Dominik Talla

The long-known presence of a sharp OH absorption band in the tetragonal fluoride mineral sellaite, MgF₂, inspired us to conduct a detailed study of the OH incorporation modes into this IR-transparent (where IR stands for Infrared) material as well as to search for hydrogen traces in two other IR-translucent halides—villiaumite (NaF) and fluorite (CaF₂). Among these three phases, sellaite is the only one to incorporate ‘intrinsic’ OH groups, most commonly as O–H∙∙∙F defects oriented nearly perpendicular to the c-axis along the shortest edge of the constituent MgF₆ polyhedra, in analogy with the isostructural mineral rutile, TiO₂. Another defect type, seen only scarcely in untreated natural material, develops when subjecting sellaite to temperatures above 900 °C. It involves an O–H∙∙∙O cluster along the 2.802 Å edge of the original MgF₆ dipyramid, as fluorine atoms are progressively expelled from the structure, being replaced by O^2- anions. This is corroborated by the appearance of spectral absorption features typical for brucite (Mg(OH)₂) and ultimately periclase (MgO), the presence of which could be proven via powder diffraction of the heat-treated material. Except for a ‘dubious’ peak most probably caused by included phases, neither villiaumite (NaF) nor fluorite (CaF₂) showed any presence of ‘intrinsic’ OH defects. They do however decompose along a similar route into the respective oxide and hydroxide phases at high temperature. This thermal decomposition of the studied halide phases is accompanied by the emission of gaseous (HF)_n species at temperatures well below their established melting point - a subject which seems to be quite overlooked.

The long-known presence of a sharp OH absorption band in the tetragonal fluoride mineral sellaite, MgF₂, inspired us to conduct a detailed study of the OH incorporation modes into this IR-transparent material as well as to search for hydrogen traces in two other IR-translucent halides – villiaumite (NaF) and fluorite (CaF₂). Among these three phases, sellaite is the only one to incorporate ‘intrinsic’ OH groups, most commonly as O–H∙∙∙F defects oriented nearly perpendicular to the c-axis along the shortest edge of the constituent MgF₆ polyhedra, in analogy with the isostructural mineral rutile, TiO₂. Another defect type, seen only scarcely in untreated natural material, develops when subjecting sellaite to temperatures above 900°C. It involves an O–H∙∙∙O cluster along the 2.802 Å edge of the original MgF₆ dipyramid, as fluorine atoms are progressively expelled from the structure, being replaced by O^2- anions. This is corroborated by the appearance of spectral absorption features typical for brucite (Mg(OH)₂) and ultimately periclase (MgO), the presence of which could be proven via powder diffraction of the heat-treated material. Except for a ‘dubious’ peak most probably caused by included phases, neither villiaumite (NaF) nor fluorite (CaF₂) showed any presence of ‘intrinsic’ OH defects. They do however decompose along a similar route into the respective oxide and hydroxide phases at high temperature. This thermal decomposition of the studied halide phases is accompanied by the emission of gaseous (HF)_n species at temperatures well below their established melting point - a subject which seems to be quite overlooked.

The technique of crystal growth in gels has nowadays become somewhat neglected in the scope of earth sciences, to the disadvantage of the experimental mineralogist. Even preparing an inorganic silica gel can prove a challenge to many, let alone successfully configure the entire experiment. Based not only on previous literature but also on our extensive experience, crystals of many substances, including supergene minerals as reference standards, can be successfully grown in gel, aiding in accomplishing various research goals in earth sciences. Instead of providing the reader with an overwhelming compendium of historical information and theoretical knowledge of the subject which can be found elsewhere, we presented herein a comprehensive, practically oriented guide to the understanding and successful use of the technique of crystal growth in gels, mentioning, in addition to the general principle, the numerous pitfalls which we encountered during our own use of the method, and the ways to overcome them. Despite that the procedure is nowadays used mainly for the laboratory synthesis of organic or metal-organic compounds, we believe it to be a valuable asset to any mineralogist, and often, the only way to obtain inorganic reference material of a particular mineral of interest.

Monohydrate sulfate kieserites (M²⁺SO₄·H₂O) and their solid solutions are essential constituents on the surface of Mars and most likely also on Galilean icy moons in our solar system. Phase stabilities of end-member representatives (M²⁺= Mg, Fe, Co, Ni) have been examined crystallographically using single-crystal X-ray diffraction at 1 bar and temperatures down to 15 K, by means of applying open He cryojet techniques at in-house laboratory instrumentation. All four representative phases show a comparable, highly anisotropic thermal expansion behavior with a remarkable negative thermal expansion along the monoclinicbaxis and a pronounced anisotropic expansion perpendicular to it. The lattice changes down to 15 K correspond to an `inverse thermal pressure' of approximately 0.7 GPa, which is far below the critical pressures of transition under hydrostatic compression (P_c≥ 2.40 GPa). Consequently, no equivalent structural phase transition was observed for any compound, and neither dehydration nor rearrangements of the hydrogen bonding schemes have been observed. TheM²⁺SO₄·H₂O (M²⁺= Mg, Fe, Co, Ni) end-member phases preserve the kieserite-typeC2/csymmetry; hydrogen bonds and other structural details were found to vary smoothly down to the lowest experimental temperature. These findings serve as an important basis for the assignment of sulfate-related signals in remote-sensing data obtained from orbiters at celestial bodies, as well as for thermodynamic considerations and modeling of properties of kieserite-type sulfate monohydrates relevant to extraterrestrial sulfate associations at very low temperatures.

The investigation of the presence and role of sulfates in our solar system receives growing attention because these compounds play a crucial role in the water budget of planets such as Mars and significantly influence melting equilibria on the icy moons of Saturn and Jupiter, leading to the formation of subsurface oceans and even cryovolcanism. Despite the dominant presence of higher sulfate hydrates such as epsomite, MgSO4·7H2O, and mirabilite, Na2SO4·10H2O, on these moons' surfaces, it is not excluded that lower-hydrated sulfates, such as kieserite, MgSO4·H2O, are also present, forming from higher hydrates under pressures relevant to the mantle of the icy moons. Given the composition of the soluble fraction in C1 and C2 chondritic meteorites, which are high in Ni content and also considered to represent the composition of the rocky cores of the Jovian icy moons, the actual compositions of potentially present monohydrate sulfates likely lie at intermediate values along the solid-solution series between kieserite and transition-metal kieserite-group end-members, incorporating Ni in particular. Moderate Ni contents are also probable in kieserite on Mars due to the planet's long-term accumulation of meteoritic nickel, although likely to a much lesser extent than Fe.Structural and spectroscopic differences between the pure Mg- and Ni-end-members have been previously documented in the literature, but no detailed crystal chemical and spectroscopic investigation along the Mg-Ni solid solution has been done yet. The present work proves the existence of a continuous (Mg,Ni)SO4·H2O solid-solution series for the first time. It provides a detailed insight into the changes in lattice parameters, structural details, and positions of prominent bands in infrared (transmission, attenuated total reflectance, diffuse reflectance) and Raman spectra in synthetic samples as the Ni/Mg ratio progresses, at both ambient as well as low temperatures relevant for the icy moons and Mars. UV-Vis-NIR crystal field spectra of the Ni end-member also help to elucidate the influence of Ni2+-related bands on the overtone- and combination modes.The (Mg,Ni)SO4·H2O solid-solution series shows Vegard-type behavior, i.e., lattice parameters as well as spectral band positions, change along linear trends with increasing Ni content. Infrared spectra reveal significant changes in the wavenumber positions of prominent bands, depending on the Ni/Mg ratio. We show that the temperature during measurement also has an influence on band position, mainly in the case of H2O-related bands. The changes observed for several absorption features in the IR spectra enable rough estimation of the Ni/Mg ratio in the monohydrate sulfate, which is applicable to present and future remote sensing data, as well as in situ measurements on Mars or the icy moons. The spectral features most diagnostic of composition are the vibrational stretching modes of the H2O molecule and a band unique to kieserite-group compounds at around 900 cm–1 in the IR spectra, as well as the pronounced ν3 and ν1 sulfate stretching modes visible in Raman spectra.

Single crystals of synthetic nickel sulfate monohydrate, α-NiSO₄·H₂O (space-group symmetry C2/c at ambient conditions), were subject to high-pressure behavior investigations in a diamond-anvil cell up to 10.8 GPa. By means of subtle spectral changes in Raman spectra recorded at 298 K on isothermal compression, two discontinuities were identified at 2.47(1) and 6.5(5) GPa. Both transitions turn out to be apparently second order in character, as deduced from the continuous evolution of unit-cell volumes determined from single-crystal X-ray diffraction. The first structural transition from α- to β-NiSO₄·H₂O is an obvious ferroelastic C2/c–P1̅ transition. It is purely displacive from a structural point of view, accompanied by symmetry changes in the hydrogen-bonding scheme. The second β- to γ-NiSO₄·H₂O transition, further splitting the O2 (hydrogen bridge acceptor) position and violating the P1̅ space-group symmetry, is also evident from the splitting of individual bands in the Raman spectra. It can be attributed to symmetry reduction through local violation of local centrosymmetry. Lattice elasticities were obtained by fitting second-order Birch–Murnaghan equations of state to the p-V data points yielding the following zero-pressure bulk moduli values: K₀ = 63.4 ± 1.0 GPa for α-NiSO₄·H₂O, K₀ = 61.3 ± 1.9 GPa for β-NiSO₄·H₂O, and K₀ = 68.8 ± 2.5 GPa for γ-NiSO₄·H₂O.

The investigation of hydrous sulfate deposits and sulfate-cemented soils on the surface of Mars is one of the important topics in the recent scientific endeavor to retrieve detailed knowledge about the planetary water budget and surface weathering processes on our neighbor planet. Orbital visible/near-IR spectra of the surface of Mars indicate kieserite, MgSO4·H2O, as a dominant sulfate species at lower latitudes. However, given the Fe-rich composition of the martian surface, it is very probable that its actual composition lies at an intermediate value along the solid-solution series between the kieserite and szomolnokite (FeSO4·H2O) end-members. Despite the known existence of significant lattice parameter changes and spectral band position shifts between the two pure end-members, no detailed crystal chemical and spectroscopic investigation along the entire kieserite–szomolnokite solid solution range has been done yet.The present work proves for the first time the existence of a continuous kieserite–szomolnokite solid-solution series and provides detailed insight into the changes in lattice parameters, structural details, and positions of prominent bands in FTIR (5200–400 cm–1) and Raman (4000–100 cm–1) spectra in synthetic samples as the Fe/Mg ratio progresses, at both ambient as well as Mars-relevant lower temperatures. Additionally, an UV-Vis-NIR (29 000–3500 cm–1) crystal field spectrum of szomolnokite is presented to elucidate the influence of Fe2+-related bands on the overtone- and combination mode region.The kieserite–szomolnokite solid-solution series established in this work shows Vegard-type behavior, i.e., lattice parameters as well as spectral band positions change along linear trends. The detailed knowledge of these trends enables semi-quantitative estimations of the Fe/Mg ratio that can be applied to interpret martian monohydrated sulfates in data from remote sensing missions on a global scale as well as from in situ rover measurements. Given the knowledge of the surface temperature during spectral measurements, the established temperature behavior allows quantitative conclusions concerning the Fe/Mg ratio. Our understanding of the kieserite–szomolnokite solid-solution series will be well applicable to the Mars 2020 and ExoMars 2020 rover missions that will focus on near IR (0.9 to 3.5 μm) and, for the first time on Mars, Raman spectroscopy.