift uplift as a consequence of phase transitions. Mantle phase transitions, in particular the spinel–garnet–plagioclase–lherzolite transitions are responsible for the most significant effects on subsidence. Differences in mantle composition are shown to be a second-order effect. Parameterized density models are derived for crustal and mantle rocks, which reproduce the main effects of the phase transitions on subsidence.

高温高圧条件下での岩石の弾性波速度測定とその地質学的意義

We review the methods of measuring the velocities of elastic-waves in rocks and summarize the temperature-dependence of elastic-wave velocities under high-temperature and high-pressure conditions. The elastic-wave velocities in rocks are strongly affected by several phenomena such as thermal cracking, phase transition of minerals, partial melting of rocks, and dehydration of hydrous minerals. These phenomena are strongly affected by pressure-temperature conditions and chemical compositions of rocks and minerals. Thus, it is very difficult to predict the elastic wave velocities of rocks and minerals under high-pressure and high-temperature conditions theoretically. Laboratory measurements of the velocities of elastic-waves in rocks under high-pressure and high-temperature conditions have provided useful data for estimating physical and geological properties in the crust and upper mantle. We also mention the next issues to be studied in relation to the velocity of elastic waves in rocks. It is important to measure elastic-wave velocities in rocks under high-temperature and high-pressure conditions in the presence of pore-fluids.

Ferroelastic phase transitions: structure and microstructure

Landau-type theories describe the observed behaviour of phase transitions in ferroelastic and co-elastic minerals and materials with a high degree of accuracy. In this review, the derivation of the Landau potential G=1/2AthetaS [coth(thetaS/T)-coth(thetaS/TC)]Q2 + 1/4BQ4 + ... is derived as a solution of the general phi4 model. The coupling between the order parameter and spontaneous strain of a phase transition brings the behaviour of many phase transitions to the mean-field limit, even when the atomistic mechanism of the transition is spin-like. Strain coupling is also a common mechanism for the coupling between multiple order parameters in a single system. As well as changes on the crystal structure scale, phase transitions modify the microstructure of materials, leading to anomalous mesoscopic features at domain boundaries. The mesostructure of a domain wall is studied experimentally using X-ray diffraction, and interpreted theoretically using Ginzburg-Landau theory. One important consequence of twin mesostructures is their modified transport properties relative to the bulk. Domain wall motion also provides a mechanism for superelastic behaviour in ferroelastics. At surfaces, the relaxations that occur can be described in terms of order parameters and Landau theory. This leads to an exponential profile of surface relaxations. This in turn leads to an exponential interaction energy between surfaces, which can, if large enough, destabilize symmetrical morphologies in favour of a platelet morphology. Surface relaxations may also affect the behaviour of twin walls as they intersect surfaces, since the surface relaxation may lead to an incompatibility of the two domains at the surface, generating large strains at the relaxation. Landau theory may also be extended to describe the kinetics of phase transitions. Time-dependent Landau theory may be used to describe the kinetics of order-disorder phase transitions in which the order parameter is homogeneous. However, the time-dependent Landau theory equations also have microstructural solutions, explaining the formation of microstructures such as tweed.

Outstanding student award

AGU's Mineral and Rock Physics Focus Group will formally present its 2004 Outstanding Student Award to Motohiko Murakami and Sergio Speziale at the M&RP‐SEDI (Study of the Earth's Deep Interior) Reception.The reception will be held 14 December, at the 2004 AGU Fall Meeting in San Francisco, California.Motohiko Murakami received his Ph.D. from Tokyo Institute of Technology (Tokyo Tech), Japan, in 2004. His thesis, “Phase transitions in lower mantle minerals and its geophysical implications,” was performed under the supervision of Kei Hirose. Murakami now holds a JSPS postdoctoral fellowship at the Department of Earth and Planetary Sciences, Tokyo Tech. Murakami's research interests include phase relations, phase chemistry, and physical properties of the Earth's deep materials at high pressure and high temperature.

Anomalous unradiogenic 87Sr//86Sr ratios in ultrahigh‐pressure crustal carbonates – evidence for fluid infiltration during deep subduction?

AbstractMarbles from Changpu (Dabie Shan, eastern China), subducted to 4.4 GPa, have 87Sr/86Sr values < 0.7040. These low 87Sr/86Sr values, which would imply a sedimentation age > 2 Ga if considered as primary signature, reflect fluid–rock interaction with a fluid from a low‐87Sr/86Sr source. The introduction of low‐87Sr/86Sr was paralleled by introduction of Mg and loss of Si, K and Na in such a way that carbonates from the purest marbles have the least evolved Sr isotopic composition. Introduction of Mg is also indicated by the distribution of calcite and dolomite. Calcite forms inclusions in garnet, whereas dolomite is restricted to the matrix. These chemical changes, inferred from the mineralogy, in combination with textural evidence require a mobile metamorphic fluid. P–T–X constraints for fluid generation and for permeability increase related to mineral reactions and phase transitions suggest that the marbles acquired their anomalous Sr‐isotopic composition during subduction below 60 km. The marbles with the least radiogenic Sr isotopic composition demonstrate that crustal rocks may lose their isotopic fingerprint during deep subduction.

Vibrational spectroscopy of phase transitions in leonite-type minerals

Infrared (IR) and Raman spectra of leonite-type minerals, K 2 Me(SO 4 ) 2 .4H 2 O (Me = Mg, Mn, Fe), confirm a succession of structural phase transitions between 277 and 120 K. Because the orientation and dynamic behaviour of the sulphate tetrahedra undergo the most conspicuous changes during these transitions, transmission IR and Raman spectra were recorded for the SO 4 bending and stretching modes from 660 to 1230 cm −1 (IR) and 350 to 1250 cm −1 (Raman), in a temperature range between 80 K and room temperature. At low temperature the leonite-type minerals show an order-disorder phase transition due to the freezing of the dynamic sulphate disorder. These phase transitions are characterised by non-linear shifts of the peak positions as a function of temperature in IR and Raman spectra. Evaluation of the peak widths of the sulphate modes in the IR and Raman spectra by autocorrelation analysis show non-linear decreases of the width parameters, confirming a tricritical ordering process according to the Landau order parameter. With decreasing temperature the crystal structures of the Mg- and Mn-endmembers switch to another ordered phase. These structural changes are accompanied by the sudden appearance of additional sulphate stretching and bending modes and by discontinuities of the peak shifts in IR and Raman spectra, indicating a first order phase transition.

A STUDY OF COAGULANT PRODUCTION FROM RED MUD AND ITS USE FOR HEAVY METALS REMOVAL

ABSTRACT Coagulant production using red mud and waste base as raw material is described together with phase and chemical composition changes in red mud during coagulant production and usage. Investigation of phase and chemical composition was carried out using X-ray diffraction analysis (XRD) and sequential extraction. Acid treatment of the red mud resulted in the removal of the exchangeable fraction, carbonates and alkalis. Base addition into acid treated and centrifuged red mud caused its flocculation and sorption by surface enlargement. Mineral phase transition in coagulant (activated red mud) with time was not found. An amount of 1g of this coagulant was able to sorb 99.347 mg of Cu, 95.002 mg of Zn and 98.695 mg of Pb from aqueous solution containing 100 mg/l of those metals. Among three metals investigated, Zn sorption was the most affected with changes in pH value of treated solution. The sorption of metal cations (Cu(II), Zn(II) and Pb(II)) from the metal–EDTA solution onto the activated red mud (...

Models of the mantle shear velocity and discontinuities in the pattern of lateral heterogeneities

Resolution of the pattern of large‐scale shear velocity variations above and below the known and postulated mantle discontinuities could provide constraints on the nature of mineral phase transitions, changes in composition, and the scale of mantle convection. To achieve good resolution across a full range of depths, we use a diversified data set consisting of body and mantle wave waveforms, travel times, and surface wave phase velocities. Our main focus is on the 670‐km discontinuity, long presumed to be an important barrier, or impediment, to whole mantle convection. Our data set has a relatively high radial resolution throughout the mantle; in the transition zone and some 200 km below it, the long period waveforms, dominated by multiple surface reflections, make a particularly important contribution. We use a local spline support to parameterize the model; this allows us to obtain a smooth model (twice differentiable) and simplifies calculation of the model and its derivatives in applications such as three‐dimensional ray tracing. In one inversion we use a continuous radial representation throughout the mantle; in the other, a discontinuity is allowed across the 670‐km boundary. Both models suggest that the long‐wavelength anomalies of the transition zone and the mantle below 750 km are significantly different. Near 670 km these two models display notable differences in the peak amplitudes and lateral scales of major anomalies. The degree 2 spherical harmonic, which dominates the large‐scale shear velocities in the transition zone, is strongly attenuated at the top of the lower mantle where the power spectrum is essentially white. Resolution tests show that these results are robust, which suggests a possible reorganization of the flow between the upper and lower mantle. At other depths the power spectra of our models as a function of depth indicate a modest change near 400 km, where the dominating effect of degree 5 (shields) is replaced by degree 2 (slabs). The power of the heterogeneity in at mid‐mantle depths is low and nearly flat as a function of spherical harmonic degree up to 𝓁 = 12, with no detectable change near 1000 km. The increase of the power in the lowermost mantle is rather gradual, not characteristic of a discontinuous change. Cross sections of our models at major subduction zones indicate that major downwelling of cold slab material may occur at some locations. On the other hand, there are numerous examples of an abrupt change of the sign of the velocity anomalies across the 670‐km discontinuity.

Phase transitions in minerals: Correlation of spectroscopic and diffraction data

Phase transitions in minerals: Correlation of spectroscopic and diffraction data

Strain and Elasticity at Structural Phase Transitions in Minerals

Almost any change in the structure of a crystal, due to small atomic displacements, atomic ordering, magnetisation, etc., is usually accompanied by changes in lattice parameters. If suitable reference states are defined, such lattice parameter variations can be described quantitatively as a combination of linear and shear strains. What was originally a geometrical description then becomes a thermodynamic description if the relationships of the strains to the elastic constants are also defined. For a phase transition in which the parent and product phases are related by symmetry, only very specific combinations of strain will be consistent with the symmetry change, and the magnitudes of each strain will depend directly on the extent of transformation. In other words, such strains provide information on the evolution of the order parameter and the thermodynamic character of a phase transition. Spontaneous strains due to phase transitions in silicate minerals can be as large as a few percent, and, with modern high resolution diffractometry using neutron or X-ray sources, variations as small as ~0.1‰ can be detected routinely. This level of resolution is illustrated by the lattice parameter and strain variations through a cubic → tetragonal → orthorhombic sequence for the perovskite NaTaO3, for example (Fig. 1⇓). Figure 1. (a) Lattice parameters (expressed in terms of reduced, pseudocubic unit cell) as a function of temperature through the cubic → tetragonal → orthorhombic sequence of NaTaO3 perovskite (powder neutron diffraction data of Darlington and Knight 1999). A straight line through the data for the cubic structure gives the reference parameter a o. (b) Linear strains e 1, e 2, e 3, parallel to reference axes X, Y and Z, respectively, derived from the data in (a). Conversion from a purely geometrical description, in terms of lattice parameters, to a …

Effect of the mid-mantle viscosity and phase-transition structure on 3D mantle convection

Recent geophysical evidence shows some sort of layering in the lower part of the mantle transition zone down to a depth of 1000 km. Seismic observations have revealed a sharp reflector surface at around 900 or 1000 km depth for which a possible explanation can be given in terms of a new phase transition of the lower-mantle constituent minerals. Furthermore, new results from the inversion of the oceanic geoid show the existence of a second low viscosity zone (LVZ) somewhere between 660 and 1000 km depth. The existence of the second LVZ may be linked to the mid-mantle phase transitions. The phase and viscosity stratification of the transition zone have been included in a series of 3D convection simulations in a 4×4×1 rectangular box with a surface Rayleigh number of 2×10 7. Beneath the well-known 400 and 660 km phase changes, we assumed a hypothetical weak endothermic transition at 1000 km in some of our models. The principal controlling factor of the style of mantle convection is still the 660 km endothermic transition, which sets up a partial or full barrier to flow, causing stratified circulation. We used various viscosity profiles with emphasis on the model containing the second LVZ. The main consequence of this zone is to enhance flow layering. Many plumes can emanate from the transition zone and small-scale instabilities develop in the second LVZ. When the 1000 km endothermic phase transition is included, these instabilities can grow only at a few places but then they form strong downwellings. Two distinct types of penetrative, deep downwellings can be present at the same time: one which crosses the whole transition zone, and another one which crosses only the 660 km discontinuity and stops at 1000 km at least temporarily. This can explain seismological observations which suggest that subducted slabs can be retarded not only by the 660 km boundary but also by some deeper obstacle near 1000 km depth.

Chemico-viscous remanent magnetization in the oxidation of siderite and its implications in paleomagnetism

Studies have been done of the secondary chemico-viscous remanent magnetization (CVRM) produced in the oxidization and decomposition of high-purity crystal siderite specimens collected from Dalizi Deposit, Jilin Province, The CVRM intensity increases to a peak value at an annealing temperature of 490?C . The direction of CVRM is always parallel to the applied field , and no self- reversal behavior has been observed in the transformation of maghemite to hematite . It is suggested that the phase transition of magnetic minerals can make great contribution to secondary magnetization. Results of successive heating at low temperature indicate that the crystal mineral siderite is basically stable. Finally , the paleomagnetic significance of CVRM is discussed.

Morse Stretch Potential Charge Equilibrium Force Field for Ceramics: Application to the Quartz-Stishovite Phase Transition and to Silica Glass

To predict phase transitions in ceramics and minerals from molecular dynamics simulations, we have developed a force field in which the charges are allowed to readjust instantaneously to the atomic configurations. These charges are calculated using the charge equilibration (QEq) method. In addition to electrostatics, a two-body Morse stretch potential is included to account for short-range nonelectrostatic interactions. This MS-Q potential is applied herein to SiO_2, where we find that it describes well the fourfold coordinated and sixfold coordinated systems (such as quartz and stishovite), silica glass, and the pressure-induced phase transition from quartz to stishovite.