Volume 94, Issue 11-12

Clays and clay minerals are common components in soils, sediments, and sedimentary rocks, and they play an important role in many environmental processes. Iron is ubiquitous in clays and clay minerals and its oxidation state, in part, controls the physical and chemical properties of these fine-grained minerals. The structural ferric iron in clay minerals can be reduced either chemically or biologically. Biological reductants include mesophilic and thermophilic microorganisms from diverse environments such as soils, sediments, sedimentary rocks, and hydrothermal hot springs. Multiple clay minerals have been used for microbial reduction studies, including dioctahedral smectite-illite series, palygorskite, chlorite, and their various mixtures in natural soils and sediments. All of these clay minerals are reducible by microorganisms under various conditions with smectite (nontronite) being the most reducible and illite the least. The rate and extent of bioreduction depends on many experimental factors, such as the type of microorganisms and clay minerals, solution chemistry, and temperature. Despite significant efforts, current understanding of the mechanisms of microbial reduction of ferric iron in clay minerals is still limited. Whereas some studies have presented evidence for a solid-state reduction mechanism, others argue that the clay mineral structure partially dissolves when the extent of reduction is high. This inconsistency may be related to several experimental conditions, and their specific effects are discussed in this paper. Whereas past experiments have been largely conducted in well-controlled laboratory systems, recent efforts have attempted to transfer knowledge to the field to improve our understanding of more complex soil systems for better agricultural practices. Biologically reduced clay minerals are also important agents in remediating inorganic and organic contaminants in soil and groundwater systems. This paper reviews the most recent developments and suggests some directions for future research

Layered anorthosite, chromiferous pyroxenite, and spatially associated mafic and felsic rocks of the 2.9 Ga Sittampundi layered complex (SLC), South India, underwent high-grade metamorphism at ca. 2.5 Ga and were subjected to amphibolite-facies metamorphism accompanied by intrusion of granitoid plutons during late-Neoproterozoic tectonothermal activity (0.72-0.45 Ga). During the latter event, anorthosite developed millimeter- to centimeter-thick compositional layers rich in clinopyroxene+amphibole+clinozoisite+chlorite±chromite and corundum+spinel+chlorite. Tiny grains of högbomite replaced only the grains of corundum and spinel and are in textural equilibrium with chlorite. The studied högbomite contains appreciable TiO 2 (>4.4 wt%) but insignificant NiO and ZnO. Cr 2 O 3 content reaches up to 0.35 wt% only in chromite-bearing samples. Systematic partitioning of Fe and Mg between högbomite and associated Fe-Mg minerals demonstrate attainment of chemical equilibrium among these phases. Integrating textural relations and algebraic analyses of the phase compositions, several reactions were constructed involving the associated oxide phases (spinel, corundum, högbomite), calcite, and silicates (amphibole, chlorite, anorthite, clinozoisite). Interpretation of the reaction reveals that (1) Mg 2+ and Ti 4+ were mobile for more than 2 cm during the formation of högbomite and chlorite, and (2) chloritization of amphibole in the clinopyroxene+amphibole-bearing layers released Ti that was transported to the spinel+corundum-bearing layers to develop högbomite. Stability fields of some critical mineral assemblages in P-X CO₂ and T-X CO₂ space combined with geothermobarometry in the associated rocks tightly constrain the growth of högbomite in the presence of aqueous fluids (X CO₂ < 0.15) to the P-T range of 7 ± 1 kbar, 650 ± 50 °C. These aqueous fluids, presumably derived from the Pan-African granitoid batholiths, chloritized amphibole grains, and transported the released Ti4+ to the spinel+corundum-bearing layers to develop högbomite. Topological relations in isothermal-isobaric fugacity diagrams (log f O₂ -log f S ₂ and log f O₂ -log f H₂O ) in the system FeO-Al 2 O 3 - TiO 2 -O 2 -S 2 -H 2 O-CO 2 (+MgO, Cr 2 O 3 ) indicate that the stability and compositional characteristics of natural högbomite are strongly influenced by f O₂ , f S₂ , f H₂O , and concentrations of other soluble species (Ti, Mg, Cr, etc.) in the metamorphic fluids

Based on single-crystal structure refinements, miguelromeroite, Mn 5 (H 2 O) 4 (AsO 3 OH) 2 (AsO 4 ) 2 , from the Ojuela mine, Mapimi, Durango, Mexico, is described as a new species corresponding to the Mn-dominant member of a series with sainfeldite, Ca 5 (H 2 O) 4 (AsO 3 OH) 2 (AsO 4 ) 2 , and type villyaellenite, (Mn,Ca)Mn 2 Ca 2 (H 2 O) 4 (AsO 3 OH) 2 (AsO 4 ) 2 , from Sainte-Marie aux Mines, Alsace, France, is redefined as an ordered intermediate species in the series. Miguelromeroite is monoclinic, C2/c, a = 18.030(1), b = 9.2715(5), c = 9.7756(5) Å, β = 96.266(2)°, V = 1624.4(2) Å 3 , Z = 4. At the Ojuela mine, miguelromeroite occurs as a compact spray of orangepink, prismatic crystals up to 4 cm in length. Crystals are elongate on [001] with forms {100}, {110} and {101}. Physical properties: pale pink streak, transparent, vitreous luster, brittle, good {100} cleavage, conchoidal fracture, Mohs hardness ~4, measured density 3.69(3) g/cm 3 , and calculated density 3.714 g/cm 3 . Optical properties: biaxial (-), n α 1.713(2), n β 1.723(2), n γ 1.729(2), 2V meas 70(5)°, 2V calc 75°, orientation X = b, Z ^ c = 40° in obtuse β, pleochroic pale pink, Z >> X > Y. Miguelromeroite is named for Miguel Romero Sanchez (1926-1997) in recognition for his dedication to documenting and preserving Mexico’s rich mineral heritage. Miguelromeroite also occurs at the Veta Negra mine, Tierra Amarilla, Copiapó Province, Chile, at Sterling Hill, Ogdensburg, Sussex County, New Jersey, and at the Gozaisho mine, Iwaki, Fukushima Prefecture, Honshu Island, Japan. Some material from the Gozaisho mine may correspond to another ordered species in the series with the formula (Ca,Mn)Mn 2 Ca 2 (H 2 O) 4 (AsO 3 OH) 2 (AsO 4 ) 2

The transformation of calaverite to gold under hydrothermal conditions was studied experimentally by probing the effects of temperature (140 to 220 °C), pH (2-12), oxidant concentration, geometric specific surface area, and solid-weight to fluid-volume ratio on the sample textures and the reaction kinetics. Under all of the experimental conditions explored, calaverite transformed to various extents to metallic gold. The replacement is pseudomorphic, as gold preserves the external dimensions of calaverite. The resulting elemental gold is porous; consisting of filament-shaped aggregates with diameters ranging from 200 to 500 nm and lengths up to 25 μm. Gold crystals appear to be randomly oriented with respect to the twinned calaverite grains. The transformation proceeds via a coupled calaverite dissolution-gold precipitation mechanism, with calaverite dissolution being rate-limiting relative to gold precipitation. Tellurium is lost to the bulk solution as Te(IV) complexes, and may further precipitate away from the dissolution site (e.g., autoclave walls) as TeO 2 (s). In contrast, gold precipitates locally near the calaverite dissolution site. Such local gold precipitation is facilitated by fast heterogeneous nucleation onto the calaverite surface. The dissolution of calaverite and the overall reaction are oxidation reactions, and oxygen diffusion through the porous metallic gold layer probably plays an important role in sustaining the reaction. A similar dissolution-reprecipitation process may be responsible for the formation of mustard gold during the weathering of gold-telluride ores. At 220°C, solid-state replacement of calaverite by gold is slow (months), but calaverite grains ~100 μm in size are fully replaced in <24 h under hydrothermal conditions, providing a possible alternative to roasting as a pre-treatment of telluride-rich gold ores

A statistically based re-evaluation of the evidence for a racemic abundance of quartz enantiomers suggests that, while this hypothesis is valid at the global scale, local deviations occur such that at any given location either l- or d-quartz may predominate. Thus, the hypothesis that the homochirality of life may have come about through interactions with a dominant quartz enantiomer at a particular location cannot be discounted

The crystal structure and crystal chemistry of two natural pollucite samples, from Buckfield, Oxford County, Maine (sample M3), and from Kanzit Mawaie, Laghman, Nooristan, Afghanistan (sample N5), have been investigated by means of wavelength-dispersive X-ray microanalysis, thermogravimetric analysis, single-crystal X-ray and neutron diffraction, and single-crystal Fourier-transform infrared spectroscopy. The X-ray and neutron diffraction patterns of the two pollucite crystals show a metrically cubic unit cell [with aM3 = 13.6914(6) Å and a N5 = 13.6808(6) Å by neutron diffraction data; the deviation from isometry is <1.5σ(li), where li is the unrestrained unit-cell length] and the reflection conditions are consistent with the space group Ia3d. Anisotropic neutron structural refinements gave final agreement indices: R 1 = 0.0543 for 32 refined parameters and 372 unique reflections with F o > 4σ(F o ) for M3 and R1 = 0.0693 for 31 refined parameters and 331 unique reflections with F o > 4σ(F o ) for N5. The structure refinements show a disordered Si/Al-distribution in the tetrahedral framework. The analysis of the difference-Fourier maps of the nuclear density confirms the presence of extraframework water molecules with oxygen sharing the Cs site (at 1/8, 1/8, 1/8, Wyckoff-16b position). However, the minima, ascribable to the proton sites, are very weak in density. Two possible proton positions, leading to a reasonable H 2 O configuration, are given, and the possible hydrogen bonding is described. Sodium is located at 1/4, 1/8, 0 (Wyckoff-24c position). The main IR absorption bands in the regions typical of H 2 O are assigned, and the presence of hydroxyls in the studied samples is ruled out. Neutron diffraction and FTIR data agree with the presence of very weak hydrogen bonds in the structure. The detailed description of the crystal structure and crystal chemistry of pollucite (e.g., Si/Al-distribution, configuration of the extra-framework content, possible hydrogen bonding scheme) reported in this study is the key to understand the high thermo-elastic stability of pollucite, the immobility of Cs at non-ambient conditions, and the extremely low leaching rate of Cs, which make this open-framework silicate a promising material with potential use for fixation and deposition of Cs radioisotopes

Homogenization temperature variations of several degrees Celsius or more are often observed within a group of fluid inclusions that appear to have all trapped the same homogeneous fluid at the same time and presumably at the same PTX conditions. For inclusions that homogenize at T ≤ ≈230°C, much of the observed variation can be attributed to the size of the inclusions. Larger inclusions homogenize at higher temperatures compared to smaller inclusions with the same density. The relationship between inclusion size and observed homogenization temperature is predicted by the Young-Laplace equation that relates the stability of a vapor bubble to the surface tension and pressure differential across the vapor-liquid interface. Vapor bubbles instantaneously collapse when the vapor bubble radius becomes less than the critical radius. During heating the critical radius of the vapor bubble is achieved at a lower temperature in the smaller inclusions. The critical vapor bubble radius varies from about 0.01 to ~3 μm for low-temperature aqueous fluid inclusions. The Gibbs surface free energy associated with the growth and collapse of vapor bubbles in pure H 2 O inclusions with critical radii ranging from 0.01 to 1 μm ranges from about 10-18 to 10-13 J/m 2 and increases with both increasing critical vapor bubble radius and homogenization temperature. As a result of surface tension effects, the highest measured homogenization temperature, obtained from the largest inclusion in the group of coeval inclusions, most closely approximate the homogenization temperature that would be expected based on the inclusion density. For inclusions ranging from a few to several tens of micrometers in diameter and having densities such that the homogenization temperatures are approximately <230°C, homogenization temperatures may vary by about 1-3°C, depending on the inclusion size

A suite of basaltic glasses were examined to determine how subtle compositional changes affect mid-infrared spectra (650 to 5400 cm -1 ). Glasses with different SiO 2 , FeO total, Fe 3+ /Fe 2+ , and alkali contents were synthesized in a gas-mixing furnace and analyzed using electron probe microanalysis, Mössbauer spectroscopy, and micro-reflectance Fourier transform infrared spectroscopy. The major mid-infrared spectral feature in silicate glasses is a broad peak located at ~900 to 1100 cm-1 arising from Si-(Al-)O asymmetric stretching vibrational modes. To accurately compare spectra of different glass compositions, we have applied the Kramers-Kronig (KK) transform to our spectra and examined the resulting absorption peak (KK abs. peak). The location of the KK abs. peak shifts to higher wavenumbers as SiO 2 content increases (1031-1054 cm -1 with SiO 2 from 47.18 to 55.57 wt%). For basaltic glasses with near-constant Al/(Al+Si), the full-width half maximum of the KK abs. peak decreases as alkali content increases (235-188 cm-1 with Na 2 O+K 2 O contents from 0.07 to 3.74 wt%). In contrast, the location and shape of the KK abs. peak are not affected by variations in total FeO (6.06-16.30 wt%) and Fe 3+ /Fe 2+ (0.05-1.17). Our results show that KK transformed mid-infrared spectra of basaltic glasses may be used to determine the SiO 2 contents in basaltic glasses, irrespective of FeOtotal and Fe 3+ /Fe 2+ , and the alkali contents if Al/(Al+Si) is known. These observations will aid in the interpretation of laboratory and remotely sensed IR spectra

The heat capacities of Sr and Ba metasilicate glasses and of a Mg silicate glass with only 44 mol% SiO 2 have been measured between 2 and 300 K with the Quantum Design Physical Property Measurement System. The derived vibrational entropies S 298 -S 0 are 50.90, 61.00, and 36.65 J/(mol⋅K) for Sr 0.5 Si 0.5 O 1.5 , Ba 0.5 Si 0.5 O 1.5 , and Mg 0.56 Si 0.44 O 1.44 glasses, respectively. Along with available data for Mg- and Ca-bearing glasses, these results indicate a regular variation of the partial molar vibrational entropy of the metal oxide as a function of the ionization potential of the cation. At very low temperatures, however, the excess heat capacity of barium metasilicate glass relative to Debye limiting T3 law is stronger than expected from such a trend, whereas Mg 1.12 Si 0.88 O 2.88 is the known glass whose C P deviates the less from this law.

The pressure evolution of bassanite (CaSO 4 ·½H 2 O) was investigated by synchrotron X-ray powder diffraction along three isotherms: at room temperature up to 33 GPa, at 109 °C up to 22 GPa, and at 200 °C up to 12 GPa. The room-temperature cell-volume data, from 0.001 to 33 GPa, were fitted to a third-order Birch- Murnaghan equation-of-state, and a bulk modulus K 0 = 86(7) GPa with K′ = 2.5(3) was obtained. The axial compressibility values are β a = 3.7(2), β b = 3.6(1), and β c = 2.8(1) GPa -1 (×10 -3 ) showing a slightly anisotropic behavior, with the least compressible direction along c axis. The strain tensor analysis shows that the main deformation occurs in the (010) plane in a direction 18° from the a axis. The bulk moduli for isotherms 109 and 200 °C, were obtained by fitting cell-volume data with a second-order Birch-Murnaghan equation-of-state, with K′ fixed at 4, and were found to be K 109 = 79(4) GPa and K 200 = 63(7) GPa, respectively. The axial compressibility values for isotherm 109 °C are β a = 2.4(1), β b = 3.0(1), β c = 2.5(1) (×10-3) GPa-1, and for isotherm 200 °C they are β a = 3.5(3), β b = 3.4(3), β c = 2.6(4) (×10-3) GP a-1 . These two bulk moduli and the 20°C bulk modulus, K 0,20 = 69(8) recalculated to a second-order Birch-Murnaghan EoS to be consistent, as well as the axial compressibilities, are similar for the three isotherms indicating that the thermal effect on the bulk moduli is not significant up to 200°C. The size variation of the pseudo-hexagonal channel with pressure and temperature indicates that the sulfate “host” lattice and the H 2 O “guest” molecule in bassanite do not undergo strong change up to 33 GPa and 200°C

Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72°C, composed of sodic and non-sodic glasses of surface activity caused by IV Al substituting in Q 4 (1Al) and Q 4 (2Al) positions, minor V Al, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H 2 O m in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H 2 O m molecules. Sodic glasses contain 71.80-77.77 wt% SiO 2 , are partially devitrified to crystallites (~5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (~10 μm size). Sodium-free glasses are more siliceous, with 74.84-83.88 wt% SiO 2 , show partial devitrification to crystallites (~5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (~10 μm size)

Simple polythermal extensions to two widely used isothermal equations of state, the Murnaghan and the Birch-Murnaghan, can lead to non-physical material behavior without proper parameterization: the thermal expansivity at high pressure can become negative. We show how this arises and propose a remedy using an approximation to the thermal relaxation of the bulk modulus. Using the revised equation of state for thermodynamic equilibrium calculations leads to low-pressure and -temperature behavior indistinguishable from the unmodified equation of state, yet extrapolates to high pressure and temperature without non-physical behavior

Enthalpies of formation of kornelite [Fe 2 (SO 4 ) 3 ·~7.75H 2 O] and paracoquimbite [Fe 2 (SO 4 ) 3 ·9H 2 O] were measured by acid (5 N HCl) solution calorimetry at T = 298.15 K. The samples were characterized chemically by an electron microprobe, and structurally by the means of single-crystal, in-house powder, and synchrotron powder X-ray diffraction. The refined structures for the two phases are provided, including estimates of the positions and concentration of non-stoichiometric water in structural channels of kornelite, location of the hydrogen atoms and the hydrogen bonding system in this phase. The measured enthalpies of formation from the elements (crystalline Fe, orthorhombic S, ideal gases O 2 and H 2 ) at T = 298.15 K are -4916.2 ± 4.2 kJ/mol for kornelite and -5295.4 ± 4.2 kJ/mol for paracoquimbite. We have used several algorithms to estimate the standard entropy of the two phases. Afterward, we calculated their Gibbs free energy of formation and constructed a phase diagram for kornelite, paracoquimbite, Fe 2 (SO 4 ) 3 ·5H 2 O, and Fe 2 (SO 4 ) 3 as a function of temperature and relative humidity of air. The topology of the phase diagram is very sensitive to the entropy estimates and the construction of a reliable phase diagram must await better constraints on entropy or Gibbs free energy of formation. Possible remedies of these problems are also discussed

Phases encountered in the hydration of monoclinic and trigonal anhydrous Fe 2 (SO 4 ) 3 and evaporation of Fe 2 (SO 4 ) 3 solutions at room temperature were determined using in situ and ex situ X-ray diffraction (XRD) under dynamic relative humidity (RH) control at room temperature (22-25 °C). Both monoclinic and trigonal forms of Fe 2 (SO 4 ) 3 remain anhydrous at 11% RH or below, and undergo the following phase evolution sequence: anhydrous Fe 2 (SO 4 ) 3 → (ferricopiapite, rhomboclase) → kornelite → paracoquimbite at RH between 33 and 53% as a function of time. Evaporation of aqueous Fe 2 (SO 4 ) 3 solutions at 40% < RH < 60% results in precipitation of ferricopiapite and rhomboclase during evaporation, followed by a transition to kornelite and then paracoquimbite. Evaporation at RH < 33% produced an amorphous ferric-sulfate phase. The presence of some iron sulfate hydrates and their stability under varying RH are not only determined by the final humidity level, but also the intermediate stages and hydration history (i.e., either ferricopiapite or paracoquimbite can be a stable phase at 62% RH depending on the hydration history). The sensitivity to humidity change and pathdependent transitions of ferric sulfates make them potentially valuable indicators of paleo-environmental conditions and past water activity on Mars. The phase relationships reported herein can help in understanding the diagenesis of ferric sulfate minerals, and are applicable to geochemical modeling of mineral solubility in multi-component systems, an endeavor hindered by the need for fundamental laboratory studies of iron sulfate hydrates

Significantly reduced detection of thermal radiation in gated spectroscopy allowed us to measure the Raman scattering of natural enstatite up to 1550 K at 1 bar. The intrinsic anharmonicity, ai = [(∂lnν i )/(∂T)] V , of the Raman-active modes in orthoenstatite (OEn) was obtained from temperature (T) shifts of vibrational frequencies measured in this study combined with previous high-pressure (P) Raman scattering data. Although the ai values of the lattice modes of OEn are similar to those for forsterite (Fo), the Si-O stretching modes have significantly lower intrinsic anharmonicity in OEn than in Fo, suggesting that the connectivity of the SiO 4 tetrahedra plays an important role in mode anharmonicity. At the phase transition at 1500 K, a doublet related to the stretching vibration of bridging O atoms in the SiO 3 chains becomes a singlet, and a doublet related to the stretching vibration of non-bridging O atoms remains as a doublet, consistent with the expected spectral change for a phase transition from OEn to protoenstatite. Two intense, low-frequency modes of OEn show a strong nonlinear decrease in frequencies with heating that cannot be explained solely by thermal expansion. This may indicate the reorganization of the structure around Mg atoms and unkinking of the SiO 3 chains at temperatures well below the phase transition

Optical absorption spectra of natural Co-bearing spinel and staurolite were studied at different temperatures and pressures. In both minerals, two broad, intense structured bands in the range 5500-8000 and 15 000-19 000 cm-1, caused by electronic spin-allowed transitions 4 A 2 → 4 T 1 (4F) and 4 A 2 → 4 T 1 ( 4 P) of IV Co 2+ are the predominant absorption features. In addition, in both cases broad bands, derived from spin-allowed electronic transitions 4E → 4T2 of IV Fe 2+ , appear in the near infrared range partly overlapping the bands caused by IV Co 2+ . In staurolite the NIR range of the spectra are complicated by intense sharp lines of OH-vibrations at around 3400 cm -1 . In spinel, with a regular tetrahedral site, the splitting of the spin-allowed bands I and II of IV Co 2+ is assumed to be caused by spin-orbit and vibronic coupling. In staurolite, the splitting is stronger due to the additional low-symmetry crystal field effect of IV Co 2+ . It is found that the effect of temperature and pressure on the behavior of the 4 A 2 → 4 T 1 ( 4 P) bands of IV Co 2+ in the two minerals are rather similar, in contrast to our findings for the spin-allowed bands of IV Fe 2+ in spinel and staurolite. This is interpreted as a manifestation of a dynamic Jahn-Teller effect for IV Fe 2+ and lack of it in case of IV Co 2+

The illitization reaction in a thick K-bentonite bed located in upper Cretaceous marine shale in the Montana disturbed belt was studied by X-ray diffraction, chemical analysis, and thermal gravimetric analysis. Modeling of the experimental XRD patterns from oriented clay specimens in air-dried and glycolated states shows that at each sample location in the bentonite bed a mixture of R0 illite-smectite (I-S) and R1 I-S coexist. Each of these phases in all samples consists of the same or similar content of illite and expandable layers independent on location in the bed. In particular, the illite content in the R0 I-S and the R1 I-S from the <0.5 μm fractions is equal to 30 and 62%, respectively. The main difference between the samples at different locations is the different weight concentrations of the coexisting I-S phases. The R1 I-S content decreases progressively from the lower and upper contacts of the bed to its center. The reverse trend was observed for the R0 I-S. The layer unit-cell parameter b increases from samples located near the middle of the bed toward samples near the bed margins. The DTG patterns of the samples contain two endothermic maxima at about 640 and 470 °C, corresponding to cis-vacant (cv) illite and trans-vacant (tv) smectite layers coexisting in the R1 I-S and R0 I-S. Analysis of the crystal-chemical features of the R1 I-S and R0 I-S shows that, in the middle of the bed, both phases are characterized by the lowest octahedral Mg and the highest tetrahedral Al contents. In the structural formula of the R1 I-S, the tetrahedral Al content is significantly higher than the (K+Na) content independent of sample location. In contrast, tetrahedral Al in the R0 I-S located near the bed boundaries is lower compared with (K+Na) content. To account for the crystal-chemical features of the coexisting I-S, a first assumption is that the initial volcanic ash was altered into tv smectite having a homogeneous Al-rich composition throughout the bed. Second, along with K, the active role in illitization was controlled by Mg. Mineralogical zonation of the K-bentonite is explained by the progressive migration of K from the margins toward the bed center with the associated decrease of K cations in the pore fluids. However, the decrease in K concentration was accompanied by a successive increase in R0 I-S content, but not a progressive decrease in illite layer content in a single I-S phase. The main role of Mg was to redistribute octahedral and tetrahedral Al in the 2:1 layers of the R0 I-S and R1 I-S in such a way that the amount of Al in the tetrahedral sheets increased at the expense of the substitution of Mg for Al in the octahedral sheet of the 2:1 layers in the initial smectite. These results demonstrate a new insight into mineralogical sequences of intermediate members of smectite illitization. Instead of a statistically homogeneous and continuous reaction associated with the increase of illite layers in I-S and the simultaneous increase of order of the layer stacking sequence, the illitization reaction in the thick K-bentonite consists of the formation of a physical mixture of two I-S having a contrasting content of layer types and their distribution. Factors responsible for the formation of the coexisting R0 I-S and R1 I-S are discussed

Kalsilite (the low-temperature form of KAlSiO 4 ) is used as the precursor of leucite, an important component in porcelain-fused-to-metal and ceramic-restoration systems, and it has also been proposed as a high-thermal expansion ceramic for bonding to metals. The present study reports the hydrothermal synthesis and characterization of pure kalsilite from kaolinite in subcritical conditions, as well as the characterization of the intermediate products by means of XRD, 29 Si and 27 Al MAS NMR, IR, SEM, and TEM. Effects of time, temperature, and pH on the reaction products are analyzed. The experimental data indicate that pure kalsilite is obtained after hydrothermal treatment of kaolinite at 300°C for 12 h in 0.5 M KOH solution. Longer reaction times increase the crystallinity of the structure, whereas lower reaction times give rise to the metastable ABW-type KAlSiO 4 polymorph. Lower temperatures are not sufficient to produce kalsilite, but zeolite W is obtained instead as the unique reaction product. Finally, the pH of the aqueous solution in contact with kaolinite is an important parameter for the synthesis of kalsilite, which must be ≥13.70

Sideronatrite, Na 2 Fe(SO 4 ) 2 (OH)·3H 2 O, is a secondary hydrated sulfate occurring in desert areas as the result of pyrite alteration. It is one of the environmental indicators of soil-water processes operating in specific landscapes, and, as a consequence, an important marker of acid mine drainage pollution. Sideronatrite has been demonstrated from its peculiar diffraction pattern to belong to a family of OD structures formed by equivalent layers. In this work, a crystal with weak diffuse streaks proved to be suitable for a single-crystal X-ray diffraction study. The crystal structure was solved by direct methods and refined by full matrix leastsquares (R = 7.4% and R W = 8.0%) in the space group P2 1 2 1 2 1 with a = 7.265(2), b = 20.522(6), c = 7.120(2) Å, V = 1061.5(5) Å 3 , and Z = 4, using 798 reflections with I > 3.0 σ(I). Sideronatrite is characterized by infinite [Fe 3+ (SO 4 ) 2 (OH)] 2- octahedral-tetrahedral chains of the type [M(TO 4 ) 2 φ] running parallel to the c axis. These chains are cross-linked by a columnar system of corner-sharing, Na-distorted octahedra along c to form corrugated sheets parallel to the (010) plane. Adjacent sheets are hydrogen-bonded through water molecules coordinated by Na atoms. The present results allow a complete description of the OD character of the structure, with the derivation of the OD groupoid and MDO polytypes. Finally, chemical and structural relationships are taken into account to explain the possible paragenetic sequence concerning several sulfates associated with sideronatrite

The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg(OH) 2 (brucite) has been calculated in the pressure range 0−22 GPa. Quantum-mechanical electronic structure (DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations. The following findings emerged: (1) The calculated dν(OH)/dP slope is -4 cm -1 /GPa, in agreement with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg(OH) 2 ]. (2) The calculated ν(OH) vs. R(O···O) correlation is linear and the slope is much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small dν/dP and dν/dR(O···O) slopes is the small electric field variation as the mineral layers are pressed toward each other. (4) At high pressure, the OH− ions show some tendency to be tilted with respect to the c axis, and a larger tilt angle leads to a larger ν(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately -1 kHz/GPa

The microscopic properties of biomineral hydrozincite [Zn 5 (CO 3 ) 2 (OH) 6 ] from Naracauli Creek (SW Sardinia) were investigated by using X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Because the biomineral hydrozincite turned out to significantly deviate from the ideal structure of hydrozincite, synthetic and geologic hydrozincite samples were also investigated for comparison. SEM imaging shows that biomineral hydrozincite is made of small platelet-shaped crystallites having a 20-50 nm long side at the shortest and other sides measuring hundreds of nanometers long. These are interlaced to form sheaths several micrometers long. HRTEM analysis of the biomineral samples shows an imperfectly oriented aggregation of the nanocrystals that is discussed in terms of mesocrystals. Transmission electron microscopy (TEM) and XRD analysis indicate a progressive decrease in the size of the particles in the biomineral compared to the synthetic and geologic hydrozincite samples, with coherent diffraction domains in the biomineral hydrozincite that are smaller by 30-50% than in the other samples investigated in this study. 13 C magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) NMR spectra show more than one peak for all the investigated samples, despite the fact that carbon atoms have a unique crystallographic position in the hydrozincite structure. The additional peaks can reflect the presence of lattice defects typical of nanocrystals as indicated by the HRTEM images, where high concentration of lattice defects, such as grain boundaries and stacking modes, can be observed both in the biomineral and in the synthetic samples. Further additional peaks in the NMR spectra of biomineral samples are attributed to organic molecules, relicts of the biomineralization process, in agreement with the filaments observed in SEM images of biomineral samples

The microstructural evolution of CaSiO 3 wollastonite subjected to carbonation reactions at T = 90 °C and pCO 2 = 25 MPa was studied at three different starting conditions: (1) pure water; (2) aqueous alkaline solution (0.44 M NaOH); and (3) supercritical CO2. Scanning and transmission electron microscopy on reacted grains prepared in cross-section always revealed unaltered wollastonite cores surrounded by micrometer-thick pseudomorphic silica rims that were amorphous, highly porous, and fractured. The fractures were occasionally filled with nanometer-sized crystals of calcite and Ca-phyllosilicates. Nanoscale chemical profiles measured across the wollastonite-silica interfacial region always revealed sharp, step-like decreases in Ca concentration. Comparison of the Ca profiles with diffusion modeling suggests that the silica rims were not formed by preferential cation leaching (leached layer), but rather by interfacial dissolution-precipitation. Extents of carbonation as a function of time were determined by quantitative Rietveld refinement of X-ray diffractograms performed on the reacted powders. Comparing the measured extents of carbonation in water (condition 1) with kinetic modeling suggests that carbonation was rate-controlled by chemical reactions at the wollastonite interface, and not by transport limitations within the silica layers. However, at conditions 2 and 3, calcite crystals occurred as a uniform surface coating covering the silica layers, and also within pores and cracks, thereby blocking the connectivity of the originally open nanoscale porosity. These crystals ultimately may have been responsible for controlling transport of solutes through the silica layers. Therefore, this study suggests that pure silica layers were intrinsically non-passivating, whereas silica layers became partially passivating due to the presence of calcite crystallites

The structure of argentopyrite, AgFe 2 S 3 , was determined for the first time with single-crystal X-ray diffraction. In contrast to the previously reported orthorhombic symmetry, our data show that argentopyrite is monoclinic with space group P1121/n (non-standard setting) and unit-cell parameters a = 6.6902(2), b = 11.4497(4), c = 6.4525(2) Å, γ = 90.2420(8)°, and V = 494.26(3) Å 3 . Similar to cubanite (CuFe 2 S 3 ), the structure of argentopyrite is also based on approximately hexagonal closepacked S atoms, with cations ordered over one half of the tetrahedral sites, forming corner-shared AgS 4 and FeS 4 tetrahedral sheets parallel to (001). The two structures differ chiefly in the linkage between the two adjacent tetrahedral sheets and the ordering patterns of cations within a tetrahedral sheet. Topologically, the structure of argentopyrite can be obtained by a displacement of a tetrahedral sheet in the cubanite structure along the (a/2 + b/6) direction relative to the sheet beneath, giving rise to a cluster of four edge-shared FeS 4 tetrahedra in argentopyrite, as compared to two in cubanite. There are two distinct Fe sites (Fe1 and Fe2) in argentopyrite, rather than only one, as in other MFe 2 S 3 sulfide minerals (M = monovalent cations). Together with published Mössbauer data, we suggest that there exists some degree of Fe 2+ -Fe 3+ order-disorder in argentopyrite, with Fe 2+ favoring the more distorted Fe2 tetrahedral site. Argentopyrite appears to possess all the features proposed by Putnis (1977) for a high-temperature ordered form of cubanite

We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite (X Fe = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite’s identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of Fe 3+ exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables T, f H₂O , f H ₂, and a SiO ₂ attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization

Molecular dynamics simulations are used to determine how a dissolved species alters the transport properties of a silicate melt. To identify the specific factors that affect the transport properties, we examine the effects of generic dissolved species for which the atomic interaction parameters can be systematically varied. We focus on the role of the size and charge of the dissolved species. Our results show that neutral dissolved species have negligible effects on the structure and bonding of the silica network, regardless of the size of the species. These neutral species are decoupled from the network, and can diffuse orders of magnitude faster than the network ions. In contrast, charged species strongly disrupt the silica network, which leads to significant enhancement of the transport properties (e.g., lower viscosity and higher diffusivity of the network ions). The effects of the charged dissolved species are strongly dependent on their size

High-resolution 29 Si and 27 Al NMR techniques have been applied to resolve the Si-Al distribution and coordination in the high-pressure CaAl 4 Si 2 O 11 (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The 29 Si MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400~1600°C show two broad, asymmetric peaks near -92.7 and -182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and Al, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The 27 Al MAS and 3Q MAS NMR spectra contain only one unresolved peak typical of octahedral Al with a range of quadrupolar coupling constants