Displays

Key-words: Super-deep diamond, Central African Republic, hydrous ringwoodite, Insitu C- and N- isotope composition, subduction, N aggregation state.

Diamonds and their inclusions are key geological materials that provide a unique opportunity to directly investigate the deepest regions of our planet.

Based on their formation depth, diamonds are classified in lithospheric, which formed between about 120 and 220 km depth and represent about 99% of worldwide diamond population, and sub-lithospheric or super-deep diamonds, extremely rare samples which crystallized from about 300 to more than 800 km depth (Stachel et al., 2008).

Here, we have investigated a 1.3 carats diamond, Type IaAB (determined by FTIR), from an alluvial deposit located in Central African Republic, close to the Ubangy River. As far as we know, this is the first study dedicated to inclusions in diamonds from this country.

The investigated diamond contains the second world finding of hydrous ringwoodite after the one found within a Brazilian diamond by Pearson et al. (2014). This finding indicates that our diamond is certainly a super-deep diamond coming from the lower part of the transition zone (between 525 and 660 km depth). Carbon isotope composition of the host diamond (δ¹³C_mean = -2.2 ± 0.3 ‰, n=16, analytical error = 0.3‰ (2σ)) is significantly enriched in heavy isotope when compare to the canonical mantle value (δ¹³C = -5‰). It is nitrogen poor (N < 44 ± 23 at.p.p.m., mean = 15 at p.p.m.) and partially aggregated (%B= 88.5 %). For N content greater that our analytical precision (23 p.p.m.) we performed N-isotope measurement and the values, although associated to large analytical uncertainties, are all positive, (d¹⁵N = 3.48 ± 3.5 ‰) and significantly enriched in heavy isotope compare with the mantle values (-5‰). These geochemical signatures are similar with those previously found in super-deep diamonds (Stachel et al., 2002). These data are consistent with a diamond forming fluid originating from a N-poor subducted source, such as carbonates, (e.g. Walter et al., 2011), in agreement with studies reporting transition-zone and lower-mantle diamonds (Nestola et al., 2018).

References

Nestola F., Korolev N., Kopylova M., Rotiroti N., Pearson D.G., Pamato M.G., Alvaro M., Peruzzo L., Gurney J.J., Moore A.M. & Davidson J. 2018. CaSiO₃ perovskite in diamond indicates the recycling of oceanic crust into the lower mantle. Nature, 555, 237-241.

Pearson D.G., Brenker F.E., Nestola F., McNeill J., Nasdala L., Hutchison M.T., Matveev S., Mather K., Silversmith G., Schmitz S., Vekemans B. & Vincze L. 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, 507, 221-224.

Stachel T., Harris J.W., Aulbach S. & Deines P. 2002. Kankan diamonds (Guinea) III: δ¹³C and nitrogen characteristics of deep diamonds. Contrib. Mineral. Petrol., 142, 465-475.

Stachel T. & Harris J.W. 2008. The origin of cratonic diamonds -Constraints from mineral inclusions. Ore Geol. Rev., 34, 5-32.

Walter M.J., Kohn S.C., Araujo D., Bulanova J.P., Smith C.B., Gaillou E., Wang J., Steele A. & Shirey S.B. 2011. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions. Science, 334, 54-57.

How to cite: Lorenzon, S., Nestola, F., Jacobsen, S., Emilie, T., Loredana, P., Alessandra, L., Martha, P., Matteo, A., Frank, B., and Paolo, N.: Super-deep diamond from Central African Republic , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10257, https://doi.org/10.5194/egusphere-egu2020-10257, 2020.

Federico Galdenzi^1,2, Giancarlo Della Ventura^1,2, Umberto Susta¹, Francesco Radica¹, Augusto Marcelli^2,3

1 Dip. Scienze, Università di Roma Tre, L. S. Leonardo Murialdo 1, 00146, Rome

2 INFN-LNF, Via E. Fermi 40, Frascati 00044 (Rome)

3 Rome International Centre for Material Science Superstripes - RICMASS, Via dei Sabelli 119A, 00185 Rome, Italy

In this work we address the diffusion of hydrogen at high temperature in a sample of riebeckite close to the end-member composition Na₂Fe³⁺₂ Fe²⁺₃Si₈O₂₂(OH)₂. We carried out isothermal experiments on both powders and single-crystals and monitored the behavior of the O-H stretching signal by FTIR spectroscopy. Two different sets of experiments were performed: in the first one we collected data on five doubly polished chips with the same thickness (85 µm) at different temperatures, in the range 520 to 560 °C. For the second set, we collected OH-stretching data at a constant T = 550 °C on six samples with thickness ranging between30 and 150 µm. In any case the target temperature was reached as fast as possible (90°C/min rate) and held constant while collecting FTIR spectra every 2 minutes, until the complete disappearance of the OH-signal. Preliminary spectra collected on amphibole powder embedded in KBr disks showed no OH loss even after prolonged heating, therefore the isothermal experiments were performed on pellets consisting of compressed pure powder. The integrated OH intensities as a function of time were fitted using the Avrami equation; for single-crystals, the data showed an initial intensity increase that was fitted testing two different procedures. The resulting parameters were plotted in the Arrhenius space to derive the activation energy (E_a) for the H⁺ diffusion in riebeckite. The final values are: 19.6±1.5 kJ/mol (from powder data), 26±3 kJ/mol or and 34±2 (from single-crystal data, depending on the fitting method). The activation energy for powders is lower than that obtained for single-crystals, and this result supports the model in which the oxidation of amphiboles occurs at the sample surface. Moreover, the E_a obtained here are considerably lower than the values reported in the literature (e.g. Ingrin and Blanchard, 2006) for pure diffusive processes of H₂ and H₂O through several different crystal matrixes. It is also consistently lower that all values reported so far for amphiboles (e.g. Johnson and Fegley, 2003). This can be related to the peculiar deprotonation mechanism in riebeckite where the OH - O²⁺ substitution at the anionic O3 site is coupled to ^M(1)Fe²⁺ - ^M(1)Fe³⁺ oxidation (e.g. Della Ventura et al., 2018, Galdenzi et al., 2018) and the transformation of the phase into an oxi-amphibole.

References cited

Della Ventura, G., Milahova, B., Susta, U., Cestelli Guidi, M., Marcelli, A., Schlüter, J., Oberti, R. (2018) Am. Min., 103, 1103-1111.

Galdenzi, F., Della Ventura, G., Cibin, G., Macis, S., Marcelli, A. (2018) Rad. Phys. Chem., 1, 1-4.

Ingrin and Blanchard (2006) Rev. Min. Geochem., 62, 291-320.

Johnson, N.M., Fegley B. (2006) Icarus, 164, 317-333.

How to cite: Galdenzi, F.: The HT diffusion of hydrogen in riebeckite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2020, https://doi.org/10.5194/egusphere-egu2020-2020, 2020.

    Knowledge of the volatiles cycles is vital to understand the evolution of the planet Earth and the life it supports. Although it is gradually accepted that water and other volatiles are recycled into the mantle through subduction, it is still not unclear how these volatiles are transported down into the deep Earth. Phlogopite is an accessory mineral frequently observed in samples from the upper mantle, thereby acting as an important carrier of fluorine and water down to >200 km depth. Previous experimental studies and textural relationships of natural samples have indicated that fluorine-rich phlogopite can be stable under ultra-high-temperature conditions. To further investigate effects of fluorine on the stability of phlogopite, here, we present an atomic level research of effects of fluorine on the structural stability using in situ high temperature infrared spectroscopy, Raman spectroscopy, and X-ray powder diffraction. Both X-ray powder diffraction and Raman spectroscopy suggests that fluorine-poor phlogopite decomposes earlier than the fluorine-rich phlogopite. Moreover, the O-H bonds and lattice modes are stiffer for the fluorine-rich phlogopite than the fluorine-poor phlogopite, which is well responsible for the mechanism of fluorine stabilizing phlogopite. Based on our studies, we propose that fluorine-rich phlogopite can effectively transport water and fluorine to the deep Earth.

How to cite: Sun, J., Yang, Y., and Xia, Q.: The role of phlogopite in the deep Earth’s water and fluorine cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2085, https://doi.org/10.5194/egusphere-egu2020-2085, 2020.

Halogens are volatile elements of great interest for the study of fluid-rock interactions between minerals of metamorphic and mantle rocks. Constraining the partitioning of these elements between minerals is also key to understanding their deep geochemical cycle. Hydrous silicates such as micas, amphiboles, chlorites, epidotes or serpentines often contain minor to trace amounts of halogens incorporated by the OH^- = X^- (X^- = F^-, Cl^-, Br^- or I^-) mechanism. Their abundance in metamorphic and mantle rocks grants them a major role in storing and transporting halogens through the subduction zone. However, low halogen concentrations hamper in situ analyses, and quantifying the partitioning of low-concentrated halogens remains then very challenging.

The present study focusses on incorporation of halogens (F^-, Cl^-, Br^-, I^-) into hydroxyl sites in phyllosilicates and amphiboles, on both analytical and theoretical grounds.

In situ measurements in minerals using electron probe microanalysis and LA-ICP-MS/MS have been carried out, allowing investigation of minor to ultra-trace halogen concentrations. Average detection limits with the electron probe are of 200 ppm for F and 35-40 ppm for Cl, Br and I. LA-ICP-MS/MS allowed simultaneous measurement of Cl, Br and I, reaching detection limits of about 50-100 ppm of Cl, 1-10 ppm of Br and well below 1 ppm for I. Calibrations have been carried out using international and house standards. Halogen ratios and partition coefficients between minerals have been measured.

Ab-initio modelling of the OH^- = X^- exchange in phyllosilicate end-members of interest (e.g. phlogopite, muscovite, clinochlore) is underway (CRYSTAL17, Dovesi et al., 2014). Halogen-bearing defects are modelled as diluted as much as possible in crystals (≤ 1 wt. %) to mimic trace concentrations. Crystal strain and energetic cost of the substitution as well as theoretical partition coefficients have been computed and compared between optimised structures. Comparison of F and Cl partitioning between Mg-biotite and muscovite shows that the effect of dioctahedral vacancies over the position of hydroxyl groups strongly influences halogen partitioning, where F and Cl distribute in favour of biotite. Forthcoming modelling will quantify the strain and energetic impact of halogen incorporation in chlorite and amphibole end-members.  

Reference

Dovesi, R.; Orlando, R.; Erba, A.; Zicovich-Wilson, C. M.; Civalleri, B., … Kirtman, B. (2014). International Journal of Quantum Chemistry, 114(19), 1287–1317.

How to cite: Figowy, S., Dubacq, B., D'Arco, P., Villemant, B., Caron, B., and Noël, Y.: Partitioning of halogens (F, Cl, Br, I) between hydrated silicates: analysis and first principles modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10422, https://doi.org/10.5194/egusphere-egu2020-10422, 2020.

Earth’s inner core likely consists of Fe-Ni alloy(s) plus a minor fraction of light element(s) to match the density and sound wave velocities of seismological models such as the preliminary reference Earth model (PREM). Among possible alloying light elements (e.g., Si, O, H, S, C), silicon is a popular candidate based on its cosmochemical abundance and potential involvement in chemical reactions at the core-mantle boundary. Previous work has shown that the solubility of Si in hcp-(Fe,Ni) alloy increases the stability field of the bcc-phase at high pressure. Comparison of sound velocity and density data of Fe-Ni-Si alloys with geophysical observations and theoretical predictions provide important constraints on the structure and dynamics of Earth’s inner core. However, knowledge of the high-pressure and high-temperature behaviour and properties of Fe-Ni alloys that contain light elements is limited. We therefore investigated bcc-Fe_0.78Ni_0.07Si_0.15 alloy to compare its sound velocity and density with ab initio calculations and PREM in order to clarify the role of Si as a light element in Earth’s inner core.

Compressional velocities and densities of bcc-Fe_0.78Ni_0.07Si_0.15 alloy have been measured using inelastic X-ray scattering (IXS) and powder X-ray diffraction at the SPring-8 synchrotron facility (BL35XU beamline). High pressure was generated using a BX90-type diamond anvil cell. The metal alloy sample was loaded together with Ne (pressure medium) in a Re sample chamber and was mechanically compressed to 75 GPa through steps of 10 GPa at room temperature. IXS data were acquired at each pressure point in the range of momentum transfer of 4.24 to 7.63 nm^-1. To determine density, we collected X-ray diffraction patterns of the sample before acquisition of each IXS spectrum using a flat panel detector installed in the optical system. All IXS spectra were fitted using Lorentzian functions. The dispersion relationship between energy (E) and momentum transfer (Q) was obtained by fitting all data with the following equation:

E (meV) = 4.192 x 10^-4 vp (m/s) x Q_max (nm^-1) x sin (π/2 x Q (nm^-1)/ Q_max (nm^-1),

where vp is the sound velocity of the sample.

Preliminary results for bcc-Fe_0.78Ni_0.07Si_0.15 show that the energy of the longitudinal acoustic phonon increases with increasing pressure. Additionally, we found that vp follows Birch’s Law, i.e., there is a linear relationship between density and sound velocity. Based on the comparison of our results and those for hcp-Fe and Fe-Si alloys reported previously with PREM, we propose that bcc-Fe_0.78Ni_0.07Si_0.15 alloy is a viable candidate as a component of Earth’s inner core.

How to cite: Dominijanni, S., McCammon, C., Eiji, O., Daijo, I., Tatsuya, S., and Ishii, T.: Composition of the Earth’s inner core from high pressure sound velocity measurements of Fe-Ni-Si alloys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12120, https://doi.org/10.5194/egusphere-egu2020-12120, 2020.

Plagioclase feldspars are the most abundant mineral in the Earth’s crust. Intermediate plagioclase feldspars commonly display incommensurately modulated structure or aperiodic structure. Both fast-cooled and slow-cooled plagioclase feldspars display the incommensurately modulated structures. The ordering pattern in the incommensurately modulated structures of e-plagioclase (characterized by the satellite diffraction peak called e-reflections) are the most complicated and intriguing. The modulated structure has a superspace group symmetry of X-1(αβγ)0 with a special centering condition of (½ ½ ½ 0), (0 0 ½ ½), (½ ½ 0 ½), and the q-vector has components (i.e., δh, δk, δl) along all three axes in reciprocal space. Displacive modulation, occupational modulation, and density modulation are observed in slowly cooled labradorite feldspars. Z-contrast images show both Ca-Na ordering and density modulation. Local structure of lamellae domains has I1 symmetry. The neighboring lamellae domains are in inversion twinning relationship.  The results from Z-contrast imaging and low-temperature single XRD provide consistent structure with density modulation.  The amplitudes of the modulation waves are new parameters for quantifying the ordering state of plagioclase feldspars. Iridescent labradorite feldspars display exsolution lamellae with average periodicity ranging from ~ 150 nm to ~350 nm. Compositional difference between the lamellae is about 10 to 15 mole % in anorthite component. Areas or zones with red iridescent color (i.e., long lamellae periodicity) always contain more Ca (~ 1 to 3 mole %) than the areas with blue (or green) iridescent color within the same labradorite crystal.  We proposed that the solvus for Bøggild intergrowth has a loop-like shape ranging from ~An₄₄ to ~ An₆₃. The Ca-rich side has higher temperature than the Na-rich side. The shapes of satellite peaks, the distances between e-reflections (modulation periods), and even the intensity of c-reflections may also be used evaluate the ordering state or cooling rate of the plagioclase feldspar. Both modulated structure and the exsolution lamellae can be used as proxies for quantifying cooling rate of a labradorite and its host rock.

How to cite: Xu, H. and Jin, S.: Incommensurately modulated structures and subsolidus phase relations of intermediate plagioclase feldspars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3662, https://doi.org/10.5194/egusphere-egu2020-3662, 2020.

Image based analytical tools in geoscience are indispensable for the characterization of minerals but most of them are 2D techniques, limited to the surface of a polished plane in a sample. X-ray micro computed tomography (micro-CT) is becoming a common analysis technique in geoscience and provides direct 3D information of the internal structure of a sample. A major drawback of micro-CT in the characterization of minerals, however, is the lack of chemical information. There have been different approaches to obtain chemical data using micro-CT but most of them are time consuming and difficult to adapt to regular use.

Therefore we introduce a potential new analytical tool: Laboratory-based Spectral X-ray Micro Computed Tomography (Sp-CT). Results from a spectral imaging detector prototype, installed inside a TESCAN CoreTOM micro-CT scanner, will be shown. This new analytical technique enables to obtain both high resolution structural and chemical information in 3D. With this information, the mineral distribution inside unbroken rocks and particles can be identified and quantified.

Based on the transmitted energy spectrum of a sample, main elements can be distinguished and minerals classified. It is also possible to quantify heavy elements within particles of complex composition and the measured sample volume is significantly larger compared to conventional analytical 2D techniques. Furthermore, Sp-CT is non-destructive and does not require sample preparation.

Sp-CT will open exciting new possibilities for mineral analysis. With this new technique, the 3D properties of the particles can now be measured and used for example in process mineralogy simulations. This is a major improvement to current simulations that predominantly use less representative 2D or bulk particle properties. Moreover, the Sp-CT could potentially be used as an alternative technique for regular characterization of ore deposits and processed ores since more representative volumes can be analyzed in a fast manner relative to existing techniques.

This research is part of the upscaling project “Resource Characterization: from 2D to 3D microscopy” and has received funding from European Institute of Innovation and Technology (EIT), a body of the European Union, under the Horizon 2020, the EU Framework Programme for Research and Innovation.

How to cite: Sittner, J., Godinho, J. R. A., Renno, A. D., Cnudde, V., Boone, M., De Schryver, T., Roine, A., and Liipo, J.: Spectral X-ray tomography for 3D mineral analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20410, https://doi.org/10.5194/egusphere-egu2020-20410, 2020.

In order to better understand the environmental behaviour of thallium, we have chosen the abandoned As–Sb–Tl–Au Allchar deposit (North Macedonia) with unique mineral composition and high thallium grades of the ore. We used pore water analyses, selective extractions, single-crystal and powder X-ray diffraction (PXRD), SEM-EDS, electron microprobe analysis (EMPA), and Raman spectroscopy to determine the distribution and speciation of thallium in waste dump material at the Tl-rich Crven Dol locality in the northern part of the Allchar deposit.

PXRD studies showed that the various solid waste samples are comprised mostly of carbonates (dolomite and calcite), gypsum, quartz, muscovite, kaolinite-group minerals followed by orpiment, realgar, pyrite, marcasite, lorandite, and various iron and calcium arsenates and iron (hydro)oxides, both amorphous and crystalline. Raman spectra, SEM-EDS and EMPA also showed the presence of Ca-Fe-, Ca-Mn-, and Ca-Mg-arsenates.

The main primary source of Tl in the waste is lorandite (TlAsS₂), which occurs as prismatic crystals and anhedral grains up to 1 mm and is frequently intergrown with realgar. Other Tl sources, included in either realgar or orpiment, are minor Tl sulphosalts such as fangite (Tl₃AsS₄), raguinite (TlFeS₂), picotpaulite (TlFe₂S₃) and jankovićite (Tl₅Sb₉(As,Sb)₄S₂₂). The Tl dissolved during weathering is precipitated as micaceous subparallel crystals of poorly crystalline to amorphous thallium arsenates (representing previously unknown mineral species), forming porous aggregates up to 100 µm. These Tl arsenates are intergrown with dolomite and Ca-Fe-arsenates and appear as two chemically different phases. The first, more common phase shows a variable Tl:As ratio ranging from ca. 2.1 to 4.1 and a variable Ca content (2.2 to 4.1 at.%). In the second, Tl-richer phase, the Tl:As ratio varies from ca. 5.1 to 8.4. Raman spectra of the Tl arsenates display broad bands and may be divided in the fingerprint region into two relevant ranges, 350–600 and 700–900 cm⁻¹, both attributed to arsenate tetrahedral complexes showing As–O(X) symmetric stretching with X = H⁺ or H₂O.

Another relatively common Tl precipitate is dorallcharite [TlFe³⁺₃(SO₄)₂(OH)₆], crystallizing in the form of tiny, well-formed platelets that are grouped into aggregates up to 400 µm in size. Tl is also accumulated in (probably cryptomelane-type) Mn oxides (up to 3.6 at.%), pharmacosiderite (up to 0.9 at.%), and jarosite (up to 0.9 at.%).

The pore water contained high aqueous concentrations of Tl (up to 660 μg·L⁻¹) and As (up to 196 mg·L⁻¹). Although these concentrations are low with respect to their total concentrations in the solid phase (Tl: 0.07-1.44 wt. %; As: 0.72-8.67 wt. %), mild extractions (ammonium nitrate and phosphate) mobilized up to 44% of the total Tl and 23% of the total As, indicating that a large amount of these toxic elements is bound weakly (sorption) to solids and can be easily mobilized into the pore water.

Financial support of the Austrian Science Fund (FWF) [P 30900-N28] is gratefully acknowledged.

How to cite: Đorđević, T., Kolitsch, U., Drahota, P., Knappová, M., Majzlan, J., Kiefer, S., Mikuš, T., Tasev, G., Serafimovski, T., Boev, I., and Boev, B.: Thallium mobility in mining wastes at the Crven Dol locality, Allchar deposit, North Macedonia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4959, https://doi.org/10.5194/egusphere-egu2020-4959, 2020.

Vanadium (V) is one of the key technologically critical elements. The slags produced by the historical mining and processing of Zn–Pb–V ores in the Waelz kiln at Berg Aukas (northern Namibia) can be potentially used as a secondary resource of V. A combination of mineralogical methods, bulk chemistry, leaching tests and speciation-solubility modeling was used to understand the binding of the major contaminants (Zn, Pb, V) in the solid phase and their potential release under the changing environmental conditions. The average concentrations of the metal(loid) contaminants in the slags are 3.78 wt% Zn, 3370 mg/kg Pb, 5880 mg/kg V, 767 mg/kg Cu, 578 mg/kg As and 92 mg/kg Sb. The mineralogy is dominated by high-temperature silicates (clinopyroxene, melilite, olivine-family phases) and Zn-bearing phases (willemite, zincite). All the primary silicates and oxides are Zn-rich, but vanadium is mainly concentrated in clinopyroxene (up to 5 wt% V2O3). Metallic Fe inclusions, formed under highly reducing conditions in the kiln, are highly weathered. Secondary Fe(III) (hydr)oxides, corresponding to the main weathering products in the slag, efficiently sequester the metal(loid)s (mainly As and Sb). The EU regulatory leaching tests indicated that the release of the metal(loid) contaminants is quite low at the natural pH (deionized water extract: 8.5–10.4) obtained by extraction in the deionized water and only Sb in all the slag samples exceeds the EU limits for the landfilling of inert waste. The pH-static leaching tests revealed up to 5 orders of magnitude higher release of Pb and Zn under acidic conditions (up to 38% and 63% of their total concentration, respectively), compared to the natural pH. In contrast, V exhibits relatively flat pH-dependent leaching patterns with only <1.6% of the total V leached. Using the slag re-processing costs by acidic (bio)leaching and the current metal prices, the recovery of V, being the most important critical metal in the Berg Aukas slags, seems to be non-economical (Ettler et al., 2020, Applied Geochemistry, DOI: 10.1016/j.apgeochem.2019.104473). This study was supported by the Czech Science Foundation project (GAČR 19-18513S).

How to cite: Ettler, V., Mihaljevic, M., Jarosikova, A., Culka, A., Kribek, B., Vanek, A., Penizek, V., Sracek, O., Mapani, B., and Kamona, F.: Mineralogy and environmental stability of vanadium-rich slags from the historical processing of Zn-Pb-V ores at Berg Aukas (Namibia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3672, https://doi.org/10.5194/egusphere-egu2020-3672, 2020.

As an accessory mineral in marine evaporites, polyhalite, K₂MgCa₂(SO₄)₄·2H₂O, coexists with halite (NaCl) in salt formations, which have been considered as potential repositories for permanent storage of high-level nuclear wastes. However, because of the heat generated by radioactive decays in the wastes, polyhalite may dehydrate, and the released water will dissolve its neighboring salt, potentially affecting the repository integrity. Thus, studying the thermal behavior of polyhalite is important. In this work, a polyhalite sample containing a small amount of halite was collected from the Salado formation at the WIPP site in Carlsbad, New Mexico. To characterize its thermal behavior, in situ high-temperature synchrotron X-ray diffraction was conducted from room temperature to 1066 K with the sample powders sealed in a silica-glass capillary. At ~506 K, polyhalite started to decompose into water vapor, anhydrite (CaSO₄) and two langbeinite-type phases, K₂Ca_xMg_2-x(SO₄)₃, with different Ca/Mg ratios. XRD peaks of the minor halite disappeared, presumably due to its dissolution by water vapor. With further increasing temperature, the two langbeinite solid solution phases displayed complex variations in crystallinity, composition and their molar ratio and then were combined into the single-phase triple salt, K₂CaMg(SO₄)₃, at ~919 K. Rietveld analyses of the XRD data allowed determination of structural parameters of polyhalite and its decomposed anhydrite and langbeinite phases as a function of temperature. From the results, the thermal expansion coefficients of these phases have been derived, and the structural mechanisms of their thermal behavior been discussed. In addition, to determine stability relations, standard enthalpies of formation of polyhalite from constituent oxides and elements were measured using high-temperature oxide-melt drop-solution calorimetry.

How to cite: Hongwu, X. and Xiaofeng, G.: Thermal behavior of polyhalite: A combined high-temperature synchrotron XRD and calorimetric study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11548, https://doi.org/10.5194/egusphere-egu2020-11548, 2020.

Mineralogy deals with the structure and related physical and chemical properties of materials in the geo- and biosphere. The knowledge about these minerals and their specific properties is increasingly used in various technical, medical and environmental fields [1]. One of the most impressive examples is garnet, a mineral which usually occurs in magmatic and metamorphic rocks. On the one hand, its chemistry is closely related to the chemistry of the host rock and, more importantly, its crystal structure reflects the pressure and temperature conditions during its formation. On the other hand, garnet is an important material for technical issues, e.g. the well-known Yttrium Aluminum Garnet (YAG)-Laser, and it finds now new applications, e.g. in the field of energy storage. Even though the chemical composition varies considerably all garnets have in common the same crystal structure. Li-oxide garnet with the composition Li₇La₃Zr₂O₁₂ (LLZO) is an excellent example to demonstrate the application of mineralogical knowledge in material science. Recently, these garnets have been identified as a promising material in the field of energy storage, as they can be used as solid state electrolyte in Li-based all solid state battery concepts. In Li-ion batteries, solid electrolytes are considered to replace polymer based electrolytes, which have disadvantages e.g. they are highly inflammable.

The presentation of garnet stands as an example for numerous other mineral groups which build the basis for their application in material science, such as spinel, perovskite, zeolite, sphalerite, chalcopyrite, kesterite, argyrodite, etc. In our contribution some of these mineral groups and their importance as basis for functional materials will be presented.

[1] S.-Heuss-Aßbichler, G. Amthauer, M. John (Eds.). Highlights in Applied Mineralogy. Dr Gruyter Berlin/Boston 2017, 344 pp.

How to cite: Amthauer, G. and Heuss-Aßbichler, S.: Minerals as basis for functional materials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21918, https://doi.org/10.5194/egusphere-egu2020-21918, 2020.

Solubility at 5 - 75 °C and thermodynamic parameters of halogenated mimetites Pb₅(AsO₄)₃X

B. Puzio^*, J. Sordyl and M. Manecki

AGH University of Science and Technology, Department of Mineralogy, Petrography and Geochemistry, Kraków, Poland (*correspondence: bpuzio@agh.edu.pl)

Mimetite Pb₅(AsO₄)₃Cl, apatite supergroup member, is a mineral of very low solubility. A very flexible structure of apatite allows for substitution of Cl by F, OH, Br, or even I. Due to lack of solubility constants K_sp, and other thermodynamic parameters (enthalpy of formation ΔH^o_f, specific heat capacity C^o_p entropy of formation S^o_f, Gibbs free energy of formation ΔG^o_f), it is unclear which of the investigated phases is the most soluble or most stable. Answers to these questions have multiple environmental and technological consequences.

The objective of this study was to run dissolution experiments of synthetic halogenated analogs of mimetite: Pb₅(AsO₄)₃F, Pb₅(AsO₄)₃OH, Pb₅(AsO₄)₃Cl, Pb₅(AsO₄)₃Br, and Pb₅(AsO₄)₃I, and determine their solubility at 5  - 75 °C which allows to calculate thermodynamic functions of state.

Pure phases have been successfully synthesized by precipitation from aqueous solutions. Batch dissolution and dissolution–recrystallization experiments were conducted for up to 9 months in triplicates at 5, 15, 25, 35, 45, 55, 65 and 75 °C, at pH = 3.5 (to avoid crystallization of secondary phases during dissolution), in a 0.05 M NH₄NO₃ background electrolyte. A plateau in the [Pb] evolution patterns was used to determine equilibrium. The ion activity products (IAP) of the mimetites were calculated based on the dissolution reaction:

Pb₅(AsO₄)₃X  < = > 5{Pb²⁺}  +  3{AsO₄^3-}  +  {X^-}

where the brackets denote activity and X means F^-, OH^-, Cl^-, Br^‑ or I^-. The new, experimentally determined values of logK_sp at 25 °C for mimetites are: -76.45±0.72; -77.71±0.38; -76.82±0.55; -76.13±0.54 and -72.48±0.45 respectively. The logK_sp of Pb₅(AsO₄)₃Cl determined here is in very good agreement with the logK_sp determined by Bajda, 2010 (the discrepancy equals to 0.62%). The nonlinear regression of logK_sp versus temperature allowed for calculation of ΔH^o_f, C^o_p, S^o_f and ΔG^o_f. The calculated ΔG^o_f for mimetites increases linearly with the increase of ionic radius of X^-. Thus, the most stable phase is F-mimetite while the least stable, in terms of Gibbs free energy of formation, is I-mimetite. The thermodynamic data reported in this study supplement existing databases used in geochemical modeling.

Financial support for the research was provided to B.P. by the Polish National Science Centre (NCN) grant No. 2017/27/N/ST10/00776.

Bajda T. (2010) Solubility of mimetite Pb₅(AsO₄)₃Cl at 5–55 °C. Environ. Chem. 7, 268–278.

How to cite: Puzio, B., Sordyl, J., and Manecki, M.: Solubility at 5 - 75 C and thermodynamic parameters of halogenated mimetites Pb5(AsO4)3X, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1241, https://doi.org/10.5194/egusphere-egu2020-1241, 2020.

The most industrial activities, that require organic dye materials for their applications, release a remarkable fraction of effluent water. As a result, serious environmental problems emerge due to the toxicity of many compounds which these industrial processes produce. The development of inexpensive and green methods for degradation of such organic dye laden water constitutes a landmark for the mitigation or even the elimination of the industrial sewage. Among many strategies, photocatalysis is regarded as the most viable one due to its usage of sunlight to decomposing organic pollutants.

The unlimited applications of nanocrystalline semiconductor materials in all sorts of technological fields, whether photochemical, biological, photovoltaic or photocatalysis have pushed to develop a new assortment of materials featuring novel properties for advanced applications. Semiconductor properties of stannite Cu₂FeSnS₄ make it a potential candidate for application in photocatalytic industry. In nature, it is a common sulfide mineral which is formed as a result of hydrothermal processes. Its crystal structure allows for numerous substitutions including replacement of Fe by Mn.

In this report, the photocatalytic activity of  Cu₂FeSnS₄ (CFTS) and Cu₂MnSnS₄ (CMTS) synthetic microspheres for degradation of environment polluting dye such as methylene blue (MB) has been explored under UV light illumination. The unique morphology of the as-synthesized nanomaterial is expected to play a major role in tailoring the optical and electrical properties for the possible photo-voltaic application. The photocatalytic activity shows the potential use of this material as an efficient photocatalyst for wastewater treatment.

Modified stannite was synthesized by hydrothermal method using reactions of metal salts and sulfur in hot ethylene glycol at presence PVP solution in an autoclave at 195^oC. The crystallinity, structural features, morphology and chemical composition were investigated using XRD, Raman, FTIR, and SEM - EDS. Synthetic Cu₂(Fe,Mn)SnS₄ solid solutions are composed of spheres 1 – 1.5 µm in size, with rough surface and concentric internal structure. The structure matches hexagonal stannite. The promising optoelectrical and photocatalytic properties of CFTS and CMTS microspheres make them a potential candidate for photovoltaics as well as for effective wastewater treatment, providing further study has to be carried out to improve specific properties.

Financial support for the research was provided by Polish Ministry of Higher Education grant No. DI2016 004946 under "Diamond Grant" program.

How to cite: Waluś, E. and Manecki, M.: Photocatalytic properties of Cu2(Fe,Mn)SnS4 microspheres synthesized via hydrothermal method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16847, https://doi.org/10.5194/egusphere-egu2020-16847, 2020.

The Nariin Sukhait mine is located in the southwest of Umnugobi province 50 kilometers from Mongolia's border with China at Shivee Khuren within the Nariin Sukhait deposit, which has relatively complex geological features. The most prominent feature relating to the Nariin Sukhait coal deposit is the arcuate, east-west trending Nariin Sukhait fault. The coal-bearing section, interpreted to be middle Jurassic in age, is exposed primarily in a window adjacent to this fault.

The chemical composition of whole indicates (variable composition, values of the ratio Th/U > 3.8-4.2, values Th/Sc 0.3-0.8, values LaN/YbN > 5 and values Eu/Eu* 0.6-0.9) indicates components derived from the active continental margin type. The low CIA values (50–60) indicate the absence or poor chemical weathering in the source area.

SEM-CL-imaging of sandstone quartz from Nariin sukhait show three types of quartz:  early Q1 cementation has gray to slightly grey luminescences, postdated compaction, and reduced intergranular porosity associated with illite formed during eogenesis. Q2 is characterized by dark luminescence overgrowths and is more voluminous in the thinly bedded sandstones than in the thickly bedded sandstones filling most of the remaining pore space during mesogenesis. Q3 was formed during the early telogenesis stage fully cementing the sandstones and the fractures were filled by hydrothermal chlorite and sulfides. Significant amounts of trace elements Al, Ti, Ca, K and Fe has been detected in quartz overgrowths. Al varies consistently between each cement with averages of 1324, 1523, and 1352 ppm for the Q1, Q2, and Q3 generations, respectively.

The geochemical, SEM-CL imaging and EPMA data results suggest a relatively igneous source, whit felsic composition. The sedimentary environment of the sandstone and argillite of these sedimentary rocks was the poor chemical weathering in the source area.

How to cite: Baigalmaa, N., Ogata, T., Jargal, L., Erdenetsogt, B.-O., and Erdenebayar, J.: Abstract title; Geology and geochemical characteristics of coal-bearing source rocks in Nariin sukhait deposit, southern Mongolia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17731, https://doi.org/10.5194/egusphere-egu2020-17731, 2020.

Carbonates appear to be one group of the main carbon-bearing minerals in the Earth’s interior. Inclusions of carbonates in diamonds of lower mantle origin support the assumption that they are present even in the Earth’s lower mantle. Although the carbonates’ phase diagrams have been intensively studied, their stability in presence of mantle silicates at deep mantle conditions (>25 GPa) remains unclear. Furthermore, the carbonate inclusions show a high REE enrichment. This raises questions on the distribution of trace elements between carbonates and silicates and on the possible role of carbonates as trace element carrier in the Earth’s mantle.

Numerous studies show that magnesite is likely to be the major solid carbonate carried by subduction into the Earth’s lower mantle. We investigated the stability of MgCO₃ in presence of mantle silicates and the Fe, Sr and La partitioning in high-pressure and high-temperature experiments. One set of experiments was conducted with multi-anvil presses at BGI, Bayreuth, at conditions ranging from 24 GPa to 30 GPa and 2000 K. The investigated reaction is between natural magnesite and (Mg,Fe)SiO₃-glasses doped with either Sr or La. Preliminary data from the multi-anvil press at 24 GPa and 2000K show the onset of carbonate melting which is consistent with the previous study of the melting curve in the enstatite-magnesite system [1]. Decomposition of MgCO₃ is not observed, in contrast to experiments using magnesite and SiO₂ as starting materials [2], suggesting that MgCO₃ is stable at these conditions in the presence of silicates phases. The silicate glass react to bridgmanite (Mg,Fe)SiO₃ as well as stishovite SiO₂ and magnesiowüstite (Mg,Fe)O. The Fe-Mg partitioning coefficient between bridgmanite and magnesite calculated in this study is ~2 and in agreement with previous experiments at similar conditions [3].
Laser-heated diamond anvil cell (LH-DAC) experiments were performed at University of Potsdam [4] at conditions 30 to 40 GPa and 1800 to 2300 K. The run products were characterized in-situ at high-pressure by XRD and XRF mapping at the P02.2 beamline at PETRA III. Our data show a transformation of the starting silicate glass into bridgmanite. We also observed stishovite and magnesiowüstite in the center of the hotspot where the temperature had reached >2000 K. In this case, the presence of magnesiowüstite might be the result of MgCO₃ decomposition at higher temperature. Additional TEM analyses on the post-mortem sample will allow us to further characterize the different phases present in the laser-heated hotspot.

[1] Thompson et al. (2014) Chemistry and mineralogy of the earth’s mantle. Experimental determination of melting in the systems enstatite-magnesite and magnesite-calcite from 15 to 80 GPa. American Mineralogist 99(8-9), 1544-1554.
[2] Drewitt et al. (2019) The fate of carbonate in oceanic crust subducted into Earth’s lower mantle. EPSL 511, 213-222
[3] Martinez, et al. (1998). Experimental investigation of silicate-carbonate system at high pressure and high temperature. Journal of Geophysical Research: Solid Earth, 103(B3), 5143-5163.
[4] Spiekermann et al. (2020). A portable on-axis laser heating system for near-90° X-ray spectroscopy: Application to ferropericlase and iron silicide. Journal of Synchrotron Radiation. (accepted)

How to cite: Libon, L., Spiekermann, G., Appel, K., Biedermann, N., Albers, C., Glazyrin, K., and Wilke, M.: Phase stabilities and Fe/Sr/La partitioning between magnesite (MgCO3) and mantle silicates at lower mantle conditions. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8879, https://doi.org/10.5194/egusphere-egu2020-8879, 2020.

Taenite, an iron-nickel alloy with 20-40 at% nickel, is a major constituent of iron meteorites that have been used to infer planetary core compositions. Many aspects of its magnetic properties are controversial, particularly near the Invar (“invariable”) composition around 36 at% nickel. Open questions include the conditions under which magnetism is lost, so to address this particular controversy, we undertook a combined magnetic remanence and Mössbauer study of synthetic taenite at high pressure. We synthesised polycrystalline iron-nickel alloy with 38 at% nickel, loaded the sample into a diamond anvil cell and collected Mössbauer spectra during decompression from 20 GPa. Our results show a clear loss of magnetism, but at pressures that differ considerably depending on the fitting model. The pressure obtained using the traditional approach involving a magnetic field distribution conflicts with results obtained from other methods, while a simple model based on magnetic field fluctuations gives results that are consistent with other data. Comparison of data from all methods provides insight that can be applied to planetary magnetism.

How to cite: McCammon, C., Wei, Q., and Gilder, S.: Magnetic properties of synthetic taenite at high pressure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5047, https://doi.org/10.5194/egusphere-egu2020-5047, 2020.

Peridotite xenoliths from the French Massif Central (FMC) have undergone a complex mantle metasomatic history by percolation of melts/fluids of variable composition. The two main points are: How do the minerals react with the percolating agent? What information can be extracted from these interactions? We present a detailed investigation of major/trace element and Li isotopic composition in fresh spinel lherzolites from the FMC (Devès area). We discuss 1) the variations in the amphibole composition with focus on the Ti behaviour; and 2) the distribution of Li and Li isotopic composition in co-existing phases.

1) Amphibole occurs as disseminated crystals generally developed at the expense of spinel ± cpx, fills cross-cutting veinlets, and forms bands with variably abundant relict spinels. Some samples are surrounded by an amphibole selvage (3 mm thick) with sharp contact with the peridotite. The amphibole composition varies from the selvage to the peridotite part. In the selvage outer part amphibole is a cumulative Cr-free Al-rich kaersutite, which shows a decrease in Ti and Al, while mg* increases towards the contact. The outer part of the selvage is the remnant of a dyke, while in the selvage inner part amphibole has reacted with the peridotite. Disseminated amphibole farther from the selvage-peridotite contact is a Cr-rich pargasite. The distinct Ti-Al trends observed in amphibole from the selvage (positive) and the peridotite (negative) are linked to distinct Ti-incorporation mechanisms in the octahedral sites of the amphibole structure: a) (Ti⁴⁺⁶Al³⁺₂₎ (M²⁺_-1 Si⁴⁺_-2) for amphibole in the selvage and b) (Ti⁴⁺ M²⁺) (⁶Al³⁺_-2) for disseminated amphibole in the peridotite. Mechanism (a) is likely to result from the crystallization of a percolating silicate melt in the mantle, whereas mechanism (b) results from hydration of the peridotite reacting with a percolating fluid emanating from the silicate melt.

  2) Li is preferentially incorporated into olivine compared to pyroxenes (1.1-1.4 ppm/0.2-0.9 ppm, average values) in the anhydrous xenoliths. Metasomatic processes increase Li abundances in all phases of the amphibole-bearing xenoliths, which deviate from the trend of equilibrium partitioning between phases, showing a preferential enrichment in cpx (2.4-5.4 ppm). In the hydrous xenoliths, the correlation between Li and REE elements in cpx and between Li in cpx and amp suggests that the carrier of the Li was a silicate melt. The  d⁷Li (‰) average values range (+5 to +15) in the anhydrous samples extend up to +35 in the amphibole-bearing xenoliths with large intra-grain variations (up to 18 ‰). These variations do not provide evidence for different sources but likely result from high temperature diffusion-related Li fractionation during metasomatism. The absence of correlation between the Li concentration and the isotopic composition in the anhydrous phases is linked to the pervasive character of the metasomatism, which allows strong Li exchanges as the melt interacts with the peridotite minerals. The preservation of the Li isotope kinetic fractionation in minerals and the sample isotopic heterogeneities implies that the Li exchange event occurs just before the extraction of the xenoliths from the mantle.

How to cite: Wagner, C., Deloule, E., Pascal, M.-L., and Boudouma, O.: Minerals as key markers of melt/fluid percolation in the lithospheric mantle from the French Massif Central, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9375, https://doi.org/10.5194/egusphere-egu2020-9375, 2020.

Opals and cryptocrystalline silica may be found in a very broad range of geological environments (Chauviré et al., 2017), systematically related to the availability of an aqueous fluid. Due to its conditions of formations, opal may contain abundant H₂O, CO₂ or both (Sodo et al., 2017), and the presence of these molecules may provide information of their genetic context. In this work we studied a series of samples from the volcanic region of Allumiere-Tolfa, north of Rome (Latium, Italy). This district has a Pliocene-Pleistocene age and is related to the Tuscan acid volcanism. It shows a very intense late-stage hydrothermal alteration that gave rise to two distinct ore basins: one to the south of the Allumiere town, consisting of sulfide (Pb, Fe, Zn, Hg) and Fe-oxide mineralizations, and a second, to the north, mainly consisting of alunite and kaolin. Both ore deposits were intensely exploited during the medieval to recent period. The hydrothermal alteration giving rise to the sulfate and clay deposits is also associated with a pervasive deposition, within the early volcanics, of opaline or microcrystalline silica, consisting of mineral replacements, veins and formation of agate druses. Although the sulphide-sulfate and clay products have been studied, due to their interest as georesources, and relevant petrological, geochemical and isotopic data can be found in the oldest literature (Lombardi and Sheppard, 1977), the silica mineralizations have never been addressed. We studied here a series of samples occurring as vein depositions or as banded crystallizations from different areas in the volcanic district. The samples were examined by using a combination of XRD, SEM-EDS and FTIR + Raman imaging. Opaline silica with different degree of order, from opal AN (hyalite) to opal A to opal CT, was identified. Some samples contain CO₂ besides H₂O/OH. The banded agates were found to consist of a layering of micro-crystalline and fibrous quartz (chalcedony) with different water contents, interbedded with moganite-rich layers; moganite, in particular was found to be associated to lower H₂O contents. The ¹⁸O and D/H isotopic data of Lombardi and Sheppard (1977) indicate temperatures around 120-100°C for the hydrothermal process responsible for the hydrothermal deposits, in close agreement with the typical range of T for the formation of opaline silica (Heaney, 1993). The ore forming process can be thus interpreted following the classical model of hydrothermal/metasomatic phenomena at low-depth, accompanied by extensive alteration of the pre-existing rocks, due to mixed magmatic/meteoric fluids, with the formation of kaolinite + alunite + sulfates + silica (Hedenquist et al., 2000).

References

Chauviré, B., Rondeau, B., Mangold, N. (2017) Eur. J. Miner. 29, 409-421

Heaney, P.J. (1993) Contrib. Mineral. Petrol., 115, 66-74.

Hedenquist, J.W., Arribas, A.R., Gonzalez-Urien, E. (2000) SEG Reviews, 13, 245-277.

Lombardi, G. and Sheppard, S.M.F., (1977) Clay Miner., 12, 147-161.

Sodo, A., Casanova Municchia, A., Barucca, S., Bellatreccia, F., Della Ventura, G., Butini, F., Ricci M.A. (2016) J. Raman Spec. 47, 1444-1451.

How to cite: Giancarlo, D. V., Camilla, N., Alessandra, C., Federico, L., Federico, G., and Benjamin, R.: Opaline and cryptocrystalline silica from the Tolfa volcanic region (Latium, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2716, https://doi.org/10.5194/egusphere-egu2020-2716, 2020.

Interface-coupled dissolution-precipitation (ICDP) reactions lead to the pseudomorphic replacement of minerals in a wide range of geological settings, exerting a significant impact in geochemical cycles (Putnis 2002). ICDP reactions play a major role in the diagenetic evolution of sedimentary rocks, specially of limestones and evaporites. Recent experimental works have studied ICDP reactions that lead to the formation of CaCO₃ pseudomorphs after anhydrite (CaSO₄), upon interaction of the latter phase with carbonated aqueous solutions. These pseudomorphs are highly porous polycrystalline aggregates that mainly consist of calcite (Roncal-Herrero et al. 2018; Altree-Williams et al. 2017). The formation of a large volume of interconnected microporosity that balances the molar volume loss associated to the anhydrite-calcite transformation as well as the specific arrangement of this microporosity, influenced by the existence of epitactic relationships between anhydrite and calcite, facilitate the progress of the ICDP reaction.

Here, we study the ICDP reaction that leads to the formation of hydroxyapatite (Ca₅(PO₄)₃(OH)) pseudomorphs after the interaction of anhydrite with phosphate-bearing aqueous solutions at temperatures 90 to180ºC during times that range from one hour to five weeks. The X-ray diffraction Rietveld analysis of the transformed samples indicates that the kinetics of the pseudomorphic transformation of anhydrite into hydroxyapatite strongly depends on temperature.  Thus, while at 180ºC a 100% transformation yield is attained in few hours, it takes five weeks of interaction at 90ºC. Scanning Electron Microscopy imagining of transformed samples shows the very good preservation of both, the original external shape and microtopographic features of anhydrite crystals. On cross-cut sections of partially replaced by hydroxyapatite anhydrite crystals we observe that the transformation advances from the surface inwards, with sharp separating the by replaced layer from the unreacted anhydrite core. Furthermore, this replaced layer is structured into a compact ~ 50 µm thick outer rim, which consists of coalescent small (~ 5 µm) hydroxyapatite crystals, and a progressively thickening inner region formed by hydroxyapatite columnar crystals in a stockade-like arrangement. This latter region is highly porous. We interpret these results taking into consideration the differences in solubility and molar volume between anhydrite and hydroxyapatite as well as the similarities/differences between the crystal structures of these phases. By comparing the characteristics of different ICDP reactions that involve anhydrite in sedimentary basins we derive implications about the diagenetic evolution of calcium sulphate evaporites. 

Altree-Williams, Alexander, et al. (2017). ACS Earth and Space Chemistry 1.2, 89-100.

Roncal-Herrero, Teresa, et al. (2017): American Mineralogist 102.6, 1270-1278.

Putnis A: (2002): Mineralogical Magazine 66.5, 689-708.

How to cite: Roza, A., Jiménez, A., and Fernández-Díaz, L.: Replacement of anhydrite by hydroxyapatite: kinetic and textural characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17842, https://doi.org/10.5194/egusphere-egu2020-17842, 2020.

In the state of Chihuahua, Mexico, a mine located in Naica is one of the most important Pb and Zn deposits in the world.  The region manifests several caves with varities of gypsum crystals (Ca4 H2O) known as selenite (the largest in the world so far, specimens up to eleven meters in length and one meter in thickness). The present abstract discusses the formation mechanism of these gigantic crystals. The review is based on the interpretations made by early workers (which includes one doctorate thesis and two Bulletins).

The interpretations were made on 19 samples of anhydrites that were collected at a depth of -345 meters. At this depth the early workers also found microscopically alternating dark dolomite and light bands of anhydrite. The dolomite-anhydrite association is generally associated with mineral recrystallization events. With the geological review carried out, two sources of sulfates were identified: one is La Virgen Formation and the second in the stratiform anhydrite of the Aurora Formation. Formation of hydrothermal anhydrite during the last stage of mineralization has been attributed to the formation of selenite mega crystals due to its dissolution. Hydrothermal minerals, such as hydrothermal anhydrite and sulphides. The most common sulfides in mineralization are galena, sphalerite, to a greater extent, pyrite and chalcopyrite. In case of weathering of these, they would generate their oxidation and result in the presence of sulfate.

According to the early workers the anhydrite was available in the late hydrothermal stage after mineralization of the mineral. The temperature during the growth of the crystals was maintained slightly below 58 ° C, the value in the solubility of the anhydrite is equal to that of the plaster. Giant selenite crystals grew from low salinity solutions with isotopic compositions compatible with the crystals formed by dissolving the anhydrite found in the mine. The kinetics of gypsum nucleation implies induction times longer than 1 m.y. for the typical temperature (54 ° C) and ~ 1 k.y. for low temperature episodes (up to 47 ° C). This mechanism provides a super saturation level that is not only small and maintained over time but is also virtually free of fluctuations (even small amplitudes).

Recent contributions have speculated that there are other caves with similar selenite or even with larger crystals exist among the tangle of underground galleries in the area of ​​the Naica mine.

How to cite: Gutiérrez Gutiérrez, A. L., Puy y Alquiza, M. D. J., and Kshirsagar, P.: Review on the formation and geochemistry of the mega crystals of Naica, México., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20593, https://doi.org/10.5194/egusphere-egu2020-20593, 2020.

Within the Samail Ophiolite, Oman, there are intervals of listvenite outcrops between layers of serpentinite zones above the basal thrust zone, atop the metamorphic sole. Near the base of the ophiolite mantle section, some peridotites underwent 100% carbonation from metasomatic introduction of CO₂-bearing fluids (~100°C) to form listvenites during the time of emplacement (97 ± 29 Ma, Falk and Kelemen, 2015). The carbonate rocks comprise mostly magnesite and/or dolomite, quartz, spinel, and Fe-(hydr)oxides; with carbonates as the sole Mg-minerals and quartz as the only silicate phase. The aim of this study is to chemically and petrographically investigate the Fe-bearing minerals within the fluid-altered mantle rocks in drill core samples from hole BT1B of the ICDP Oman Drilling Project. We investigated the quantities of Fe-oxide/hydroxide phases through a series of chemical extractions (Poulton and Canfield, 2005) via atomic absorption spectroscopy in addition to optical microscope/ SEM/EDS analysis. Sequential chemical extractions are useful for recognizing iron pools based on the minerology. Extractions preformed at room temperature show varying proportions of carbonate-associated Fe (sodium acetate), reducible oxides (citrate-dithionite), magnetite (oxalate), and HCl-extractable Fe(II). The amount of Fe in carbonates based on sodium acetate extraction ranges from 17-54% of the overall extracted iron (12-28 ‰) in the samples. The same extraction performed at 50°C for twice as long resulted in higher proportions of carbonate-associated Fe extracted with a range of 44-85% of the total extracted iron (15-35 ‰). Easily reducible iron quantities from a diluted HCl solution extraction display the lowest overall Fe fractions at 6.2-25% following the room temperature acetate and 2.6-6.2% after the 50°C acetate extraction. Reducible oxides extracted by dithionite were wide ranging (8.3-49%) as a proportion of the overall extracted iron, with similar results following the 50°C acetate step (5.3-48%). Oxalate extraction succeeding the room temperature acetate revealed magnetite proportions of 13-28%, while the increased temperature and time in the first step (acetate extraction) resulted in significantly lower proportions of Fe extracted by oxalate (3.1-10%). Falk and Kelemen (2015) suggest significant amounts of poorly crystalline Fe-phases or amorphous oxides within the listvenites not detected by X-ray diffraction, but we do not see evidence of this based on the relatively small HCl fractions. Further examination of the total elemental compositions of the individual solutions and electron microprobe analyses will reveal more details about the Fe-minerals dissolved in each extract and weather they represent separate Fe-oxide/hydroxide phases.    

Falk, E. S., & Kelemen, P. B. (2015). Geochemistry and petrology of listvenite in the Samail ophiolite, Sultanate of Oman: Complete carbonation of peridotite during ophiolite emplacement. Geochimica et Cosmochimica Acta, 160, 70-90.

Poulton, S. W., & Canfield, D. E. (2005). Development of a sequential extraction procedure for iron: implications for iron partitioning in continentally derived particulates. Chemical Geology, 214(3-4), 209-221.

How to cite: Severin, Z., Till, J. L., and Phase 1 Science Party, O. D. P.: Sequential geochemical extractions and mineralogy of Fe-bearing minerals of mantle rocks in the Samail Ophiolite, Oman , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20945, https://doi.org/10.5194/egusphere-egu2020-20945, 2020.

Prehistoric slags (Late Bronze Age to early Iron Age, ca. 1300 – 1000 BC) from copper metallurgy were sampled at the archaeological site no.2 in Timna, Israel. A classical combination of analytical methods for this kind of samples (optical and scanning electron microscopy, X-ray diffraction analysis, and electron microprobe analysis) was complemented with Raman microspectroscopy.

Raman microspectroscopy is a strong tool for phase or mineral identification in general, and when coupled with the methods for determination of the chemical composition such as electron probe microanalysis, it provides a comprehensive phase description of the sample. Slags are generally composed of both crystalline and amorphous glass-like phases and include metals, intermetallic compounds and alloys, sulfides, oxides, silicates, silicate glasses and carbonaceous fuel residues. With the exception of pure metals and their respective alloys, all these phases can be theoretically analyzed using Raman microspectroscopy. However, laser-induced fluorescence can become a major issue, owing to a presence of many different metallic elements. Selection of appropriate laser excitation wavelength can reduce the amount of fluorescence. Using Raman microspectroscopy it was possible to identify major silicate phases such as olivine (fayalite) and clinopyroxene (hedenbergite). Using this technique the crystallinity of iron oxides was identified and magnetite and hematite were differentiated. Despite the fact that Cu sulphides have simple Raman spectra with only few diagnostic bands, digenite and chalcopyrite were confirmed in the Timna slags. This study was supported by the Czech Science Foundation project (GAČR 19-18513S). The sampling campaign was carried out in the framework of Erasmus+ Mobility exchange programme between Charles University, Prague, Czech Republic (CUNI) and Hebrew University in Jerusalem, Israel (HUJI).   

How to cite: Culka, A., Natherová, V., Jehlička, J., and Ettler, V.: Application of Raman spectroscopy for understanding the mineralogical composition of ancient copper slags (Timna, Israel), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7209, https://doi.org/10.5194/egusphere-egu2020-7209, 2020.

On the role of U/ThO₈ polyhedral distortions in controlling the high-pressure zircon→reidite type transition in U_xTh_1-xO₄

Sudip Kumar Mondal^1,2, Pratik Kr Das^2,3, Nibir Mandal² and Ashok Arya⁴
1 Department of Physics, Jadavpur University, Kolkata 700032, India
2 Faculty of Science, High Pressure and Temperature Laboratory, Jadavpur University, Kolkata 700032, India
3 The Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, N-0315, Norway
4 Material Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Coffinite (USiO₄) and thorite (ThSiO₄) are conspicuous radiogenic silicates in the geonomy. They form U_1-xTh_xSiO₄ (uranothorite) solid solutions in zircon-type phase. Investigating the phase-evolution of these minerals is of utmost significance in realizing their applicability in the front-as well as at the back-end of nuclear industries and also from geological perspective, such as geochronology. We carried out a systematic study of zircon- to reidite-type (tetragonal I41/amd to I41/a) structural transitions of U_1-xTh_xSiO₄ solid solution, and investigated their mechanical behaviour. Our ab-initio calculations revealed a unique interconnection of phase transition pressure (p_t) with the change in U-Th concentration in the solid solution. The transition pressure is found to be minimum (6.82 GPa) for x = 0.5 whereas for the endmembers coffinite and thorite p_t’s are 8.52 and 8.68 GPa, respectively. We developed a novel method to estimate the longitudinal and angular distortions of the highly irregular U/ThO₈-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: δ (longitudinal distortions) and σ² (angular distortions) to quantify the polyhedral distortions. A detailed analysis of the snub-disphenoidal distortions demonstrates that the difference in angular distortion of UO₈ and ThO₈ polyhedra (i.e. σ_U² and σ_Th²) between zircon- and  reidite-type phases becomes minimum when U and Th percentage are equal, leading to the structural phase transition at the minimum hydrostatic pressure for the unique chemical composition: U_0.5Th_0.5SiO₄. Our result is also substantiated by the minimum compressibility observed for the zircon-type U_0.5Th_0.5SiO₄. It is worthwhile to note that the distortions parameters, δ and σ² are defined without any attribute to external parameters. They are also independent to the elements occupying the polyhedra. Thus, we propose that these parameters: δ and σ² can also be used to calculate the distortions of similar AB₈-type snub-disphenoids observed in zircon-, reidite-, fergusonite- and wolframite-type mineral phases.

How to cite: Mondal, S. K.: On the role of U/ThO8 polyhedral distortions in controlling the high-pressure zircon→reidite type transition in UxTh1-xO4, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15299, https://doi.org/10.5194/egusphere-egu2020-15299, 2020.

Five Pb-As bearing apatites Pb₅(AsO₄)₃X, where X stands for F, Cl, Br, I and OH, were synthesized by precipitation from an aqueous solution and analyzed with powder X-ray diffraction and Raman spectroscopy. High-resolution high-quality powder diffraction data were obtained at the 11-BM beamline of the Advanced Photon Source at Argonne National Laboratory, Argonne, IL, USA and the structure Rietveld refinements of mimetites with different halogenic substitutions were provided.

Mimetites precipitated from aqueous solutions crystallize in hexagonal crystal system (space group P6₃/m). The lattice parameters a and c, as well as the volume of the unit cell, increase with the increasing ionic radius of halogen substitution: a = 10.081Å, 10.247Å, 10.310Å, 10.351Å; c = 7.426Å, 7.442Å, 7.473Å, 7.528Å; V = 653.515ų; 676.716ų, 688.019ų, 698.402ų for Pb₅(AsO₄)₃F, Pb₅(AsO₄)₃Cl, Pb₅(AsO₄)₃Br, Pb₅(AsO₄)₃I, respectively. The OH^- ion has similar effect on the lattice parameter a = 10.187Å but much stronger effect on parameter c = 7.525Å and overall volume V = 676.274ų than halogens.

Systematic linear relations between the unit cell parameters and the Pb(2) – Pb(2) distance as well as the size of AsO₄ tetrahedra are observed. The distance between the Pb(2) – Pb(2) increases (from 4.093Å for Pb₅(AsO₄)₃F to 4.674Å for Pb₅(AsO₄)₃I) indicating the systematic increase in the radius of hexagonal channels occupied by halogens and OH. In contrast, the size (volume) of AsO₄ tetrahedra decreases (from 2.474ų for Pb₅(AsO₄)₃F to 2.025ų for Pb₅(AsO₄)₃I) with the substitution and with the increasing size of the unit cell.

These structural effects affect the Raman spectra of substituted mimetites resulting in systematic shift of the position of the bands. The most sensitive to isomorphic substitutions are symmetric stretching vibrations ν₁ of the As-O bond in [AsO₄] tetrahedra, position of which range from 813 cm^-1 for Pb₅(AsO₄)₃F to 810 cm^-1 for Pb₅(AsO₄)₃I. This, however, is not due to the increase in the mass of substituted anion. The position of the bands is directly affected by the increasing length of As-O bond: the increase in As-O bond length shifts the position of v₁ vibrations towards lower wavenumbers.

Financial support for the research was provided by the Polish National Science Centre (NCN) grant No. 2017/27/N/ST10/00776.

How to cite: Sordyl, J., Puzio, B., Borkiewicz, O., and Manecki, M.: X-ray diffraction and Raman spectroscopy study of F, Cl, Br, I and OH substitutions in lead arsenate apatites (mimetites) Pb5(AsO4)3X, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16686, https://doi.org/10.5194/egusphere-egu2020-16686, 2020.

Low temperature aqueous synthesis of Rare Earth Element (REE) carbonates show extensive variability in the resulting minerals. Precipitated mineral phases and crystallisation rates vary depending, in part, on the REE used. Indeed, much of the work to date on REE aqueous geochemistry focuses on the individual behaviour of discrete REEs. 

We present a low temperature aqueous geochemical investigation of REE carbonate crystallisation pathways, which takes into consideration the influence of multiple REEs in solution. This serves to mimic more realistic conditions that are found in natural geological settings propitious to REE mineralisation. Our experiments focus on the behaviour of La, Ce, Nd, Dy carbonates at 30^oC.

Concordant with previous studies, our results suggest that the crystallisation process of REE carbonates begins with the formation of an amorphous phase that transitions into a crystalline phase after a lag time that depends on the element and the proportions in the mixture. 

This lag time is REE specific and is shorter for lighter REE compared to their heavier counterparts. In particular, the presence of another REE in the system affects the crystallisation timings and the morphology of the resulting crystals. For example, samples of mixed La/Nd carbonates begin their phase transition at lag times in between that of the two end-members (i.e. La and Nd) carbonate compositions. Furthermore, we find that the resulting growth rates and crystal habits are unique to the ratio of the REE mixture, with the underlying ionic potential of the mixture linked to the growth rates. In addition, observations throughout the crystallisation process also show that growth begins with flocculation of nanoparticles followed by crystal growth via Ostwald ripening.

REEs are sought after due to their unique properties and are integral to modern technologies such as lasers, catalytic converters, batteries, electro-magnets and wind turbines. Considering how the crystallisation behaviour with REE mixtures differs from that of discrete REE in solution, this work gives insights into the fundamental chemistry of REEs in aqueous solutions - relevant for studies of REE mineralisation and materials processing. 

How to cite: Price, D., Butler, I., Ngwenya, B., and Kirstein, L.: Crystallisation of REE carbonates from aqueous solutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19019, https://doi.org/10.5194/egusphere-egu2020-19019, 2020.

Manganese oxy-hydroxides are ubiquitous in soils and sediments where they occur as fine-grained aggregates and coatings on other mineral particles. Owing to large surface areas these minerals are very reactive and have long been known for their enormous adsorption capacity for Co and Ni. This is now of great relevance for Co and Ni extraction from lateritic ores, where Co and Ni bearing Mn oxy-hydroxides can be found in the most highly oxidised parts of weathering profiles. Detection and characterisation of these minerals however is very challenging, as they present with low bulk concentrations often within mineralogically complex, fine-grained mixtures of poorly crystalline phases.

In this study we identified and characterised a number of Mn oxy-hydroxides in samples from a variety of laterite deposits: Shevchenko (Khazakstan), Acoje (Philippines), Nkamouna (Cameroon), Piauí (Brazil), and Penamax and Tiebaghi (New Caledonia). Bulk chemical and mineralogical characterisation was undertaken with ICP-OES/MS and XRD, followed by spatially resolved imaging at the micron scale using µXRD, EPMA, SEM, µRaman and synchrotron-based µXRF. The chemical state and local environment of Co and Ni were determined using X ray spectroscopy (μXANES and μEXAFS).

The total concentrations of Co and Ni in the bulk samples ranged from 420 mg/kg (Piaui) to 1.245 wt% (Tiebaghi) and 0.5 wt% (Nkamouna) to 1.74 wt% (Piaui) respectively. The low abundance in addition to the poorly crystalline nature of the manganese oxy-hydroxides made them undetectable with XRD with the exception of the Cameroon and New Caledonia samples, where lithiophorite was detected. Following spatially resolved electron microscopy, Mn-rich grains were localised in the bulk samples and further studied with µRaman spectroscopy, µXRD and EPMA. In Shevchenko, asbolane was identified containing Co with concentrations varying from 0.25 to 12.4 wt% (average 6.3%) and Ni from 3.2 to 16.9 wt% (average 11.7 wt%) In samples from Nkamouna Co varied widely from below 1 wt% in romanechite and pyrolusite, average of 5.5% in lithiophorite and up to 21 wt% in lithiophorite-asbolane intermediates. In the Piaui samples asbolane and asbolane-lithiophorite intermediates were identified and found to carry from 0.35 to 14.2 wt% (average 3.4 wt%) of Co and 0.35 to 18.8 wt% of Ni (average 8.5 wt%). In addition, unusually Co-rich barium manganese oxide was found with Co varying from 1.5 to 10.4 wt% (average 3.0 wt%) and Ni from 0.3 to 1.98 wt% (average 0.6 wt%). In the New Caledonia samples asbolane-lithiophorite intermediates were identified with 1.1 to 9.6 wt% Co (average 5.3 wt%) and 1.9-11.4 wt % of Ni (average 6.9 wt%).

X-ray spectroscopy revealed that Co is bound in a range of Mn oxide minerals as Co³⁺ while Ni is present as Ni²⁺. The crystal chemistry of Co was very similar in the various minerals with Co structurally incorporated by substituting Mn in the manganiferous layer. The crystal chemistry of Ni was more variable. In asbolane Ni was found to build Ni(OH)₂ layers, in lithiophorite it was structurally incorporated in the Al(OH)₃ layer while in the lithiophorite-asbolane intermediates it was found partly in the Al(OH)₃ layer and partly adsorbed.

How to cite: Dybowska, A., Schofield, P., Mosselmans, F., and Herrington, R.: Diversity of manganese oxy-hydroxides and their sorption capacity for Co and Ni in lateritic deposits worldwide , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20147, https://doi.org/10.5194/egusphere-egu2020-20147, 2020.