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In this work, the crystal structure of a natural specimen of the "Messbach-type" variscite from Tooele Co. (Utah, U.S.A.) was determined, and the dehydration process leading to the transition to a new AlPO SB 4 sb phase was monitored at high temperature. Crystal structure description of variscite 2O and comparison with variscite 1O Like variscite I 1O i , the variscite I 2O i crystal structure belongs to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms ([30]). The Al-coordination change from octahedral to tetrahedral that accompanies dehydration, clearly demonstrated by the SP 27 sp Al NMR data, confirms the removal of the Al-bound H SB 2 sb O molecules as the dehydration mechanism, transforming the variscite octahedral-tetrahedral framework to a purely tetrahedral framework. Keywords: Variscite; X-ray powder diffraction; charge-flipping; nuclear magnetic resonance; polytypism; AlPO4 EN Variscite X-ray powder diffraction charge-flipping nuclear magnetic resonance polytypism AlPO4 1385 1395 11 07/05/22 20220701 NES 220701 Introduction Interest in hydrated aluminophosphates and in compounds that derive from their dehydration spans a wide spectrum of disciplines. [Extracted from the article]

Magmatic conditions prior to explosive eruption are often investigated using geochemical signatures in glassy components of pyroclastic deposits and related to magmatic processes at depth. One important process is fractional crystallization, which causes systematic changes to the SiO2/Al2O3 ratio of the residual melt that can be determined by observation of the mineralogy of fully crystallized lavas, by experimental petrology, and by magmatic modeling. However, for many alkaline-mafic pyroclastic deposits, the record of residual melt compositions is obscured by alteration, commonly affecting more than 50% of pyroclastic rock components including reactive glass and some susceptible minerals. In this study, melt signatures of SiO2/Al2O3 represented heterogeneously by the scarce fresh glass and abundant, zeolitized proxy-glass in the alkaline deposits of a major, caldera-forming eruption were used in conjunction with a model system (Rhyolite-MELTS) to reconstruct residual melt compositions and characteristics that existed immediately prior to explosive eruption. Through the model, full major oxide compositions of residual melts and fractionally crystallizing minerals become accessible, with associated constraints on volatiles and physical characteristics (melt temperature, density, viscosity). The use of zeolitized proxy-glass signatures relies on established and deposit-specific evidence for "hydrologically closed" systems that suggests the SiO2/Al2O3 ratio is closely retained through initial alteration reactions and therefore closely representative of SiO2/Al2O3 in the precursor glass (erupted melt). The relationship is supported by a review of available, paired data (R2 = 0.94). Therefore, magmatic system data for the abundant and pervasive fine ash fraction of pyroclastic deposits can be investigated using this method and can progress more deeply beyond the widely used simple affiliation to igneous rock classification. Model-predicted magmatic mineral compositions (clinopyroxene, spinel, and nepheline as demonstrated here) serve to validate a case study reconstruction by comparison with compositions reported from natural and experimental samples. This predictive capability of the novel procedure is demonstrated in the case of a major caldera-forming eruption, the 355 ka Villa Senni event of the quiescent Colli Albani volcano, Rome, Italy, and its pervasively zeolitized Tufo Lionato deposit (>50 km3). The key finding is that a more-evolved residual melt fraction has been revealed, based on a reconstructed SiO2/Al2O3 ratio of 2.05 relative to that of the parent magma at 2.68, with implications for a reappraisal of pre-eruptive conditions and eruption mechanisms, and potentially for similar patterns across the volcanic stratigraphy and for other alkaline volcanoes. [ABSTRACT FROM AUTHOR]

The Mars Science Laboratory (MSL) rover Curiosity has documented a section of fluvio-lacustrine strata at Yellowknife Bay (YKB), an embayment on the floor of Gale crater, approximately 500 m east of the Bradbury landing site. X-ray diffraction (XRD) data and evolved gas analysis (EGA) data from the CheMin and SAM instruments show that two powdered mudstone samples (named John Klein and Cumberland) drilled from the Sheepbed member of this succession contain up to ~20 wt% clay minerals. A trioctahedral smectite, likely a ferrian saponite, is the only clay mineral phase detected in these samples. Smectites of the two samples exhibit different 001 spacing under the low partial pressures of H2O inside the CheMin instrument (relative humidity <1%). Smectite interlayers in John Klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 Å, but largely remain open in the Cumberland sample with a basal spacing of ~13.2 Å. Partial intercalation of Cumberland smectites by metal-hydroxyl groups, a common process in certain pedogenic and lacustrine settings on Earth, is our favored explanation for these differences. The relatively low abundances of olivine and enriched levels of magnetite in the Sheepbed mudstone, when compared with regional basalt compositions derived from orbital data, suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine. Mass-balance calculations are permissive of such a reaction. Moreover, the Sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim. Early diagenetic fabrics suggest clay mineral formation prior to lithification. Thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants, allowing circum-neutral pH. The kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years. Mineralogical evidence of the persistence of benign aqueous conditions at YKB for extended periods indicates a potentially habitable environment where life could establish itself. Mediated oxidation of Fe2+ in olivine to Fe3+ in magnetite, and perhaps in smectites provided a potential energy source for organisms. [ABSTRACT FROM AUTHOR]

Sulfate minerals are important indicators for aqueous geochemical environments. The geology and mineralogy of Mars have been studied through the use of various remote-sensing techniques, including thermal (mid-infrared) emission and visible/near-infrared reflectance spectroscopies. Spectral analyses of spacecraft data (from orbital and landed missions) using these techniques have indicated the presence of sulfate minerals on Mars, including Fe-rich sulfates on the iron-rich planet. Each individual Fe-sulfate mineral can be used to constrain bulk chemistry and lends more information about the specific formational environment [e.g., Fe2+ sulfates are typically more water soluble than Fe3+ sulfates and their presence would imply a water-limited (and lower Eh) environment; Fe3+ sulfates form over a range of hydration levels and indicate further oxidation (biological or abiological) and increased acidification]. To enable better interpretation of past and future terrestrial or planetary data sets, with respect to the Fe-sulfates, we present a comprehensive collection of mid-infrared thermal emission (2000 to 220 cm-1; 5-45 µm) and visible/near-infrared (0.35-5 µm) spectra of 21 different ferrous- and ferric-iron sulfate minerals. Mid-infrared vibrational modes (for SO4, OH, H2O) are assigned to each thermal emissivity spectrum, and the electronic excitation and transfer bands and vibrational OH, H2O, and SO4 overtone and combination bands are assigned to the visible/near-infrared reflectance spectra. Presentation and characterization of these Fe-sulfate thermal emission and visible/near-infrared reflectance spectra will enable the specific chemical environments to be determined when individual Fe-sulfate minerals are identified. [ABSTRACT FROM AUTHOR]

We present an X-ray diffraction and multi-nuclear (²H and 43Ca) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H2O interface. Variable-temperature 43Ca MAS NMR and controlled humidity XRD indicate that Ca2+ occurs as proximity-restricted outer-sphere hydration complexes between -120 and +25 °C in a two-layer hydrate and at T ≤ -50 °C in a 2:1 water/solid paste. Changes in the 43Ca NMR peak width and position with temperature are more consistent with diffusion-related processes than with dynamics involving metal-surface interactions such as site exchange. The ²H NMR signal between -50 and +25 °C for a two-layer hydrate of Ca-hectorite is similar to that of Na- and other alkali metal hectorites and represents 2H2O molecules experiencing anisotropic motion describable using the ²H C2/C3 jump model we proposed previously. ²H T1 relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for ²H2O dynamics, yielding GHz-scale rotational reorientation rates compatible with the C3 component of the C2/C3 hopping model. The apparent activation energy for ²H2O rotational diffusion in the two-layer hydrate is greater for Ca-hectorite than Na-hectorite (25.1 vs. 21.1 kJ/mol), consistent with the greater affinity of Ca2+ for H2O. The results support the general principle that the dynamic mechanisms of proximity-restricted H2O are only weakly influenced by the cation in alkali metal and alkaline earth metal smectites and provide critical evidence that the NMR resonances of charge-balancing cations in smectites become increasingly influenced by diffusion-like dynamic processes at low temperatures as the charge density of the unhydrated cation increases. [ABSTRACT FROM AUTHOR]

Changes in the mechanisms of formation and global distribution of phyllosilicate clay minerals through 4.567 Ga of planetary evolution in our solar system reflect evolving tectonic, geochemical, and biological processes. Clay minerals were absent prior to planetesimal formation ~4.6 billion years ago but today are abundant in all near-surface Earth environments. New clay mineral species and modes of clay mineral paragenesis occurred as a consequence of major events in Earth's evolution--notably the formation of a mafic crust and oceans, the emergence of granite-rooted continents, the initiation of plate tectonics and subduction, the Great Oxidation Event, and the rise of the terrestrial biosphere. The changing character of clay minerals through time is thus an important part of Earth's mineralogical history and exemplifies the principles of mineral evolution. [ABSTRACT FROM AUTHOR]

Hydrated evaporite minerals have the ability to hold large amounts of H2O, making them a potential source of H2O in cold, low-... environments such as the surface of Mars. Many of these hydrated evaporite minerals experience a reversible change in hydration state in response to changes in temperature (T) and relative humidity (RH). Such phases may thus have the potential to interact with the martian atmosphere on a daily or seasonal basis. The Na2Mg(SO4)2⋅nH2O system was previously thought to contain three hydrated phases: a decahydrate (n = 10), konyaite (n = 5), and blödite (n = 4). We examined this system using temperature- and RH-controlled X-ray powder diffraction (XRD) methods, as well as temperature-controlled single-crystal X-ray diffraction. When blödite was exposed to sub-freezing conditions, T ≤ -10 °C, a new phase was produced (n = 16, 52 wt%H2O). Similar low-temperature behavior has been documented in the MgSO4⋅nH2O system, through the presence of meridianiite (Peterson et al. 2007). The hydration and dehydration behavior of phases in the Na2Mg(SO4)2⋅nH2O system was evaluated with powder XRD from -30 to >25 °C and from ~99 to near 0% RH, and single-crystal XRD data were collected for the n = 16 phase at -120 °C. The 16-hydrate is triclinic, space group P..., with unit-cell parameters a = 6.5590(12), b = 6.6277(14), c = 14.441(3) Å, α = 87.456(15)°, β = 79.682(15)°, γ = 65.847(13)°, and a unit-cell volume of 563.3(2) ų. The existence of this new phase at low temperatures, its high hydration state, and its ability to form reversibly from blödite all suggest that if phases in this system exist on the martian surface, they will participate in the Mars H2O cycle. [ABSTRACT FROM AUTHOR]

Although the transition of the blueschist to greenschist facies has been modeled by the reaction glaucophane (Gln) + paragonite (Pg) + water (V) = clinochlore (Chl) + albite (Ab) + quartz (Qtz), we have found in the system Na2O-MgO-Al2O3-SiO2-H2O that smectite (Smec) forms repeatedly and persistently through the approximate reaction Gln + Chl = Smec ± Ab. The reaction was verified using both synthetic and natural Gln and Chl, where in all cases complete reaction of Chl occurs in the presence of Gln to yield Smec. We have, therefore, investigated the reaction Gln + Pg + Qtz ± V = Smec + Ab, as a possible alternative to the first reaction. The reaction Gln + Pg + Qtz ± V = Smec + Ab was studied over the P-T range of 600--800 °C and 1.4--2.0 GPa keeping the water content in the range of 0--4 wt% to avoid the direct reaction of Gln to Smec + Ab at water contents above 4 wt%. The was experimentally reversed at 600 °C and 1.60(5) GPa, 700 °C and 1.70(5) GPa, and at 800 °C and 1.90(5) GPa (at 0 wt% water content), but with partial melting at 800 °C and pressure ≤2.0 GPa with water ≥3 wt%. The compositions of the phases and the state of A1-Si disorder in the albite were determined. The amphibole showed minor increase in Al and decrease in Mg and Si relative to the glaucophanic amphibole starting material, whereas paragonite showed minor enrichment in Mg and Si at the highest temperatures, complementary to the change in amphibole composition. Albite showed only limited ordering with decrease in temperature. Smectite at 600--700 °C has the average composition Na0.25Mg2.21A11.19Si3.44 O10(OH)2·nH2O and shows slight Mg depletion and Al enrichment at 800 °C. Evidence for at least some Mg enrichment in the interlayer cation sites was supported by powder X-ray diffraction with in situ heating stage analyses using changes in the d001 reflection at 200 °C as an approximate indication of Mg for Na substitution. The composition of smectite formed in this study is quite similar to that of other natural trioctahedral phyllosilicates reported in the literature. This, together with the experimental reversibility of the reaction observed here, indicates that smectite can form as a progressive metamorphic mineral at greenschist- and amphibole-facies conditions, particularly at the expense of chlorite and glaucophane. [ABSTRACT FROM AUTHOR]

Progress in understanding mineral evolution, Earth's changing near-surface mineralogy through time, depends on the availability of detailed information on mineral localities of known ages and geologic settings. A comprehensive database including this information, employing the mindat.org web site as a platform, is now being implemented. This resource will incorporate software to correlate a range of mineral occurrences and properties vs. time, and it will thus facilitate studies of the changing diversity, distribution, associations, and characteristics of individual minerals as well as mineral groups. The Mineral Evolution Database thus holds the prospect of revealing mineralogical records of important geophysical, geochemical, and biological events in Earth history. [ABSTRACT FROM AUTHOR]

The crystal structures of natrolite and its dehydrated high-temperature phases (α1- and α2-metanatrolite) have been determined from powder X-ray diffraction measurements as a function of temperature and partial pressure of water (PH2O) to characterize the phase transition behavior. The evolution of crystal structure as a function of temperature shows two different phase transitions, depending on the PH2O, with α1-metanatrolite occurring at elevated PH2O and α2-metanatrolite occurring at low PH2O. Our discovery of α2-metanatrolite implies the existence of more than one transition mechanism, which we correlate with the migration of Na+ ions and the rate of evolution of H2O molecules. The transition behavior is rationalized in terms of two cooperating mechanisms: (1) dehydration-induced processes, which determine the phase transition temperature; and (2) thermally induced processes, which determine how the framework and its extraframework cations are modified. [ABSTRACT FROM AUTHOR]

Yucca Mountain, Nevada, has been approved by the U.S. Congress as the site of the first high-level radioactive waste repository in the U.S., paving the way for the Department of Energy to submit a license application to the Nuclear Regulatory Commission. The U.S. concept of a radioactive waste repository involves both engineered (man-made) barriers and geologic barriers to minimize the long-term migration of waste from the repository. The minerals at Yucca Mountain are an integral part of the geologic barrier to the migration of radionuclides, and understanding the distribution of reactive and potentially sorptive minerals is crucial to modeling the long-term performance of the repository. We have determined the three-dimensional distribution of minerals at Yucca Mountain by analyzing a number of drill cores using quantitative X-ray diffraction methods. Clinoptilolite and mordenite are locally abundant in the rhyolitic tuffs in both the unsaturated and saturated zones. They formed by the alteration of glassy rhyolitic tuffs, and their occurrence is stratified, often following the distribution of nonwelded, initially vitric tuffs near and below the water table. Because of their large cation-exchange capacities, these minerals have traditionally been considered to be important in retardation of the migration of cationic radionuclides such as [sup137]Cs and [sup90]Sr. However, analysis of radionuclides that may be contained at Yucca Mountain reveals that, for the most part, the radioactive species of concern are not those that would be readily sorbed by natural zeolites. Although clinoptilolite and mordenite are highly selective for Cs and Sr, the half-lives of these radionuclides (∼30 years) are sufficiently short that the man-made containment at any radioactive waste repository is expected to outlive these radionuclides. The most important radionuclides are thus anionic species and long-lived actinides that often form large, complex aqueous species, both of... [ABSTRACT FROM AUTHOR]

An isosymmetric displacive structural transformation in the hydrous layer silicate dickite [Al[sub2]Si[sub2]O[sub5](OH)[sub4], monoclinic Cc, a = 5.161(3), b = 8.960(6), c = 14.459(10) Å, β = 96.77(1)°], occurring under hydrostatic compression above 2.0 GPa, has been studied using single-crystal X-ray diffraction and diamond-anvil cell techniques. The structure of the high-pressure phase, determined in situ, is monoclinic with space group Cc with unit-cell parameters a = 5.082(3), b = 8.757(6), c = 13.771 (9) Å, and β = 89.60(2)° at 4.1 GPa. The positions of all hydrogen atoms at both ambient and high pressure have been determined by a combination of simulated annealing and energy minimization. The mechanism of the transformation, which may be general for other hydrous layered materials, involves a shift of the 1:1 layers with respect to each other by the vector [1/6, 1/6, 0] and is accompanied by the formation of new hydrogen bonds. [ABSTRACT FROM AUTHOR]

Describes an experimental study of site-specific thermodynamic properties of hydration of the W1 and W5 sites in laumonite that allows predictions of water site occupancies and the water content of laumonite at elevated temperatures and pressures. Background to the study; Methods; Results and analyses; Discussion.

Presents information on a study which evaluated the response of the laumontite crystal structure to hydration/dehydration using Rietveld refinements with x-ray diffraction data collected. Crystal chemical background; Experimental setup; Rietveld refinements; Results and discussion.

Reports on the hydration, expansion, structure and dynamics of layered double hydroxides. Effects of hydration state on the structure and dynamical behavior of interlayer and surface anions; Factors controlling the expansion behavior of layered double hydroxides; Interpretation of sorption isotherms.

Presents the use of a recursive calculation technique that allows for ordered and disordered intergrowths of planar units to model X-ray diffusion patterns of opal-CT. Calculation of diffraction patterns; Calculated patterns suggesting several low-intensity diffraction bands.

Presents results of a quantitative phase analysis of a number of multicomponent standard and natural mineral mixtures using an adaptation of the Rietveld method. Digital powder X-ray diffraction data; Application in geology.

Investigates equilibrium in the clinoptilolite-H20 system. Chemical analyses of purified natural and cation-exchanged clinoptilolite from Fish Creek Mountains, Nevada; Water vapor pressure; Thermodynamic relations; Data analysis; Linear regression results.

Focuses on a study which examined the complete set of (001), (010), and (100) of a Saint Claire dickite specimen called orientation, and the complementary relationship between IR and Raman results. Use of variations in intensities of the v(OH) bands to polarized spectra to confirm previous band assignments; Methods and materials used to conduct the study; Results of the study.

Ferrian saponite from the eastern Santa Monica Mountain, near Griffith Park (Los Angeles, California), was investigated as a mineralogical analog to smectites discovered on Mars by the CheMin X-ray diffraction instrument onboard the Mars Science Laboratory (MSL) rover. The martian clay minerals occur in sediment of basaltic composition and have 02l diffraction bands peaking at 4.59 A, consistent with tri-octahedral smectites. The Griffith saponite occurs in basalts as pseudomorphs after olivine and mesostasis glass and as fillings of vesicles and cracks and has 02l diffraction bands at that same position. We obtained chemical compositions (by electron microprobe), X-ray diffraction patterns with a lab version of the CheMin instrument, Mössbauer spectra, and visible and near-IR reflectance (VNIR) spectra on several samples from that locality. The Griffith saponite is magnesian, Mg/(Mg+ΣFe) = 65-70%, lacks tetrahedral Fe3+ and octahedral Al3+, and has Fe3+ /ΣFe from 64 to 93%. Its chemical composition is consistent with a fully tri-octahedral smectite, but the abundance of Fe3+ gives a nominal excess charge of +1 to +2 per formula unit. The excess charge is likely compensated by substitution of O2- for OH-, causing distortion of octahedral sites as inferred from Mössbauer spectra. We hypothesize that the Griffith saponite was initially deposited with all its iron as Fe2+ and was oxidized later. X-ray diffraction shows a sharp 001 peak at 15 Å, 00l peaks, and a 02l diffraction band at the same position (4.59 A) and shape as those of the martian samples, indicating that the martian saponite is not fully oxidized. VNIR spectra of the Griffith saponite show distinct absorptions at 1.40, 1.90, 2.30-2.32, and 2.40 mu m, arising from H2+O and hydroxyl groups in various settings. The position of the ~2.31 μm spectral feature varies systematically with the redox state of the octahedrally coordinated Fe. This correlation may permit surface oxidation state to be inferred (in some cases) from VNIR spectra of Mars obtained from orbit, and, in any case, ferrian saponite is a viable assignment for spectral detections in the range 2.30-2.32 μm. [ABSTRACT FROM AUTHOR]

Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO4 tetrahedra mimicking the hierarchy of polymerization of SiO4 tetrahedra seen in silicate minerals. To augment previous Mössbauer studies of individual phosphate species and groups of species, this paper presents new Mössbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mössbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated M össbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mössbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mössbauer spectra acquired on Mars. [ABSTRACT FROM AUTHOR]

The response of the mesolite crystal structure (Na16Ca16Al48Si72O240·64H2O) to dehydration was evaluated as a function of temperature and partial pressure of water (i.e., PH2O or relative humidity, RH) using laboratory X-ray powder diffraction (XRD; CuKα radiation) and synchrotron X-ray pair distribution function (PDF) methods. At 425 °C under low-PH2O conditions (PH2O ≤ ~1.3 mbar), dehydrated mesolite preserved the long-range ordered aluminosilicate framework structure, which has not been previously observed. This new dehydrated phase, x-metamesolite, has unit-cell parameters [a = 16.731(3) Å, b = 17.822(2) Å, c = 6.312(1) Å, V = 1882.5(5) ų, and possible space group Fdd2] similar to those of other dehydrated natrolite phases (either α1- or α2-metanatrolite). Conversely, under high-PH2O conditions (PH2O > ~1.3 mbar), dehydrated mesolite became amorphous (amorphous T5O10) at 425 °C. The local structure of amorphous T5O10 was characterized by PDF analyses, which showed the formation of twisted T5O10 nano-fibers [with dimensions (LWH) of ~6.9 x 6.9 x 6.3 Å] resulting from breakage of the mesolite aluminosilicate framework. The two distinct high-temperature PH2O-dependent phase transition paths illustrate the importance of considering the combined effects of temperature and PH2O in mesolite. In addition, the low-temperature phase transition in mesolite, involving order-disorder of the extraframework cations, also showed a PH2O-dependent transition temperature. Although, there is no path dependence on PH2O for this transition, the local arrangement of Na, Ca, and vacancies in disordered metamesolite (formed through the extraframework cation order-disorder phase transition) likely influences the thermal stability of the aluminosilicate framework during further heating. [ABSTRACT FROM AUTHOR]