Single-phase Oxidation Research Articles

Tracing titanomagnetite alteration with magnetic measurements at cryogenic temperatures

SUMMARY Titanomagnetite containing up to 0.6–0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3−xTixO4 (x ≈ 0.46) at temperatures of 355, 500 and 550 °C for up to 375 hr, by examining their magnetic properties over a wide range of temperatures. The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30 per cent for the samples annealed for 375 hr at 355 °C, and for 110 hr at 500 and 550 °C. The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205 °C) by up to 350 °C. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, that is, not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties.

Effect of Single-Phase Oxidation of Titanomagnetite in Basalts on the Determination of Intensity and Direction of Paleomagnetic Field

Effect of Single-Phase Oxidation of Titanomagnetite in Basalts on the Determination of Intensity and Direction of Paleomagnetic Field

The Properties of Thermochemical Remanent Magnetization Acquired by Slow Laboratory Cooling of Titanomagnetite-Bearing Basalt Samples from Different Temperatures and the Results of the Thellier Method

—The experiments have been carried out on the acquisition of thermochemical remanent magnetization (TCRM) in basalt samples containing titanomagnetite (TM) with the Curie temperature Тс ~200°C by their rapid heating to maximum temperatures Т* from 450 to 530°C followed by slow cooling in the laboratory magnetic field Blab. At different stages of the preliminary thermal treatment of the initial samples, a set of magnetomineralogical studies including electron microscopy, X-ray diffraction and thermomagnetic analyzes, and measurements of magnetic hysteresis parameters were performed. It is shown that as early as the very beginning of the cooling process, all samples demonstrate explosive growth of TCRM corresponding to the stage of rapid single-phase oxidation of the initial titanomagnetite fraction of basalt, and that TCRM is acquired by the increase of Тс and volume of single-phase oxidized parts of TM grains as well as by the growth of the volume of Ti-depleted (relative to the initial TM) cells of microstructure of the subsequent oxidative exsolution. The Arai–Nagata diagrams for the samples carrying TCRM have a form of a broken line consisting of two linear segments. The low-temperature interval T < Т* corresponds to a mixture of thermochemical and thermoremanent (TRM) magnetizations and gives a slightly overestimated Blab because of the effect of a low cooling rate during the acquisition of TCRM and TRM. The high-temperature interval corresponds to pure TCRM and the Blab strength determined from this interval is underestimated by 20–27%. It is recommended to reject samples whose Araii–Nagata diagram has two or more linear segments against the background single-component NRM.

Single Phase Oxidation of Ferrimagnetic Grains as a Cause of L-Shaped Arai–Nagata Diagrams

Abstract—The experiments on the determination of paleointensity Banc of the Earth’s magnetic field by Thellier method on basalts of the Berd (Jurassic) and Paravakar (Cretaceous) collections sampled in the northeastern Armenia in 2006 revealed an anomalous behavior of the Arai–Nagata diagrams. The anomaly manifests itself by a sharp drop in the intensity of natural remanent magnetization (NRM) under heating of samples to about 400°C, which is accompanied by a very weak acquisition of partial thermoremanent magnetizations (pTRMs) in this temperature interval. The further increase in temperature leads to the opposite phenomenon—an unexpectedly sharp rise in pTRMs intensity with almost no decrease in NRM, which creates the L-shape of the Arai–Nagata diagram. A similar phenomenon of a steep decline during thermal demagnetization was also established for remanent saturation magnetization Mrs(T). We carried out a number of magnetomineralogical experiments from which we conclude that, consistent with the hypothesis suggested by (Kosterov and Prevot, 1998), the sharp drop in Mrs(T) and NRM(T) curves is caused by the transitions of the domain state from a metastable configuration to a more stable one. These transitions are initiated by the processes of single- and heterophase oxidations of primary titanomagnetites under the laboratory heating of samples to moderate temperatures.

Determination of the Formation Field of Artificial CRM and pTRM by the Thellier Method at Different Oxidation Stages of Natural Titanomagnetite

Artificial chemical remanent magnetization (CRM) created on a basalt rock from the Red Sea rift zone, originally containing titanomagnetite (TM) with a Curie temperature of 203–208°С, is studied at various oxidation stages. CRM was formed by annealing the samples at temperature T = 355°C during 4.5–350 h in the magnetic field with induction of 100 μT. The thermomagnetic analysis has shown that with the increase of the annealing time, a single-phase oxidation process takes place, the degree of the oxidation increases and reaches its maximum after annealing during 110 hours. It is established that the subsequent heating of the samples to 600°C in argon leads to the homogenization of the single-phase-oxidized titanomaghemite. When the annealing time increases to 350 h, oxi-exsolution of titanomaghemite begins. CRM as well as the sum of CRM and partial thermoremanent magnetization (pTRM) have been investigated by the Thellier–Coe procedure. The magnetic field Bcalc calculated in the temperature interval 355–475°C from chemical magnetization formed at the annealing times of 4.5, 16.5, and 40.5 h (CRM4.5, CRM16.5 and CRM40.5) proved to be underestimated by 55, 48–52, and 16–34%, respectively. The difference between the calculated and true field decreases with the growth in the degree of single-phase oxidation. The Bcalc values calculated in the temperature interval 475–580°С from CRM40.5 and CRM110 corresponding to the high degree of single-phase oxidation turned out to be underestimated by 69–72 and 62–63%. The last result is caused by the mineral transformations of titanomaghemite at heatings in the Thellier–Coe experiments. During paleointensity determination, it is proposed to control mineral transformations not only using the check-point procedure but also based on the difference in the removal of remanent magnetization in the Thellier experiments and by thermal demagnetization.

The magnetization of the ocean floor: stress and fracturing of titanomagnetite particles by low-temperature oxidation

SUMMARY The natural remanent magnetization (NRM) of the ocean floor is carried by titanomagnetite grains that undergo low-temperature oxidation after initial cooling. Progressing oxidation is known to generate shrinkage cracks in grains larger than approximately 5 μm, and is suspected to control the long wavelength variation of NRM-intensity across the ocean floor. Here we develop a quantitative theory of single-phase oxidation and crack formation by solving the vacancy-diffusion equation that describes the oxidation process for spherical titanomagnetite particles, where the diffusion coefficient strongly decreases with vacancy concentration. The latter dependence has been experimentally demonstrated and is essential to explain the peculiarities of the observed variations of oxidation-degree with ocean-floor age. The calculated diffusion profiles provide the exact stress distributions inside oxidized titanomagnetite spheres, and predict a size limit for shrinkage-crack formation that agrees with microscopic observations of crack appearance in ocean-floor basalt samples. The new diffusion model provides a unified explanation of long-known experimental facts that (1) temperatures for the onset of low-temperature oxidation during laboratory heating are theoretically estimated as 200–400 ○C, depending on grain size and (2) that heating to 400–500 ○C is required to obtain a sufficiently high degree of oxidation z ≈ 0.8 for the development of high-temperature exsolution lamellae. Calculations for ocean-floor conditions quantitatively suggest that a rapid decrease of NRM intensity during the first 40 ka results from a deflection of magnetization by strong stresses that emerge in titanomagnetite grains of subcritical sizes, and randomization of domain-state by crack formation in larger grains.

The Stability of Titanomagnetite Basalt of the Red Sea during Heating in Air and Argon

To optimize heating conditions during the Thellier procedure and to improve the reliability of paleofield determination, thermal stability experiments with titanomagnetite and titanomaghemite obtained by laboratory heating were conducted. The experiments were performed on P72/2 and P72/4 basalts of the rift zone of the Red Sea that contain titanomagnetite with a concentration of the magnetite component equal to (44.7 ± 3.9)% and (46 ± 5)%, respectively. It has been shown that the single-phase oxidation process prevails in annealing of titanomagnetite for 10 hours in a temperature range of 290–410°C in air. An increase in the annealing temperature up to 460–535°C means that single-phase oxidation is replaced by oxi-exsolution. The temperature range of thermal stability of the single-phase oxidized titanomagnetite expands with the degree of oxidation. In particular, at an oxidation state close to 0.9, titanomaghemite is stable up to temperatures of 410–460°C. The use of the Thellier technique for studying the properties of the chemical remanent magnetization of single-phase oxidized titanomagnetite with an oxidation state above z ≥ 0.6 is limited to approximately 20% of its value. It is possible to stabilize the single-phase oxidized state of titanomagnetite at temperatures above 460°C and thereby to improve the quality of CRM research using the Thellier method by adjusting the value of the oxygen partial pressure.

REMOVAL OF LEAD FROM POLLUTED WATER BY USING CLAY MINERALS

Clay minerals or natural materials such as zeolite play an important role in reducing the potential hazards of toxic elements. They cannot completely destroy heavy metals but can only be transformed from a single phase oxidation or organic compound to another. In this study clay minerals used to reduce the hazardous of lead on aqueous solutions. The used minerals were zeolite, bentonite and montmorillonite. Each mineral was applied at the rates of 1, 3 and 5% (W/V) of waste water. The concentrations used of lead were 60, 120 and 180ppm. The results observed that wherever increased the rate of the clay minerals added to the contaminated liquid has increased the amount adsorbed of lead. Therefore, the highest amount adsorbed of lead was obtained with 5% of soil minerals in all cases. The highest amount of lead adsorbed was obtained with zeolite at the rate of 5% (177 ppm) followed by montmorillonite and bentonite (165 and 145 ppm), respectively. There is a positive relationship with both the amount adsorbed of lead and the rate of soil mineral added to the solution of contaminated. On the other hand, the highest lead removal efficiency was found as about 98.7% with the rate of 5% zeolite mineral. Whereas, the lowest lead removal efficiency was found as about 39% with the rate of 1% bentonite mineral. The arrangement of the soil minerals ability to adsorb lead was followed this order: zeolite < montmorillonite < bentonite. This study suggests that using of available natural materials could be an economic and promising alternative solution in contaminated water to minimize hazards of heavy metals. The clay minerals are suitable materials for heavy metal removal from the industrial waste water.

Modeling the implications of chemical transformations for the magnetic properties of a system of titanomagnetite nanoparticles

Implications of the breakdown and oxidation processes for the magnetic properties of finegrained titanomagnetites are theoretically studied in the model of two-phase nanoparticle system. It is shown that the decomposition of titanomagnetites is accompanied by the increase in the blocking temperature, coercive force, spontaneous saturation magnetization, and chemical magnetization, whereas the remanent chemical magnetization behaves nonmonotonically and may undergo self-reversal. The single-phase oxidation of titanomagnetites reduces the coercive force and saturation magnetization and thus leads to the growth in chemical magnetization.

Magnetic properties of basalts and geodynamics of the rift zone in the southern Red Sea

Magnetic properties of young rift basalts in the southern Red Sea are studied in detail. It is found that basalt samples dredged in the rift zone are characterized by a large spread in magnetic parameters. The magnetic properties of the basalts are shown to indicate a complex evolution of the Red Sea rift zone. Titanomagnetite grains of the basalts are mostly affected by single-phase oxidation processes and have preserved paleomagnetic information. However, basalts discovered near the rift axis yield evidence of multiphase oxidation of titanomagnetite grains, which is untypical of young oceanic basalts. These basalts have high Curie points and large values of the natural remanent magnetization and Koenigsberger parameter. The corresponding samples were taken in nontransform zones where rocks have experienced the action of significant tectonic forces and, moreover, anomalous geomagnetic field patterns correlate with the position of these zones. Using the magnetic properties of the basalts, the northern segment of the rift axis is relocated.

Properties of the chemical remanent magnetization formed at a single-phase oxidation and a subsequent decomposition of titanomagnetites

Regularities of influence of a single-phase (SP) oxidation and a subsequent decomposition of synthetic titanomagnetites (TM) of a dierent original composition (Fe3 xTixO4 (where x = 0.2, 0.4 and 0.6)) upon the change of their hysteresis parameters and on properties of the resultant chemical remanent magnetization (CRM) were determined as a result of the laboratory researches. Calculations were made of the changes of magnetization of the igneous rocks similar to the investigated TM of a composition with x = 0.4 taking into account SP changes of the primary titanomagnetite at a high partial pressure of oxygen. The influence of CRM on safety of the primary (thermoremanent) magnetization at dierent stages of the SP oxidation of TM and decomposition of the cation-deficient TM was assessed. The results which were obtained may be used for interpretation of changes in the magnetic properties of the submicron grains of TM in the course of their thermal transformation on the earth surface.

Nanoclusters as precursors to (MoVW)5O14: In situ and chemical characterisation of the systems of a single phase oxidation catalyst

Abstract The ternary oxide (MoVW)5O14 is considered to be highly significant for catalytic, mild selective oxidation reactions. By using UV/Vis, ESR and 95Mo NMR spectroscopies the coordination chem-istry of the molybdate species in the precursor solutions was studied. This work has established that mixing the precursor solutions (ammonium hepta molybdate, ammonium meta tungstate and vanadyl oxalate) forms a polymeric network in which the vanadyl species act as a linker between the molybdate species. Addition of tungsten atoms significantly enhances the polymerisation proc-ess

Kinetics of single-phase oxidation of titanomagnetite

Abstract Cell-edge-determined compositions and their change with time ( t ) during the single-phase oxidation in air of 0.3 ± 0.15 μm synthetic titanomagnetites (TM) (Fe 3 − x Ti x O 4 ; where x = 0.2, 0.4, 0.6 or 0.8) to titanomagnetite are discussed in detail. The data indicate that the rate of TM oxidation in isothermal conditions is dependent on the degree of oxidation ( Z ) and particle composition ( X ): an increase of Z decreases the rate of oxidation; TM grains with x = 0.4 undergo the slowest oxidation, presumably as a result of cation ordering. For titanomagnetite with x = 0.8 the dependence of kinetics on the grain size ( d ) is studied in the range 0.3–1.2 μm. The influence of the above-mentioned parameters could be accounted for in terms of the distribution of vacancies and cations. Extrapolation of oxidation rates to low temperatures suggests that high oxidation ( Z ⩾ 0.8) requires time commensurate with the Earth's age, but that some oxidation would be expected for titanomagnetite maintained at 20° C in air for 1 Ma.

Two types of chemical remanent magnetization during the oxidation of magnetite

The chemical remanent magnetization (CRM), M CRM, resulting from oxidation of equidimensional magnetite to a mixture of 90% hematite and 10% cation-deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization, ( M ARM), of the parent magnetite and the field H CRM applied during oxidation. M CRM lies in the plane defined by H CRM and M ARM (which were perpendicular in our experiments) in an intermediate orientation determined by the relative strengths of H CRM (0.05, 0.1 and 0.2 mT) and M ARM (0.002–0.12 × saturation remanence). The CRMs are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization. Similar experiments in which acicular single-domain cation-deficient magnetite was oxidized to maghemite gave quite different results. M CRM was always parallel to M ARM. Its direction was unaffected by field strengths as high as H CRM = 1.5 mT. The essential factors controlling CRM direction seem to be the number and crystallographic structure of oxidation products. For single-phase oxidation without change in crystal lattice, CRM remains parallel to the initial remanence of the parent mineral. For multiphase oxidation to a mixture of rhombohedral and spinel phases, both initial remanence and the field acting during oxidation influence the CRM direction, which therefore has an intermediate orientation that parallels neither M ARM nor H CRM.

O 1s studies of the oxidation of InP(110) and GaAs(110) surfaces

Two major outstanding issues in the oxidation of InP and GaAs(110) surfaces are addressed: (1) single phase versus multiphase oxidation and (2) surface versus bulk oxidation. From the analysis of O 1s, In 4d, and P 2p core level spectra it is concluded that InP oxidizes predominantly in a single phase at room temperature. Its stoichiometry is enhanced upon annealing to give InPO4. There is no evidence for any other oxide phase for the coverage range investigated (to ∼1 monolayer of adsorbed oxygen). It appears that the oxidation is nucleated, but forms a fairly homogeneous oxide layer at the surface. GaAs(110), on the other hand, oxidizes from submonolayer oxygen coverages on into separate gallium and arsenic oxide phases. There is evidence for several oxidation states of the As. These do not consolidate upon annealing into a stoichiometric phase, but rather are reduced by the presence of GaAs to form additional gallium oxide. Reference spectra obtained from β-Ga2O3 crystals were used to confirm the absence of this oxide in the oxidized GaAs surfaces and to calibrate the oxygen uptake. The experimental results confirm the issue of subsurface oxidation prior to a monolayer uptake of oxygen.