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An In-Situ Mössbauer Study of the High-Temperature Behavior of Siderite under Reduction Conditions

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04 January 2024

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Abstract
The thermal decomposition of siderite and its decomposition products in a vacuum atmosphere was investigated for the first time using in situ high-temperature transmission Mössbauer spectroscopy. These measurements were supported by X-ray diffraction, Raman spectroscopy, X-ray fluorescence, and magnetic measurements. In situ Mössbauer spectra were collected from room temperature (RT) to 750 oC with a temperature step of 50 oC. After that, the sample was cooled down to RT with step 100 oC. Raw siderite samples contain siderite as the only iron-bearing mineral, and after heating the samples at high temperatures, hematite and magnetite are only Fe-bearing phases. In the temperature range of 300 oC - 500 oC, siderite decomposition occurs. The analysis shows that siderite transforms directly into magnetite, and then hematite generation occurs as long as siderite is present in the sample. Changing hyperfine parameters like isomer shift, quadrupole splitting, or hyperfine magnetic field versus temperature were obtained for siderite and iron oxides (hematite, magnetite). The measurements also show that magnetite formed during the decomposition of siderite is a composition of particles of various sizes and a low degree of crystallization.
Keywords: 
Subject: Physical Sciences  -   Applied Physics

1. Introduction

The carbon exchange between the Earth’s interior and the surface occurs over time scales of billions of years and constitutes the geodynamical carbon cycle [1]. Carbon is recycled through subduction into the deep Earth, mainly as carbonates [1,2]. The three major carbonate components in the crust and upper mantle are CaCO3 (calcite), MgCO3 (magnesite), and FeCO3 (siderite) [3]. Throughout Earth’s history, these sediments have formed from divalent metals released from silicate weathering and atmospheric CO2. In the long-term carbon cycle, this process has played a critical role in regulating the atmospheric CO2 content [2,4]. Siderite is an iron-bearing carbonate mineral common in oxygen-free environmental conditions, including lakes, rivers, and marine sediments [5,6,7]. Natural samples of siderite often contain significant substitution of Mg, Ca, and Mn for Fe in the lattice [5,8], and pure siderite is seldom found. Siderite crystallizes in the trigonal crystal system and is rhombohedral in shape, typically with curved and striated faces, and the color of siderite ranges from yellow to dark brown or black, the latter being due to the presence of manganese [9]. This carbonate is an important mineral that finds application in various industries and has gained commercial importance over the years [10]; for example, it is one of the primary sources of iron in Europe [11], in petroleum drilling fluids as a scavenger for H2S and in processes for making ferrous catalyst materials [12,13,14] or in the combustion of the coast [15,16].
The thermal decomposition of siderite is significant because of the industrial relevance of this mineral [17,18]. The mechanism of this process depends on siderite composition and experimental conditions like temperature, atmosphere, or sample microstructure. Many studies are reported to conduct the mechanism of the thermal decomposition of the siderite in the air atmosphere. Many of these studies have used thermomagnetometry and thermogravimetry [5,17,18] or X-ray diffraction analysis [5,19]. As they show, the final product of the thermal decomposition of siderite is hematite (α-Fe2O3). The final product decomposition of FeCO3 was Fe3O4 in a CO2 atmosphere [17,20], while the FeO and Fe3O4 were only observed in an inert atmosphere or a vacuum [10,17,19,20,21]. The studies mentioned above were concentrated mainly on the final products of the thermal decomposition of siderite under different conditions. The process of thermal decomposition of siderite [22,23,24] and the products of this transformation under various external conditions are relatively well-described. Still, only limited work has been performed in-situ on this thermal reaction process.
In this work, the siderite high-temperature decomposition and products of this process were characterized using an in-situ 57Fe Mössbauer spectroscopy method (nuclear resonance technique). It is well known that this technique is very sensitive to the local atomic structure, its local deformation, and atomic or lattice defects when treating the Fe nucleus as a probe of its local surroundings [25]. We use this method among many other analytical techniques because it can provide information about the 57Fe hyperfine interactions in various minerals and compounds, which are related to the iron oxidation states, the iron local microenvironments, the iron magnetic states, the relative fractions of iron-bearing components [26,27]. The results of this method will also be the key to understanding the thermodynamic relationship between the Fe-bearing minerals in the transformation sequence. Such measurements will be presented for the first time. This study has profound implications for processing siderite ore through metallurgy and ore dressing as well as the quality of waste from the coal mining industry. Also, it can be helpful to understand siderite pyrolysis behavior further and may impact the exploitation of Fe-bearing carbonates as a source of iron ore as well as the direction of siderite alteration in thermally active coal mining tailings dumps. The Mössbauer investigation was supported by magnetic measurement, X-ray diffraction, X-ray photoelectron, and Raman spectroscopy.

2. Materials and Methods

Various carbonate rocks are present in the Upper Silesian Coal Basin, Poland. These are mainly siderites in the form of spherosiderites, shoal siderites, and carbonaceous siderites. The studied siderites come from the Ruda coal seams’ upper parts, in the Chwałowice Trough’s eastern part, in the Upper Silesian Coal Basin. Siderites in the seams occurred in claystone layers, forming intergrowths with a 0.10-0.80 m thickness. Macroscopically, siderite is gray or dark grey, sometimes with a beige tinge [28]. It has a fine crystalline structure. The texture is usually massive and chaotic, sometimes directional, and poorly marked due to the presence of thin carbon laminae with a thickness of ~0.5 mm. The rocks are cut with carbonate veins 0.5-5 mm thick. These minerals are white. The research was carried out on four rock samples taken from different seams. These samples were designated Sd1, Sd2, Sd4 and Sd5.

2.1. X-ray Fluorescence

The chemical composition of all samples was determined by X-ray fluorescence (XRF) with a ZSX Primus II Rigaku spectrometer. The spectrometer, equipped with the 4 kW, 60 kV Rh anode and wavelength dispersion detection system, allowed for the analysis of the elements from Be to U. No external standards were necessary. Only the internal standards coupled with the fundamental parameters (theoretical relationship between the measured X-ray intensities and the concentrations of elements in the sample) were implemented. The samples for the analysis were prepared in the form of pressed tablets.

2.2. X-ray Diffraction

Minerals present in the investigated raw siderite samples and after annealing at 750 oC and cooling to room temperature were determined using X-ray diffraction (XRD) method. These studies were conducted at room temperature using a Siemens D5000 X-ray diffractometer and CuKα radiation. Rietveld refinement was performed in a licensed X’Pert High Score Plus with a PDF-4 crystallography database. The percentages of individual components in initial samples were determined using the Rietveld method. The content of a given phase should be greater than 2% by volume. It is then assumed that the phase is correctly identified.

2.3. Raman spectroscopy

Structural characterization of siderite degradation products performed using a WITec confocal Raman microscope CRM alpha 300R. A solid-state laser (λ = 532 nm, P = 5 mW) was coupled to the microscope through a single-mode optical fiber with a 50 μm diameter. The laser radiation was focused onto the sample via an air Olympus MPLAN (100x/0.76NA) objective, while the scattered light passed through a multi-mode fiber (100 μm diameter). Raman spectra were gathered using 20 scans with an integration time of 20 s and a resolution of 3 cm-1. The spectrometer monochromator working with a 600 line/mm grating was checked before the measurements using a silicon plate (520.7 cm-1). A post-processing analysis, including cosmic ray removal and baseline correction, was performed in WITec Project Five Plus. Gauss-Lorentz band fitting analysis used the Grams 9.2 software package to estimate the real bands’ position.

2.4. Magnetic measurements

The magnetic measurements of the investigated samples were performed using magnetic balance in the temperature range of 100 oC – 800 oC in the magnetic field of 1.2 T. Based on these plots, the Curie temperatures (Tc) and Neel temperature (TN) for all samples were determined by the maximum of the first derivative of the temperature-dependent magnetization (M-T) curves.

2.5. Mössbauer Spectroscopy

The Mössbauer spectroscopy method is a powerful technique for structural and magnetic characterization at the atomic level. 57Fe Mössbauer transmission spectra were recorded at room temperature using a POLON-type spectrometer and a linear arrangement of source 57Co:Rh (25 mCi), a multichannel analyzer, an absorber, and a detector. A gas proportional counter LND-45431 was used as a gamma-ray detector. The 2 keV escape peak and 14.4 keV gamma-ray pulses were selected with a multichannel analyzer. The spectrometer velocity scale was calibrated at room temperature with a 25 μm thick α-Fe foil. Data were collected with a 1024-channel analyzer (before folding). All Mössbauer measurements were carried out on powdered samples, and the amount of the sample was chosen to get a thickness of ~8 mg Fe/cm2. The Mössbauer absorber was placed in a boron nitride container in a vacuum oven. The sample temperature was controlled with an accuracy of 0.1 oC. Temperature measurements were made in steps of 50 oC, starting from 50 oC up to 750 oC, and then the sample was cooled down to room temperature with steps 100 oC. Each high-temperature measurement took 12 hours. This time was the optimal time to obtain spectra with good statistics. The Mössbauer spectra were evaluated by least-square fitting of the lines using the MossWinn4.0i program. The spectral parameters such as isomer shift (IS), quadrupole splitting (QS) and quadrupole shift for magnetically split spectra (ε), magnetic hyperfine field (B), full line width at half maximum (Γ), and relative subspectrum area (A) were determined. Some parts of the spectra were fitted with a hyperfine magnetic field (B) distribution. The model and implementation are based on the Voigt-based fitting method [29].

3. Results

3.1. X-ray Fluorescence

Table 1 presents the contents of the elements in the investigated raw siderite samples. These data concern elements whose concentration in the sample was higher than 0.05 %wt. Of course, the analyzed materials contained very small amounts of elements like S, Cl, Cr, Ni, Zn, Sr, or Ba, but their total concentration did not exceed 0.5 % of the total weight. Large amounts of Fe (>33.5 %wt), Si (>3.4 %wt), and Al (>2.12 %wt), just as C and O, are elements of minerals present in rock siderite samples (Table 2).

3.2. X-ray Diffraction

Measurements by XRD revealed that rock siderite samples contained siderite as the main component and small amounts of accessory minerals (Figure 1a, Table 2). A small concentration of quartz, illite, and kaolinite was detected in all non-heated samples. In particular, samples Sd4 and Sd5 contain quite a lot of dolomite.
After heating the samples to 750 oC, and then cooling down from 750 oC to room temperature, hematite (Fe2O3) and magnetite (Fe3O4) are the only Fe-bearing phases in the samples (Figure 1b). The highest concentration of Fe2O3 (~83%) was in the sample Sd1 and the smallest (~39%) in the sample Sd4 (Table 2). The reflections associated with the Fe3O4 visible in the diffractograms are quite broad. Such wide lines may indicate a high disorder of this phase and/or small crystallite sizes. All samples after heating also contain quartz. The content of this silicate is quite significant, especially in sample SD4.

3.3. Raman spectroscopy

Employing a statistical approach to data collection, Figure 2 summarizes the three most representative Raman spectra in the 100 - 1600 cm⁻¹ region obtained from distinct areas of siderite decomposition products for samples Sd1 and Sd4. All collected Raman spectra exhibited a prominent arrangement of hematite (α-Fe₂O₃) bands in the 200 - 700 cm⁻¹ region, featuring A1g and Eg modes (Figure 2) correlated with symmetric stretching and bending Fe-O vibrations [30,31]. Around 1320 cm⁻¹ observed a strong band attributed to two-magnon scattering arising from the interaction of two magnons created on antiparallel closed spins [32]. Notably, the positions of the hematite bands varied slightly based on the collection site. Some areas indicated a crystalline bulk phase (α-Fe₂O₃b) [30,31], while others suggested a Ti or Al-doped or more nanocrystalline form of hematite (α-Fe₂O₃n) [33]. Interpretative challenges in Raman spectra of investigated samples arose concerning an additional band observed in the 650 - 670 cm⁻¹ range. Multiple hypotheses exist for the origin of this band, including surface resonance of incomplete symmetry of hematite, stress-induced defects, the first overtone of the two-magnon scattering band [34], and contaminant or impurities [35,36]. Some papers reported that this band is also related to the A1g mode in orthorhombic magnetite structure [37,38,39,40]. Some instances suggested surface resonance or stress-induced defects in hematite, while in others, their nature was related to the coexistence of hematite with magnetite (grey-colored band in Figure 2). The latter hypothesis suggests that the A1g-magnetite band position around 663 or 664 cm-1 is related to a less defective phase, while its shift toward lower frequencies points to a more defective spinel structure. These findings underscore the complex interplay of factors influencing Raman spectra and microscopic phase heterogeneity with different degrees of structure order depending on the locations.

3.4. Magnetic measurements

The temperature-dependent magnetization curves (M-T) of all the investigated siderites were measured during heating and cooling in an external magnetic field of 1.2 T, as shown in Figure 3. Above temperature about 360 oC for all samples one can observe a small decrease in magnetization, which indicates the initial decomposition temperature of the studied siderites. This temperature is very close to that (350 ◦C) observed for pure siderite [41]. Note that thermal stability of siderite can be affected by partial substitution of Fe2+ in the cationic sublattice. A rapid increase in magnetization is observed at above 400 oC for samples Sd4 and Sd5, above 440 oC for Sd1 and Sd2, with broad and even double maximum in temperature range from ~470 oC to ~530 oC, which indicative of some new magnetic phases generation. We assume that up to this temperature range and under vacuum pressure siderite transformation into magnetite according to equation [42]:
3FeCO3 → Fe3O4 + 3C + 5/2O2,
The above reaction also allows for the formation of hematite, but two paths are possible, illustrated by the equations below [42,43]:
4FeCO3 + O2 → 2Fe2O3 + 4CO2
9Fe3O4 + 2O2 → 12Fe2O3 + (Fe3O4)surface
Based on equations (2) and (3), we can conclude that hematite is formed only when siderite is present in the sample. Here equation (3) requires a brief comment. At higher temperatures, oxidizing magnetite leads to hematite but magnetite and hematite have different crystal symmetries, from this reason hematite domains must nucleate before they can grow [43,44]. As was shown [43], the high-temperature oxidation grows magnetite at the surface and hematite in the interior. It means that different reactions occur in the interior (where hematite forms) and at the surface (where new magnetite forms) [43]. As shown in equation (3), for every 12 formula units of Fe2O3 created, one unit of Fe3O4 is added to the surface. The hematite and magnetite thus formed will have different degrees of crystallization.
Above ~530 oC, with the temperature increasing, a decrease in magnetization is visible (Figure 3), which suggests some magnetic transition. The Curie temperatures were calculated based on these slopes and marked on the plots. These temperatures ​​are slightly lower than those for bulk magnetite (583 oC) [45]. This may be because the Curie temperature decreases with decreasing particle size [46] and the substitution of Fe ions by magnesium [47]. After that, the magnetization slowly decreases with the increase in temperature, and the minimum is close to 800 oC (Figure 3). The magnetic transition temperatures were also calculated based on these slopes (Figure 3). Such high temperatures can indicate the magnetic transition of hematite (680 oC) [48], but these values are a bit too high on this iron oxide. This effect can be explained based on the Dzialoshinskii theory [49], which assumes that the temperature at which the basal plane anisotropy passes zero is shifted with the substitutions, and the transition at ~680 oC can be shifted considerably by partially substituting Cr, Al, Ga, or Mn for the iron in α-Fe2O3. Additionally, for the siderite samples, Sd2 at temperature ~715 oC and Sd4 at ~730 oC are visible some maxima on magnetization curves. This effect can be related to the decomposition of dolomite [50], which is in higher amounts of these samples. However, such a maximum was not observed for sample Sd5, where the concentration of dolomite is higher than in sample Sd2. For this reason, we can assume that this peak is likely Hopkinson-peak effects and was observed for hematite-containing samples [51,52]. Hopkinson peak is usually associated with the unblocking of single-domain grains, making them superparamagnetic or increased domain wall motion in larger multi-domain grains due to enhanced thermal energy; both mechanisms cause an increase in magnetization, though this is particularly enhanced in fine single-domain grains [52].
During cooling back to room temperature (RT), the plots show that the magnetization values were not reversible with temperature. The calculated Neel temperature for hematite present in all samples are marked on the plots (Figure 3). Further, a sharp decrease in magnetization curves in the temperature range from ~300 oC to ~400 oC is observed which indicates some magnetic transition in these samples. This transition is likely related to the magnetic transition in magnetite, where Mg replaces some of the Fe ions. In such a case, we can talk about Mg-rich magnetite, the so-called magnesio-magnetite, where magnetic transitions were observed in this temperature range for such type of compounds [47,53].

3.5. Mössbauer Spectroscopy

The in-situ Mössbauer spectra of investigated siderites were obtained during heating up to temperature 750 oC and then cooling to room temperature. Figure 4 shows selected in-situ spectra obtained during heating the sample Sd1, and Figure 5 presents spectra obtained during cooling this sample up to room temperature. For other samples, these spectra are collected in Figures S1–S3 (Supplementary Materials). The hyperfine parameters of the Mössbauer components for all samples and temperatures are listed in Table S1 (Supplementary Materials).
Up to temperature 300 oC, all spectra were fitted with a single doublet associated with Fe2+ ions in siderite. Obtained isomer shift (ISSd) and quadrupole splitting (QSSd) for initial samples of siderite are in very good agreement with literature data [54,55,56]. In the temperature range of 300 oC - 500 oC, the concentration of this component decreases to zero (Figure 6a). In this temperature range, siderite decomposition takes place. Changing hyperfine parameters (ISSd(T) and QSSd(T)) characterizing this doublet versus temperature (T) are illustrated in Figure 6a. Based on these relationships, the temperature dependence of the isomer shift and quadrupole splitting for siderite was determined, as presented in the equations below:
ISSd(T) = - 0.00070 T + 1.237
QSSd(T) = - 0.00115 T + 1.822
Based on equations (4) and (5), it is easy to determine the ISSd and QSSd values for siderite in a wide temperature range, in particular at 0 oC, they will be respectively 1.237 mm s-1 and 1.822 mm s-1.
Between temperatures 300 oC and 600 oC, the most significant changes are observed in the Mössbauer spectra. The results of magnetic measurements also indicate this (Figure 3). At a temperature of 350 oC, a sextet with a hyperfine magnetic field of approximately 38 T also begins to be visible. Temperature 400 oC is the temperature at which magnetic components dominate the spectra. Marked sextets are associated with hematite (Figure 4). Obviously, the room temperature Mössbauer spectrum of this iron oxide is represented by one sextet associated with only crystalline lattice site for Fe3+ in hexagonal α-Fe2O3. However, these sextets will be related to the formation process of this iron oxide, i.e., hematite is "produced" in portions from magnetite and siderite decomposition. Therefore, these sextets represent different degrees of crystallinity of this iron oxide. The possibility of substituting Fe ions by, among others, Al, Ti, Mn or Mg should also be allowed at high temperatures. These substitutions will cause a decrease in the hyperfine field in hematite [57,58]. Both, the size of the crystallites and the impurities in the hematite will cause the magnetic transition to occur in a broader range of temperatures, and in the Mössbauer spectra, we observe this as the appearance of components with smaller fields. Figure 6b shows the change of average isomer shift ISHem and hyperfine magnetic field BHem versus the temperature of all magnetic components representing hematite during the heating and cooling of all siderite samples. The average value means a weighted average for all components representing Fe2O3. Based on changes in the value of the isomer shift of hematite with temperature, the dependence of this parameter on temperature was determined, as shown in equation (6):
ISHem(T) = - 0.00070 T + 0.386
The isomer shift value determined from equation (6) at room temperature (RT) is 0.370 mm s-1. This is a typical value for this iron oxide at RT [57,58,59,60], thus confirming the correctness of the interpretation and way of fitting of the Mössbauer spectra. Similarly to the values of the hyperfine magnetic field (BHem) in RT, which are slightly above 51 T, the dependence of the BHem on temperature is consistent with the literature [57,58,59,60,61]. Additionally, for the BHem(T) relationship for hematite, it can be seen that the values ​​are almost identical for all samples during cooling. Slightly lower BHem values ​​were obtained during heating, probably due to the different degrees of crystallization of this oxide with increasing temperature and substitution Fe3+ by other ions [49,58].
Apart from the sextets associated with hematite, its remaining magnetic part is visible in the spectra but has no clear absorption maxima. X-ray diffraction, Raman spectroscopy, and magnetic measurements indicate that this part of the spectrum is related to magnetite. These measurements also show that it is low-crystallinity magnetite. It is worth noticing that such magnetite is formed in all heated siderite samples. According to equation (1), this magnetite is formed due to siderite decomposition but is simultaneously transformed into hematite (equation (2)), and a small part of this spinel is again created during hematite generation. It is impossible to fit this part of the spectrum with two sextets characterizing Fe ions in the tetrahedral and octahedral coordination in the structure of this spinel [61]. This part of the spectra was fitted with a magnetic field distribution. The obtained value of the isomer shift of this distribution (Figure 6c) will be the average value characterizing two positions (tetrahedral and octahedral) of Fe ions, similar to the value of the hyperfine field (Figure 6c). For samples Sd2 and Sd5, the values of isomer shift are smaller in comparison to the two other samples. Such lower values of this parameter can result from the substitution of iron for magnesium in octahedral sites [62]. During the cooling process, the concentration of this distribution in the spectra is almost constant up to a temperature of 400 oC, where a slight increase in the contribution of this component up to room temperature was visible. At the same time, the hyperfine field decreases slightly in this temperature range. This effect can be explained by considering the doublet observed in the Mössbauer spectra of cooled samples (Figure 5 and Figures S1–S3). The concentration of this doublet (Table S1) remains constant, but starting from a temperature of about 400 oC, it begins to decrease with temperature decreasing (Figure 6d). Below 200 oC, its content is close to zero. This doublet is probably associated with magnesio-magnetite. High Mg content in the magnetite structure significantly reduces the magnetic transition temperature [53], as indicated by the results of magnetic measurements. It is also worth noting that even when the samples are cooled to room temperature, the spectra have no clear lines related to balk magnetite. This fact would indicate a high degree of disorder in this phase as well as small sizes of crystal grains.
The ferric doublet visible in the Mössbauer spectra during heating and cooling siderite samples (Figure 4, Figure 5, Figures S1–S3, Table S1) is multi-faceted but generally represents non-magnetic and superparamagnetic iron oxides particles and is therefore described as NPS. Changing of isomer shift ISNPS versus temperature for this component (Figure 6d) while heating and cooling are linear and fitted, as shown in equation (7):
ISNPS(T) = - 0.00065 T + 0.335
As shown above, in the cooling process, from a temperature of about 600 oC, this ferric doublet represents the paramagnetic form of Mg-rich magnetite. At high temperatures (above 600 oC), it constitutes deposits of the paramagnetic phase of both magnetite and hematite. Below a temperature of 600 oC, the nature of this doublet appears to be different in the cooling and heating processes, as indicated by the quadrupole splitting QSNPS dependence on the temperature of this component (Figure 6d). The values of this parameter obtained during the heating of the samples are much higher than those calculated during the cooling process. Higher quadruple splitting values are characteristic of hematite nanoparticles [63,64].

4. Discussion

The X-ray diffraction and Mössbauer measurements of all investigated carbonate rocks show that siderite (FeCO2) is the only mineral containing iron. The most significant amount of this mineral (Table 2), approximately 94%, is found in the Sd1 sample and the least in the Sd5 sample (~79%). Other minerals in the samples are mainly dolomite (CaMg(CO3)2) and quartz (SiO2), which contribute to the significant content of Mg, Ca and Si, especially in the Sd4 and Sd5 samples (Table 1). The fact that iron is located only in siderite allows the use of the Mössbauer spectroscopy method to study the transformations that this mineral undergoes at high temperatures in a vacuum.
Above a temperature of 300 oC, the siderite decomposition process begins (Figure 3 and Figure 6). Magnetite (Fe3O4) is an oxide mineral belonging to the group of iron spinels, which is formed as a result of this process. Magnetite is then oxidized to hematite (Fe2O3), but the process of magnetite formation itself allows for the formation of hematite from siderite. These processes are perfectly illustrated by equations (1)-(3). The abovementioned processes occur in a narrow temperature range, up to 500 oC. This is the temperature at which siderite completely decomposes. The temperature dependence of hyperfine parameters characterizing the doublet associated with siderite was calculated (Figure 6a). Linear dependencies of the isomer shift ISSd and quadrupole splitting QSSd on temperature are presented in equations (4) and (5). These equations make it possible to determine the hyperfine parameters of siderite in a wide temperature range.
The observed changes occurring during heating the samples at temperatures higher than 500 oC are related to magnetic transitions in the formed iron oxides. Based on the analysis of the in situ Mössbauer spectra, it was possible to determine the temperature dependencies of the isomer shift or hyperfine field for hematite. The linear correlation of the isomeric shift ISHem with temperature allows us to determine the value of this parameter over a wide temperature range, and the value of 0.37 mm s-1 for room temperature obtained based on this relationship is in perfect agreement with the well-known IS value for this iron oxide.
Magnetite, which is formed during siderite decomposition and a small amount of it is generated during the formation of hematite, attracts particular attention. X-ray, Raman, and Mössbauer (Figure 1, Figure 2, Figure 4 and Figure 5) measurements show that the iron oxide formed in the abovementioned processes is slightly crystalline during heating and remains in such form even after the sample is cooled. In samples containing more significant amounts of Mg (Table 1), we also observe the incorporation of this element into the structure of magnetite and the formation of the so-called magnesio-magnetite.

5. Conclusions

The presented study shows the results of the thermal decomposition mechanism of siderite in vacuum atmosphere. The phase analysis of iron-bearing minerals formed during these processes is shown. Most of the conclusions were drawn based on the results of in situ Mössbauer measurements. This technique is an excellent tool for identifying iron-containing minerals and tracking the processes they undergo, such as temperature. Importantly, we get a macroscopic image of the whole sample and the local information that composes it because, as we know, we use the Fe nucleus as a probe of the local environment in this technique.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org, Table S1: Hyperfine parameters of siderite rock samples (Sd1, Sd2, Sd4 and Sd5) heated and cooled at different temperatures (T) in vacuum atmosphere. IS - isomer shift, QS - quadrupole splitting, ε - quadrupole shift for magnetically split spectra, B - magnetic hyperfine field, Γ - line width (full-width at half maximum), and A - relative subspectrum area; Figure S1: The in-situ Mössbauer spectra measured at selected temperatures during heating (a) and cooling (b) the sample Sd2. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles; Figure S2: The in-situ Mössbauer spectra measured at selected temperatures during heating (a) and cooling (b) the sample Sd4. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles; Figure S3: The in-situ Mössbauer spectra measured at selected temperatures during heating (a) and cooling (b) the sample Sd5. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles.

Author Contributions

M.K.-G. did the conceptualization; designed the methodology; and performed the measurements/investigations, and validation. She also prepared the original draft and reviewed and edited the manuscript, secured the resources; investigation, M.K.-G., M.D., P.G. and M.W.; formal analysis, M.K.-G., Z.A., and M.D. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data may be made available by the authors upon individual request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of the initial siderite samples (a) were heat-treated up to 750 oC and cooled to room temperature (b). The interplanar spacing values are shown in the diffractograms. Sd—siderite, Kln—kaolinite, Dol—dolomite, Mag—magnetite, Hem—hematite, Qtz—quartz.
Figure 1. XRD patterns of the initial siderite samples (a) were heat-treated up to 750 oC and cooled to room temperature (b). The interplanar spacing values are shown in the diffractograms. Sd—siderite, Kln—kaolinite, Dol—dolomite, Mag—magnetite, Hem—hematite, Qtz—quartz.
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Figure 2. Raman spectra of heat-treated siderite Sd1 (a) and Sd4 (b) up to 750 oC and cooled down. Spectra were gathered from a few points (red, blue, and green), while color crosses corresponded to the individual spectra. Grey-colored bands corresponded to Fe3O4, while other bands originated from α-Fe2O3 (a nanocrystalline or Ti,Al-dopped form of hematite α-Fe₂O₃n and crystalline bulk phase α-Fe₂O₃b).
Figure 2. Raman spectra of heat-treated siderite Sd1 (a) and Sd4 (b) up to 750 oC and cooled down. Spectra were gathered from a few points (red, blue, and green), while color crosses corresponded to the individual spectra. Grey-colored bands corresponded to Fe3O4, while other bands originated from α-Fe2O3 (a nanocrystalline or Ti,Al-dopped form of hematite α-Fe₂O₃n and crystalline bulk phase α-Fe₂O₃b).
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Figure 3. Temperature-dependent versus magnetization (M-T) plots for siderite samples Sd1, Sd2, Sd4, and Sd5. The open points characterize the values of magnetization during heating, and the hollow points describe these values during the cooling of the samples. Arrows indicate the temperatures of magnetic transitions.
Figure 3. Temperature-dependent versus magnetization (M-T) plots for siderite samples Sd1, Sd2, Sd4, and Sd5. The open points characterize the values of magnetization during heating, and the hollow points describe these values during the cooling of the samples. Arrows indicate the temperatures of magnetic transitions.
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Figure 4. The in-situ Mössbauer spectra measured at selected temperatures during heating the sample Sd1. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles.
Figure 4. The in-situ Mössbauer spectra measured at selected temperatures during heating the sample Sd1. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles.
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Figure 5. The in-situ Mössbauer spectra measured at selected temperatures during cooling the sample Sd1. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles.
Figure 5. The in-situ Mössbauer spectra measured at selected temperatures during cooling the sample Sd1. The experimental points, fitting curves and spectral components corresponding to particular iron sites in different structural and magnetic phases (colored lines) are presented; line – siderite, orange line – hematite, gray line – magnetite, blue line – iron oxide nanoparticles.
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Figure 6. Concentration of components in in-situ Mössbauer spectra and their hyperfine parameters versus temperature for siderite Sd (a), hematite Hem (b), magnetite Mag (c), and iron oxides nanoparticles NPS (d). The open points characterize the parameters during heating, and the hollow points describe the parameters during the cooling of the sample. The lines indicate the trends of the data and act as a guide to the eye.
Figure 6. Concentration of components in in-situ Mössbauer spectra and their hyperfine parameters versus temperature for siderite Sd (a), hematite Hem (b), magnetite Mag (c), and iron oxides nanoparticles NPS (d). The open points characterize the parameters during heating, and the hollow points describe the parameters during the cooling of the sample. The lines indicate the trends of the data and act as a guide to the eye.
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Table 1. Elemental concentrations for rock siderite samples.
Table 1. Elemental concentrations for rock siderite samples.
Sample Element concentration (%wt)
C O Mg Al Si P K Ca Ti Mn Fe Sum
Sd1 6.17 48.60 0.80 2.63 4.58 0.17 0.32 0.86 0.09 0.60 34.96 99.78
Sd2 6.93 49.21 0.67 2.38 3.86 0.23 0.31 0.73 0.08 0.48 34.85 99.73
Sd4 6.90 49.01 1.57 2.17 3.44 0.16 0.25 1.83 0.08 0.58 33.53 99.52
Sd5 5.21 45.39 1.42 2.66 6.75 0.24 0.47 1.41 0.11 0.43 35.57 99.66
Table 2. Mineral composition of investigated siderite samples based on XRD analyzes by Rietveld method in vol.%.
Table 2. Mineral composition of investigated siderite samples based on XRD analyzes by Rietveld method in vol.%.
Component Formula Sample
Sd1 Sd2 Sd4 Sd5
Initial samples Siderite FeCO3 94.2 90.3 85.5 79.4
Quartz SiO2 Trace Trace 0.3 7.3
Illite K0.65Al2.0[Al0.65Si3.35O10](OH)2 1.0 1.2 0.9 0.4
Kaolinite Al2Si2O5(OH)4 4.8 2.2 0.2 1.6
Dolomite CaMg(CO3)2 Trace 6.3 13.1 11.3
Sum 100 100 100 100
Heated at 750 oC Hematite Fe2O3 83.2 74.9 38.8 71.4
Magnetite Fe3O4 15.5 24.7 19.4 14.6
Quartz SiO2 1.4 0.3 41.8 14.0
Sum 100.1 99.9 100 100
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