Hydrous Environments Research Articles

On the Attributes of Mineral Paragenetic Modes

Abstract The mineral kingdom has experienced dramatic increases in diversity and complexity through billions of years of planetary evolution as a consequence of a sequence of physical, chemical, and biological processes. Each new formational environment, or “mineral paragenetic mode,” has its own characteristic attributes, including the stage of mineral evolution and geological age, ranges of T, P, duration of formation events, and other environmental influences on mineral formation. Furthermore, the minerals associated with each paragenetic mode have a wide range of average properties, including hardness, density, and chemical and structural complexity. A survey of attributes of 57 mineral paragenetic modes representing the full range of mineral-forming processes reveals systematic trends, including: (1) minerals documented from older paragenetic processes are systematically harder on average than those from more recent processes; (2) minerals from paragenetic modes formed at lower T (notably <500 K) display greater average structural complexity than those formed at high T (especially >1000 K); and (3) minerals from paragenetic modes that display greater average chemical complexity are systematically less dense than those from modes with lesser average chemical complexity. In addition, minerals formed in anhydrous environments and/or by abiotic processes are, on average, significantly denser and harder than those formed in hydrous environments and/or by biotic processes.

Dynamics of Proton Transfer on Boehmite (010) Surface in Anhydrous and Hydrous Environments

Boehmite (010) surface has a high density of ordered hydroxyl groups but no intrinsic hydrogen bonds. This represents a special case for which proton‐transfer mechanism is unknown and different from those along a continuous hydrogen‐bond network. In this study, probable pathways for proton transfer on boehmite (010) surface in both anhydrous and hydrous environments are explored, and energy barriers of these processes are calculated using the climbing‐image nudged elastic band method based on density‐functional theory. The results reveal that under anhydrous conditions, long‐distance proton transfer is unlikely on clean (010) surface, but excess protons are mobile with a low proton‐transfer energy barrier of 31.4 kJ mol−1. Subsurface Zn dopants (for charge balance with excess protons) reduce the energy barrier to 21.1 kJ mol−1. In contrast, water‐mediated proton‐transfer processes only need to overcome a low‐energy barrier of 10.0 kJ mol−1. The low‐energy barriers suggest boehmite (010) surface can conduct protons, particularly in hydrous environments, making boehmite a promising filler material for proton exchange membranes.

RAMAN SPECTRAL ANALYSIS FROM SHERLOC ONBOARD MARS 2020

Abstract. The Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) is one of the instruments mounted on the Perseverance rover’s robotic arm. SHERLOC focuses on searching for mineral species and organics that may contain spectral signatures of hydrous environments and past microbial life. It measures the Raman scattering in a narrow wavelength range (246–357 nm) with a significantly stronger fluorescence around 274–355 nm. Here the 40 measurements of Raman spectra from three detectors of SHERLOC, captured on Sol 4, were analyzed along with Matscam-Z images. Principal component analysis was applied to check the mineralogical diversity. The results include fluorescence signatures with maxima at 276 nm and Raman peaks similar to amorphous silicates.

Kinetic and Thermodynamic Factors Influencing Palladium Nanoparticle Redispersion into Mononuclear Pd(II) Cations in Zeolite Supports

Palladium cations and nanoparticles supported on oxides and zeolites are used as catalysts and adsorbents for a wide range of chemical reactions of practical importance, with various and distinct active site requirements. Consequently, sintering and redispersion processes that interconvert site-isolated Pd cations and agglomerated nanoparticles underpin catalyst activation and deactivation phenomena, yet the influence of Pd nanoparticle size distribution and gas conditions on the thermodynamic and kinetic factors influencing such interconversion is imprecisely understood. Here, we prepare Pd nanoparticles of different particle size and distribution (normal, log–normal) supported on high-symmetry crystalline zeolites to access well-defined materials whose Pd site distributions can be quantitatively characterized by experiments and modeled accurately by theory. Ab initio thermodynamic modeling and isothermal (593–973 K) wet and dry air treatments of Pd-zeolites reveal that the widely observed deactivation in low-temperature hydrous environments reflects site interconversion thermodynamics that favor the agglomeration of isolated Pd cations into nanoparticles. Under high-temperature anhydrous conditions, experimental kinetic measurements and kinetic Monte Carlo simulations evince the preeminence of kinetic factors on Pd nanoparticle redispersion into cations, which proceeds at rates that are strongly influenced by the initial Pd particle size distribution and via a substrate diffusion-mediated Ostwald ripening process whereby Pd monomers are captured in an atom trapping process at anionic exchange sites (framework Al) in the zeolite support. These findings resolve longstanding questions regarding the roles of H2O and support interactions in Pd redispersion processes and identify strategies to enhance or suppress Pd site interconversion by modifying oxide supports and gas conditions.

Comparative Compressibility of Smectite Group under Anhydrous and Hydrous Environments.

High-pressure synchrotron X-ray powder diffraction studies of smectite group minerals (beidellite, montmorillonite, and nontronite) reveal comparative volumetric changes in the presence of different fluids, as pressure transmitting media (PTM) of silicone oil and distilled water for anhydrous and hydrous environments at room temperature. Using silicone oil PTM, all minerals show gradual contraction of unit-cell volumes and atomistic interplane distances. They, however, show abrupt collapse near 1.0 GPa under distilled water conditions due to hydrostatic to quasi-hydrostatic environmental changes of water PTM around samples concomitant with the transition from liquid to ICE-VI and ICE-VII. The degrees of volume contractions of beidellite, montmorillonite, and nontronite up to ca. 3 GPa are ca. 6.6%, 8.9%, and 7.5% with bulk moduli of ca. 38(1) GPa, 31(2) GPa, and 26(1) GPa under silicone oil pressure, whereas 13(1) GPa, 13(2) GPa, and 17(2) GPa, and 17(1) GPa, 20(1) GPa, and 21(1) GPa under hydrostatic and quasi-hydrostatic environments before and after 1.50 GPa, respectively.

Identification of the hydrate gel phases present in phosphate-modified calcium aluminate binders

The conversion of hexagonal calcium aluminate hydrates to cubic phases in hydrated calcium aluminate cements (CAC) can involve undesirable porosity changes and loss of strength. Modification of CAC by phosphate addition avoids conversion, by altering the nature of the reaction products, yielding a stable amorphous gel instead of the usual crystalline hydrate products. Here, details of the environments of aluminium and phosphorus in this gel were elucidated using solid-state NMR and complementary techniques. Aluminium is identified in both octahedral and tetrahedral coordination states, and phosphorus is present in hydrous environments with varying, but mostly low, degrees of crosslinking. A 31P/27Al rotational echo adiabatic passage double resonance (REAPDOR) experiment showed the existence of aluminium–phosphorus interactions, confirming the formation of a hydrated calcium aluminophosphate gel as a key component of the binding phase. This resolves previous disagreements in the literature regarding the nature of the disordered products forming in this system.

Transport processes at quartz–water interfaces: constraints from hydrothermal grooving experiments

Abstract. We performed hydrothermal annealing experiments on quartzite samples at temperatures of 392 to 568 °C and fluid pressures of 63 to 399 MPa for up to 120 h, during which hydrothermal grooves developed on the free surfaces of the samples. An analysis of surface topology and groove characteristics with an atomic force microscope revealed a range of surface features associated with the simultaneous and successive operation of several processes partly depending on crystal orientation during the various stages of an experiment. Initially, dissolution at the quartzite-sample surface occurs to saturate the fluid in the capsule with SiO2. Subsequently, grooving controlled by diffusion processes takes place parallel to dissolution and precipitation due to local differences in solubility. Finally, quench products develop on grain surfaces during the termination of experiments. The average groove-root angle amounts to about 160°, varying systematically with misorientation between neighboring grains and depending slightly on temperature and run duration. The grooving is thermally activated, i.e., groove depth ranging from 5 nm to several micrometers for the entire suite of experiments generally increases with temperature and/or run time. We use Mullins' classical theories to constrain kinetic parameters for the transport processes controlling the grooving. In the light of previous measurements of various diffusion coefficients in the system SiO2–H2O, interface diffusion of Si is identified as the most plausible rate-controlling process. Grooving could potentially proceed faster by diffusion through the liquid if the fluid were not convecting in the capsule. Characteristic times of healing of microfractures in hydrous environments constrained from these kinetic parameters are consistent with the order of magnitude of timescales over which postseismic healing occurs in situ according to geophysical surveys and recurrence intervals of earthquakes.

Can hematite nanoparticles be an environmental indicator?

Environmental indicators play an important role in assessing the state of the environment and understanding the trend of environmental changes. Iron oxide nanostructures are widespread in multiple ecosystems, and are therefore suitable for providing environmental indicators with measurable physical or chemical properties that are sensitive to the environmental conditions. It has been found that the morphologies of natural hematite (α-Fe2O3) nanocrystals have large variations depending on thermodynamic and chemical conditions of their formation environments. To assess the applicability of nanomorphology as an environmental indicator, we construct a morphology map of hematite nanocrystals in hydrous environments, characterized with different temperatures, humidity and supersaturation of oxygen. Our model indicates that the fractional surface areas of morphologically significant facets change with humidity and temperature, and can be used to track the variation of these environmental conditions.

Environmentally dependent stability of low-index hematite surfaces

Nanoparticulate hematite is a promising material for catalytic and photoelectrochemical applications, where the surfaces are engineered to improve efficiency in different chemical environments. In the presence of water, the surfaces are typically passivated by hydroxyl groups, which modify the surface stability and reactivity. We use density functional theory and first principles thermodynamics to investigate the low-index surfaces (001), (101), and (104) in hydrous environments. For each of the surfaces, we build various hydroxylation configurations and compare their thermodynamic stability under different environmental conditions (temperature, humidity, and supersaturation of oxygen). The results enable us to construct surface phase diagrams, which provide guidance to the selection of surface structures, and the control of environmental conditions for specific applications.

Gale Crater: Formation and post-impact hydrous environments

Gale Crater, the landing site of the 2011 Mars Science Laboratory mission, formed in the Late Noachian. It is a 150km diameter complex impact structure with a central mound (Mount Sharp), the original features of which may be transitional between a central peak and peak ring impact structure. The impact might have melted portions of the substrate to a maximum depth of ∼17km and produced a minimum of 3600km3 of impact melt, half of which likely remained within the crater. The bulk of this impact melt would have pooled in an annular depression surrounding the central uplift, creating an impact melt pool as thick as 0.5–1km. The ejecta blanket surrounding Gale may have been as thick as ∼600m, which has implications for the amount of erosion that has occurred since Gale Crater formed. After the impact, a hydrothermal system may have been active for several hundred thousand years and a crater lake with associated sediments is likely to have formed. The hydrothermal system, and associated lakes and springs, likely caused mineral alteration and precipitation. In the presence of S-rich host rocks, the alteration phases are modelled to contain sheet silicates, quartz, sulphates, and sulphides. Modelled alteration assemblages may be more complex if groundwater interaction persisted after initial alteration. The warm-water environment might have provided conditions supportive of life. Deep fractures would have allowed for hydraulic connectivity into the deep subsurface, where biotic chemistry (and possibly other evidence of life) may be preserved.

Metastable structures and isotope exchange reactions in polyoxometalate ions provide a molecular view of oxide dissolution

Reactions involving minerals and glasses in water are slow and difficult to probe spectroscopically but are fundamental to the performance of oxide materials in green technologies such as automotive thermoelectric power generation, CO2 capture and storage and water-oxidation catalysis; these must be made from geochemically common elements and operate in hydrous environments. Polyoxometalate ions (POMs) have structures similar to condensed oxide phases and can be used as molecular models of the oxide/water interface. Oxygen atoms in POM exchange isotopes at different rates, but, at present, there is no basis for predicting how the coordination environment and metal substitution influences rates and mechanisms. Here we identify low-energy metastable configurations that form from the breaking of weak bonds between metals and underlying highly coordinated oxygen atoms, followed by facile hydroxide, hydronium or water addition. The mediation of oxygen exchange by these stuffed structures suggests a new view of the relationship between structure and reactivity at the oxide/solution interface.

Surface phase diagram of hematite pseudocubes in hydrous environments

Hematite nanoparticles often display the pseudocubic morphology enclosed exclusively by the (012) surface. The surface chemistry on these facets is important to understand the formation and properties of these nanoparticles in varying chemical environments. The surface is typically terminated by hydroxyl groups in water or humid atmospheres, and the various terminations differ largely in composition, structure, thermodynamic stability, and chemical reactivity. When we compare a large variety of chemical configurations, we find that three specific terminations are thermodynamically stable under aerobic conditions. The termination by singly and triply coordinated hydroxyl groups (S-,T-OH) is stable at low temperatures and in hydrous environments, the stoichiometric clean termination is stable at high temperatures and in dry environments, and the termination by doubly coordinated hydroxyl groups (D-OH) is stable under intermediate conditions. The S-,T-OH termination can convert topologically to the clean surface by dissociative adsorption of water, whereas the conversion from these two terminations to the D-OH termination requires re-organization of several atomic layers at the surface. Therefore, the surface may reversibly change between S-,T-OH and the clean surface depending on the temperature and humidity. Based on these findings we have constructed surface phase diagrams to predict the termination types in different hydrous and humid environments.

Surface Structure and Environment-Dependent Hydroxylation of the Nonpolar Hematite (100) from Density Functional Theory Modeling

Hematite (α-Fe2O3) nanoparticles are typically synthesized, stored, or used in hydrous environments, and the mineral/water interfaces are important for the surface stability and reactivity of these nanoparticles. Under such conditions the exposed facets are often passivated by hydroxyl groups. The configurations of surface hydroxylation vary with environmental conditions and affect the morphology and surface chemistry. Among the low-index hematite surfaces, the {100} are the only nonpolar surfaces and are often present on nanorods or nanotubes elongated along the [001] direction. In this paper we explore the relaxation and hydroxylation of this surface using first principles thermodynamics. Our results reveal that depending on the supersaturation of water and oxygen, various extents of hydroxylation may appear. In humid or hydrous environments, undercoordinated subsurface oxygen atoms are hydrogenated. In water singly and doubly coordinated hydroxyl groups coexist with chemisorbed water molecules at the s...

3. Formation and alteration of clay materials

3.1. Introduction The formation and alteration of clay minerals and their accumulation as clay materials can occur by a very wide range of processes. In one way or another, however, most of these processes and the environments in which they operate involve the chemical actions and physical movement of water. As such, clay minerals can be considered the characteristic minerals of the Earth;s near surface hydrous environments, including that of weathering, sedimentation, diagenesis/low-grade metamorphism and hydrothermal alteration (Fig. 3.1). Simply defined, the weathering environment is that in which rocks and the minerals they contain are altered by processes determined by the atmosphere, hydrosphere and the biosphere. Soil formation, also known as pedogenesis, occurs in the weathering environment. The sedimentary environment is the zone in which, soil, weathered rock and mineral (and biogenic) materials are eroded, mixed and deposited as sediments by water, wind and ice. Diagenesis involves all those physical and chemical processes that occur between sedimentation and metamorphism, whilst hydrothermal alteration encompasses the interactions between heated water and rock. In this chapter, the origins of the various clay minerals that may occur in each of these environments are reviewed along with the processes that may lead to their accumulation and alteration, usually together with other components, to form clay materials. In many instances, clay materials are formed in one environment by the accumulation or alteration of clay minerals formed in others. Thus the geological history of a clay material, and consequently its properties and behaviour, may depend on many

The Structure Sensitivity of HxMoO3Precipitation on MoO3(010) during Reactions with Methanol

Atomic force microscopy has been used to examine the effect of surface morphology on the formation of HxMoO3during the reaction of MoO3with methanol. By heating MoO3crystals in hydrous environments, pits can be formed on the (010) surface. We have found that pit formation is activated by elastic strain from extended defects and because of this, it is possible to exercise some degree of control over the density, crystallography, and size of these features. This phenomenon was exploited to create different and well characterized morphological features on pairs of mated surfaces, cleaved from the same single crystal. The mated pairs were then reacted simultaneously in methanol–N2mixtures at temperatures between 300 and 330°C. Surfaces with pits that expose {h0l} type facets containing undercoordinated Mo sites were observed to intercalate more H than relatively flat surfaces. This result, together with the observation that acicular HxMoO3precipitates grow from the edges of the pits, suggests that the undercoordinated surface Mo on the {h0l} surfaces are active for the dissociative chemisorption of methanol at temperatures between 300 and 330°C.

Potassium-argon ages of coexisting minerals from pyroxene-bearing granitic rocks in the Sierra Nevada, California

Potassium-argon dates were determined for pyroxene, plagioclase, orthoclase, biotite, and hornblende from two granitic rocks of different thermal histories. The pyroxenes from both rocks yielded low ages relative to the other mineral phases. The argon deficiency in the pyroxenes suggests that lattice imperfections due to exsolution or to alteration to hornblende may make these minerals from hydrous plutonic environments poor retentive reservoirs for internally produced radiogenic argon.