Volume 100, Issue 2-3

Gaseous sulfur compounds are critical climate active volatile components released by volcanoes. Volcanic eruptions can emit massive amounts of H 2 S and SO 2 into the atmosphere, which react with oxygen and H 2 O to sulfuric acid. Formation of H 2 SO 4 aerosols, which have on a geological timescale short-term residence times of month or years, may have global climatic impact. The phase stability of sulfur-bearing minerals such as anhydrite in erupting magmas may be a key controlling factor limiting SO 2 emission during subaerial volcanic activity (Huang and Keppler, January 2015 issue of American Mineralogist).

Tweed, twin, and porous microstructures are traditionally studied in mineralogy to understand the thermal history of minerals, and to identify their properties such as chemical transport and elastic behavior. Recently, the same research area has blossomed in material sciences and physics with the aim to design and build devices that are based on the properties of nano-structures. Only the very existence and the properties of tweed, twins, and holes matters in this quest while the crystalline matrix plays only a minor role in the current search for novel device materials. This development has largely bypassed mineralogists while physicists did not profit from the age-long experience of mineralogists in dealing with such materials. In this Invited Centennial article, I will first discuss some key findings and approaches to foster the transfer of ideas in both directions: mineralogists can potentially inspire material scientists while the physics of the fine structure of twin walls and tweed can help mineralogists understand mineral properties in much more detail than hereto possible. Besides the observation that novel physical properties can spring from microstructures, most recent work also includes the dynamics of microstructures under external stress or electric fields. The dynamics is virtually always non-smooth or “jerky.” One of the best studied jerk distribution is that of collapsing porous minerals under stress, where the main focus of research is the identification of precursor effects as warning signs for larger events such as the collapse of mines, boreholes, or even regional earthquakes. The underlying physics is the same as in large earthquakes (which can be modeled but not observed in laboratory experiments). The agreement between laboratory experiments of porous collapse and large-scale earthquakes goes well beyond each quake’s statistics and includes waiting-time distributions and the Omori law of after-shocks. The same approach is used to characterize high-tech materials in aircraft industry and functional materials such as used in electronic memory devices, ferroelectric sensors and non-volatile memories and ferromagnets.

This review combines fluid inclusion data from (HP-)UHP rocks with experimental research and thermodynamic models to investigate the chemical and physical properties of fluids released during deep subduction, their solvent and element transport capacity, and the subsequent implications for the element recycling in the mantle wedge. An impressive number of fluid inclusion studies indicate three main populations of fluid inclusions in HP and UHP metamorphic rocks: (1) aqueous and/or non-polar gaseous fluid inclusions (FI); (2) multiphase solid inclusions (MSI); and (3) melt inclusions (MI). Chemical data from preserved fluid inclusions in rocks match with and implement “model” fluids by experiments and thermodynamics, revealing a continuity behind the extreme variations of physico-chemical properties of subduction-zone fluids. From fore-arc to sub-arc depths, fluids released by progressive devolatilization reactions from slab lithologies change from relatively diluted chloridebearing aqueous solutions (±N2), mainly influenced by halide ligands, to (alkali) aluminosilicate-rich aqueous fluids, in which polymerization probably governs the solubility and transport of major (e.g., Si and Al) and trace elements (including C). Fluid inclusion studies point to a reconsideration of the petrological models explaining deep volatile liberation, and their flux into the mantle wedge.

In situ X-ray diffraction study of the pyroxene to majorite transition in Na 2 MgSi 5 O 12 was carried out in Kawai-type high-pressure apparatus coupled with synchrotron radiation. The phase boundary between Na-pyroxene and Na-majorite was determined over the temperature interval of 1073-1973 K and was described by a linear equation P (GPa) = 12.39 + 0.0018×T (K). The Clapeyron slope (dP/ dT) determined in this study is similar to the one predicted by computer simulations (Vinograd et al. 2011) but smoother than the one obtained by quenched experiments (Dymshits et al. 2010). The presence of sodium in the system lowers the pressure of pyroxene-to-majorite transformation. For the first time Na-majorite was characterized using Raman spectroscopy. Raman peaks of Na-majorite are broader than pyrope due to the substitution of Mg 2+ for Na + at the X site. Both Si-O symmetric stretching (A 1g -n 1 ) and O-Si-O symmetric bending (A 1g -n 2 ) modes of Na-majorite are significantly shifted to higher frequencies relative to corresponding bands of pyrope. In contrast the A 1g -R (SiO 4 ) mode of Na-majorite (342 cm -1 ) displays a lower frequency than that of pyrope (365 cm -1 ). Our results enable further understanding of the mechanisms responsible for phase transformations in the Earth’s transition zone and lower mantle.

Crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent. Syn-eruptive crystallization, in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling, strongly influences the texture of erupted magma. This stage is difficult to isolate for study in natural rocks, but well-suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated. This study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression-driven degassing. Experimental samples in which crystal growth at both near-equilibrium and far-from equilibrium conditions were examined using electron backscatter diffraction (EBSD) analysis to ascertain crystallographic lattice orientations of individual crystals. The crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density: contiguous crystals with parallel faces are crystallographically continuous, whereas contiguous crystals with non-parallel faces have unrelated crystal lattice orientations and as such, represent separate crystals. In the highly undercooled sample, twinning is identified in ~87% of the crystals examined; in the near-equilibrium sample, 38% of the crystals are twinned. We find that the observed twinning is unlikely to be the result of deformation, transformation, or synneusis, but rather a result of growth defects introduced during the incipient stages of crystallization. We suggest internal structural defects (twins) control macroscopic morphological defects (embayments, swallowtails, and melt inclusions) as a result of the high energy of the twin plane boundary. Formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression, and plays a role in the development of some plagioclase-hosted melt inclusions.

The northern part of the Lunar Crater Volcanic Field (central Nevada, U.S.A.) contains more than 100 Quaternary basaltic cones and maars and related eruptive products. We focused on four informal units of different ages and locations in the field to test the compositional variability and magma ascent processes within the time span of an individual eruption and the variability between very closely spaced volcanoes with different ages. Based in whole-rock chemistry, mineral chemistry and the calculation of intrinsic properties (pressure, temperature, and oxygen fugacity) we found that individual magma batches were generated in the asthenospheric mantle from a heterogeneous garnet lherzolite/olivine websterite source by ~3-5% partial melting. Each magma batch and temporary deep reservoir was a separate entity rather than part of a continuous long-lived reservoir. Magmas ascended relatively fast, stalled and crystallized in the uppermost several kilometers of the mantle near the base of the crust and some also stalled at mid-crustal levels with minor or no geochemical interaction with surrounding rocks. Our data also suggest that volcanoes erupting within certain time windows had similar source characteristics and ascent processes whether they were located within a few hundred meters of each other or were separated by many kilometers.

Among the lunar samples that were returned by the Apollo missions are many cumulate plutonic rocks with high Mg# [molar Mg/(Mg+Fe) in %] and abundances of KREEP elements (potassium, rare earth elements, phosphorus, U, Th, etc.) that imply KREEP-rich parental magmas. These rocks, collectively called the magnesian suite, are nearly absent from sampling sites distant from Imbrium basin ejecta, including those of lunar highlands meteorites. This absence has significant implications for the early differentiation of the Moon and its distribution of heat-producing elements (K, Th, U). Here, we analyze a unique fragment of basalt with the mineralogy and mineral chemistry of a magnesian suite rock, in the lunar highlands meteorite Allan Hills (ALH) A81005. In thin section, the fragment is 700 × 300 μm, and has a sub-ophitic texture with olivine phenocrysts, euhedral plagioclase grains (An97-70),and interstitial pyroxenes. Its minerals are chemically equilibrated. Olivine has Fe/Mn ~ 70 (consistent with a lunar origin), and Mg# ~80, which is consistent with rocks of the magnesian suite and far higher than in mare basalts. It has a rich suite of minor minerals: fluorapatite, ilmenite, Zr-armalcolite, chromite, troilite, silica, and Fe metal (Ni = 3.8%, Co = 0.17%). The metal is comparable to that in chondrite meteorites, which suggests that the fragment is from an impact melt. The fragment itself is not a piece of magnesian suite rock (which are plutonic), but its mineralogy and mineral chemistry suggest that its protolith (which was melted by impact) was related to the magnesian suite. However, the fragment’s mineral chemistry and minor minerals are not identical to those of known magnesian suite rocks, suggesting that the suite may be more varied than apparent in the Apollo samples. Although ALHA81005 is from the lunar highlands (and likely from the farside), Clast U need not have formed in the highlands. It could have formed in an impact melt pool on the nearside and been transported by meteoroid impact. Lunar highlands meteorites should be searched for rock fragments related to the magnesian-suite rocks, but the fragments are rare and may have mineral compositions similar to some meteoritic (impactor) materials.

Trinitite is the glass formed during the first atomic bomb test near Socorro, New Mexico, on July 16, 1945. The protolith for the glass is arkosic sand. The majority of the glass is bottle green in color, but a red variety is found in the northern quadrant of the test site. Glass beads and dumbbells, similar in morphology to micro-tektites, are also found at the Trinity site. The original description of this material, which appeared in American Mineralogist in 1948, noted the presence of two glasses with distinctly different indices of refraction (n = 1.46 and 1.51-1.54). Scanning electron microscopy (SEM) and Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN) analysis is used to investigate the chemical composition and fine-scale structure of the glass. The glass is heterogeneous at the tens of micrometer scale with discrete layers of glass showing flow-like structures. The low index of refraction glass is essentially SiO 2 (high-Si glass), but the higher index of refraction glass (low-Si glass) shows a range of chemical compositions. Embedded in the glass are partially melted quartz (α-quartz as determined by X-ray diffraction) and feldspar grains. The red trinitite consists of the same two glass components along with additional Cu-rich, Fe-rich, and Pb-rich silicate glasses. Metallic globules are common in the red trinitite. In terms of viscosity, the high-Si and low-Si glasses differ by several orders of magnitude, and there is minimal mixing between the two glasses. QEMSCAN analysis reveals that there are several chemical subgroups (that can be characterized as simple mixtures of melted mineral components) within the low-Si glasses, and there is limited mixing between these glass subgroups. The red trinitite contains regions of Fe-rich glass, which show sharp contact with surrounding Fe-poor glass. Both the textural and chemical data suggest that these two glasses existed as immiscible liquids. The metallic droplets in the red trinitite, which consist of variable amounts of Cu, Pb, and Fe, show textural evidence of unmixing. These metals are largely derived from anthropogenic sources-Cu wire, Pb bricks, and the steel tower and bomb casing. The combination of mineralogical and chemical data indicate that temperatures on the order of 1600 °C and pressures of at least 8 GPa were reached during the atomic detonation and that there was a reducing environment during cooling, as evidenced by the presence of native metals, metal sulfides, and a low-Fe 3+ /Fe 2+ ratio. Independent estimates of maximum temperature during the detonation are on the order of 8000 K, far higher than suggested by the mineral data. This discrepancy is probably due to the very short duration of the event. In all respects, the trinitite glasses are similar to tektites and fulgurites, and by analogy one conclusion is that temperature estimates based on mineralogical observations for these materials also underestimate the maximum temperatures.

Pauloabibite (IMA 2012-090), trigonal NaNbO3, occurs in the Jacupiranga carbonatite, in Cajati County, São Paulo State, Brazil, associated with dolomite, calcite, magnetite, phlogopite, pyrite, pyrrhotite, ancylite-(Ce), tochilinite, fluorapatite, “pyrochlore”, vigezzite, and strontianite. Pauloabibite occurs as encrustations of platy crystals, up to 2 mm in size, partially intergrown with an unidentified Ca-Nb-oxide, embedded in dolomite crystals, which in this zone of the mine can reach centimeter sizes. Cleavage is perfect on {001}. Pauloabibite is transparent and displays a sub-adamantine luster; it is pinkish brown and the streak is white. The calculated density is 4.246 g/cm 3 . The mineral is uniaxial; n(mean)calc is 2.078. Chemical composition (n = 17, WDS, wt%) is: Na 2 O 16.36, MgO 0.04, CaO 1.36, MnO 0.82, FeO 0.11, SrO 0.02, BaO 0.16, SiO2 0.03, TiO2 0.86, Nb 2 O 5 78.66, Ta 2 O 5 0.34, total 98.76. The empirical formula is (Na 0.88 Ca 0.04 Mn 2+ 0.02 )S Σ0.94 (Nb 0.98 Ti 0.02 ) Σ1.00 -O 3 . X-ray powder-diffraction lines (calculated pattern) [d in Å(I)(hkl)] are: 5.2066(100)(003), 4.4257(82)(101), 3.9730(45)(012), 2.9809(54) (104), 2.3718(88)(21̄3), 1.9865(28)(024), 1.8620(53)(21̄6), and 1.5383(30)(300). It is trigonal, space group: R3̄, a = 5.3287(5), c = 15.6197(17) Å, V = 384.10(7) Å 3 , Z = 6. The crystal structure was solved (R 1 = 0.0285, wR 2 = 0.0636 for 309 observed reflections). Pauloabibite is isostructural with ilmenite and is polymorphic with isolueshite (cubic) and lueshite (orthorhombic). The name is in honor of Paulo Abib Andery (1922-1976).

Kojonenite, Pd 7-x SnTe 2 (where 0.3 ≤ x ≤ 0.8), is a new mineral (IMA 2013-132) discovered in specimens from the Stillwater Layered Igneous Intrusion, Stillwater Valley, Montana, U.S.A. (45°23′11′′N and 109°53′03′′W). Synthetic kojonenite is tetragonal, space group I4/mmm, with a = 4.001(1) Å and c = 20.929(3) Å giving V = 335.0(1) Å 3 with Z = 2. Strongest lines in the synthetic powder pattern [PDF 01-073-5652] are (d in Å, I, hkl) 10.465, 29, 002; 2.496, 52, 114; 2.1986, 100, 116; 2.0930, 18, 00 10; 2.0025, 48, 200. Identity between natural and synthetic kojonenite is illustrated by an electron backscattered diffraction (EBSD) study. Reflectance data for natural kojonenite for the four COM wavelengths are [λ (nm), Ro, Re′] 470, 55.0, 52.7; 546, 58.5, 56.5; 589, 61.0, 58.3; 650, 63.9, 60.1 and the mineral is uniaxial negative. Electron microprobe analysis of natural kojonenite yields a simplified formula of Pd 6 SnTe 2 that extends the range reported for the synthetic of Pd 7-x SnTe 2 (where 0.3 ≤ x ≤ 0.8).

A collection of 42 feldspar mineral samples from a wide geographical range of North America was examined by cold-cathode cathodoluminescence (CL) spectroscopy. Characteristic wavelength peaks, which were determined to be independent of geographic origin, were associated with each feldspar phase. Most of these peaks were attributed to previously assigned Mn 2+ and Fe 3+ luminescent centers and structural defects. An unattributed set of infrared (IR) peaks was observed in many samples; one uncommon ultraviolet (UV) peak was observed in samples from two locations. The peak centroids associated with the Mn 2+ and Fe 3+ luminescent centers vary with stoichiometric changes in the K-Na- Ca composition of the feldspars. For both alkali and plagioclase feldspars, shifts in CL peak centroids correlate well with lattice size, as measured by X-ray diffraction (XRD). Additional analyses of the feldspar samples by electron microprobe analysis (EMP), particle-induced X-ray emission spectroscopy (PIXE), energy-dispersive micro-X-ray fluorescence spectroscopy (μXRF), and/or laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were conducted for confirmation of elemental composition. These results demonstrate the potential of CL spectroscopy, a relatively nondestructive analytical technique, to facilitate rapid discrimination between feldspar samples. The addition of CL spectroscopy of feldspars to existing forensic analytical protocols for geologic materials has the potential to provide support for casework, both to discriminate sources in a forensic comparison, as well as to constrain the provenance of an unknown sample.

Tourmaline nodules occurring in the Capo Bianco (Elba Island, Italy) aplitic rocks are here investigated by X-ray microtomography 3D imaging. This non-invasive technique provides 3D images of the tourmaline nodules, revealing an irregular morphology consisting of branches that extend radially from the cores. The nodules present scale-invariant features that can be described by a box-counting fractal dimension. The value of the fractal dimension is proportional to the size of the nodules and tends asymptotically to a value of 2.5, in agreement with the results obtained from the simulation of virtual nodules, by means of a diffusion-limited aggregation model based on a Monte Carlo Metropolis algorithm, in which the growth probability at the tips of the nodule is an inverse function of the diffusion coefficient. The results support the hypothesis that tourmaline formed by a disequilibrium magmatic process, in which diffusion represents the rate-limiting step, inducing the formation of nodules with irregular shapes. This study shows the potential of X-ray microtomography, in combination with numerical modeling, as a probe for accessing the 3D microstructural information of complex mineral morphologies with a non-invasive approach. The combination of numerical and experimental, non-invasive, 3D techniques represents a fundamental step forward in bridging the gap between the observation of microstructures and the interpretation of the associated processes.

Water can form different chemical bonds with the ionic entities composing silicate melts. Because of that, its influence on the physico-chemical properties of magmas can vary with silicate composition and water content, temperature, and pressure. To further our understanding of how silicate chemical composition governs proton distribution in magmas, the environment of protons in hydrous alkali (Li, Na, K) silicate glasses was varied as a function of the type of alkali metal and total water content. From 1 H MAS NMR spectroscopy, H + are distributed among five different structural environments in alkali silicate glasses. One of these environments is in the form of H 2 O molecules (H 2 O mol ). The four others are the proton environments associated with Si-OH bonding, and perhaps also with M-OH bonding (with M = Li, Na, or K). Those environments differ in their O-O distance and extent of hydrogen bonding. H 2 O mol species are located in an environment with an O-O distance of ~290 pm. OH- groups are in environments with O-O distances from 240 to 305 pm. The ionic radius of the alkalis, and hence their ionic field strength, determines the fraction of water dissolved as H 2 O mol and OH - groups, as well as the distribution of protons in the various OH - environments. The mean volume of the H + oxygen coordination sphere was calculated using the 1 H + NMR signal intensity and the mean O-O distance around H + . Increasing ionic radius of the alkali metal in silicate glasses results in a decrease of this mean volume. The partial molar volume of water in the corresponding melts determined through other technics seems to vary in a comparable way. Therefore, the chemical composition of silicate melts may control the partial molar volume of dissolved water because of its influence on the structural environment of protons. This probably also plays a role in determining water solubility.

In situ mineralization of CO 2 in ultramafic rock-hosted aquifers is one of the promising solutions for decreasing CO 2 concentrations in the atmosphere. Naturally altered ultramafic rocks suggest that carbonation processes are controlled by local heterogeneities in the structure of the rock and fluid transport at the water-rock interfaces. We studied the role of rock crystallographic anisotropy relative to the global fluid flow direction on the mineralization of CO 2 by means of electron microscope analyses from the macro- to the micrometer scale (EBSD-FIB). The sample used for the measurements was a hot pressed olivine core percolated by water enriched in CO 2 (pCO 2 = 10 MPa) at 180 °C. During the percolation experiment, olivine was dissolved and two types of carbonates, dolomite, and magnesite, were precipitated on olivine surfaces. The results showed that the dissolution of olivine is controlled by its crystallographic properties as shown by the development of etch-pits only on the (010) ol planes and with elongated shapes parallel to the [010] ol axes. In contrast, the precipitation of carbonates is governed by hydrodynamic properties. Carbonates are heterogeneously distributed in the percolated rock. They are mainly located along the moderate (for dolomite) and the minor (for magnesite) flow paths, both oriented parallel to the principal fluid flow direction, which allow carbonates to be supplied with divalent cations (e.g., Ca 2+ , Mg 2+ , and Fe 2+ ). In these flow paths, carbonate growth is systematically oriented normal to the flow that facilitates the development of chemical gradients with cationic supersaturation conditions for carbonate precipitation near the walls. In natural systems, the (010) ol planes are parallel to the Moho and the (100) ol planes are vertical; our study suggests that flow of CO 2 -rich fluids will induce precipitation of carbonates localized along, and preferentially clogging, vertical flow paths while favoring olivine dissolution along horizontal fluid pathways. This dual control of structure and fluid flow on carbonation mechanisms could be an important parameter allowing sustainable CO 2 storage in peridotites, while limiting the risks of leakage toward the surface.

Because of its rare occurrence in modern sediments, as well as the difficulty in synthesizing it under low-temperature conditions in the laboratory, the origin of sedimentary dolomite has remained a long-standing enigma, often referred to as the “dolomite problem.” Recently, anaerobic microorganisms, such as sulfate-reducing bacteria and methanogens, have been recognized for mediating dolomite precipitation. However, the exact role of microorganisms in dolomite crystallization is still under debate and the possible involvement of anaerobic fermenting bacteria has not been studied. In this study, we characterized the effect of purified non-metabolizing biomass and bound extracellular polymeric substances (EPS) of a natural consortium of anaerobic microorganisms dominated by fermenting bacteria and sulfate-reducing bacteria on Ca-Mg carbonate precipitation. This natural consortium was enriched from sediments of Deep Springs Lake, California, where dolomite is still precipitating. Our data show that disordered dolomite, a precursor of some sedimentary stoichiometric ordered dolomite, can be precipitated in calcite-seeded Ca-Mg carbonate solutions containing purified non-metabolizing consortium biomass. Bound EPS extracted from the consortium culture were shown to be the active component that triggered the crystallization of disordered dolomite. Further experiments show that purified non-metabolizing biomass from pure cultures of both anaerobic fermenting and sulfate-reducing bacteria closely related to those organisms present in the consortium could also catalyze the precipitation of disordered dolomite. This study contributes to the understanding of the “dolomite problem” by revealing (1) the catalytic effect of bound EPS on Ca-Mg carbonate crystallization and (2) the possible involvement of anaerobic fermenting bacteria in sedimentary dolomite formation, which has not been reported previously.

Amphiboles have played a leading role in metamorphic petrology, from helping to define several metamorphic facies to forming the basis of geothermobarometry, if their thermodynamic mixing properties can be calibrated to the temperature and pressure of formation. Compositional variations of sodium- and sodium-calcium-amphiboles may reveal important information about paleo-subduction zones but have not been studied as much as the more common calcium-amphiboles. In this study we investigate the mixing properties of amphibole solid solutions between magnesio-hornblende and glaucophane [ B Ca 2 C (Mg 4 Al) T (AlSi 7 )O 22 (OH) 2 - B Na 2 C (Mg 3 Al 2 ) T (Si 8 )O 22 (OH) 2 ] as a binary sub-join within the ternary amphibole system tremolite-glaucophane-tschermakite where the principal substitutions are Ca for Na at the B, Al for Mg at the C, and Al for Si at the T crystallographic sites. Amphiboles were made from mixtures of reagent oxides at 10 mol% increments between magnesio-hornblende and glaucophane, formed in a piston-cylinder press at 735-860 °C and 1.3-2.5 GPa for 72-216 h giving good yields (92-100 wt%). A positive deviation is present in the volume-composition plot, even after correcting volumes for non-binary components, supporting the presence of a positive deviation in the enthalpy of mixing (ΔH mix ) along this join. Fourier transform infrared spectra (FTIR) were obtained in the range of 350-4000 cm -1 for the mid-infrared spectra (MIR) for the purpose of estimating the extent of short-range ordering and for autocorrelation analysis, and, in the 650-50 cm -1 , for far-infrared spectra (FIR) for autocorrelation analysis. Autocorrelation analysis gave δΔCorr values, which further support a positive deviation in the ΔH mix along the magnsio-hornblende-glaucophane join, although the δΔCorr maximum did not occur at the calcium-poor (i.e., glaucophane-rich) portion of the join as expected. Synthetic end-member glaucophane and magnesio-hornblende were mixed in a molar ratio of 1:1 and allowed to equilibrate by homogenization for variable durations in the range of 600-800 °C at 2.0 GPa to determine the maximum-width of the miscibility gap. These compositional re-equilibration experiments suggested the presence of an asymmetric miscibility gap (steeper toward glaucophane) with a critical-point below 700 °C. Combining the results of this study with previously published results on the tremolite-glaucophane join allowed refinement of several asymmetric formalism mixing parameters (i.e., W Gl,Ts = 20 kJ, α Ts = 1.2) and modeling of the miscibility gap within the tremolite-glaucophane-tschermakite ternary system. The results showed that the composition of the critical point is very close to the maximum in the autocorrelation parameter δΔCorr, as one would predict. An important implication of this study is that low-temperature immiscibility between calciumand sodium-rich amphiboles may be more important than the role of pressure, as proposed by Brown (1977), in accounting for the change in B-site Na contents of metamorphic amphiboles.

Ca-Na ordering and structural polarity of subcells in an intermediate plagioclase with modulated structure have been observed using Z-contrast imaging with an aberration-corrected scanning transmission electron microscope. Neighboring lamellar domains with I1̄ symmetry are related by inversion twin operation, instead of anti-phase domain boundaries (or APBs) as in all previously reported structure models. The boundaries between lamellar domains have I1̄ symmetry instead of C1̄ symmetry. Modulated plagioclase has unique Ca-Na and Al-Si ordering structure that is different from those in end-member structures of anorthite and low albite. The modulated structures of intermediate plagioclase are not metastable structures formed during phase transition, but instead thermodynamically stable structures at low temperature due to Ca-Na ordering within the subcells with I1 symmetry.

We analyze here the technical connection between the (104) calcite and (001) montmorillonite surfaces at the atomic level, as well as the conditions to obtain a stable assembly. To this end, the appropriate force field is used to describe the bonding character at the interface. Results show that, in the x direction, the elastic energy provided by calcite to ensure good adhesion at the interface is lower than the one of clay. While in the y direction, it is rather the clay’s atoms that provide energy for the accommodation to calcite. Furthermore, we have evaluated, additionally, the adhesion energy, the interfacial distance and the work of separation. We also show that the presence of water molecules at the interface absorb the excess energy and thus contribute to the stabilization of the interface.

Hydrothermal experiments were conducted at ca. 1 to 7000 bars and 700 to 1250 °C in 121 rhyolitic to basaltic systems to determine Cl solubility in silicate melts, i.e., the maximum Cl concentration in melts that are saturated in a hydrosaline liquid with or without an aqueous or aqueous-carbonic vapor. The Cl concentration of melts increases with the Cl contents of the fluid unless the melt coexists with vapor plus hydrosaline liquid at fixed pressure and temperature; this phase assemblage buffers the Cl content of each phase with increasing Cl in the system. The Cl content of fluid(s)-saturated melts is independent of the CO 2 concentration of the saline liquid ± vapor with up to 21 wt% CO 2 in the fluid(s). The experiments show that Cl dissolution in aluminosilicate melts increases with temperature and pressure. Chlorine solubility is also a function of melt composition; it increases with the molar ([Al 1/2 +Ca 1/2 +Mg 1/2 +Na]/Si) of the melt. These experimental data have been integrated with results involving 41 other experiments (Webster and De Vivo 2002) to develop a broadly expanded model that supports calculation of Cl solubility in 163 aluminosilicate melts. This empirical model applies to Cl dissolution in melts of most silicate magmas at depths as great as 25 km. It determines the exsolution of hydrosaline liquid, with or without a coexisting vapor, as magmas ascend from depth, cool, crystallize, and differentiate from mafic to felsic compositions. In combination with H 2 O solubility models, our model supports determination of H 2 O-Cl solubility relations for most aluminosilicate magmas and is useful for barometric estimations based on silicate melt inclusions containing low CO 2 and moderate to high-Cl concentrations. The model is applied to the phase relations of fluids in volatile-enriched magmas of Augustine volcano, Alaska. The Cl and H 2 O concentrations of melt inclusions from 14, basaltic to dacitic eruptive units are compared with modeled solubilities of Cl and H 2 O in Augustine melts. The majority of these eruptions involved magmas that first exsolved aqueous to aqueous-carbonic vapors when the melts were dacitic in composition (i.e., before the residual melts in these magmas had evolved to felsic compositions) and well prior to the eruptions. Hydrosaline liquid with or without a vapor phase exsolved from other, more-felsic fractions of Augustine melts at low, near-surface pressures of several tens of bars.

A suite of Fe-bearing natural and synthetic grossular-rich [(Ca,Fe) 3 (Al,Fe) 2 Si 3 O 12 ] and pyrope-rich [(Mg,Fe) 3 Al 2 Si 3 O 12 ] garnets were investigated using 27 Al and 29 Si MAS NMR and 57Fe Mössbauer spectroscopy. This was done to study the state of cation order-disorder in garnet solid solutions by analyzing paramagnetically shifted resonances in high-resolution NMR spectra. The Mössbauer spectra, along with electron microprobe results, give the concentrations of Fe 2+ and Fe 3+ and their site occupancies, even in grossular samples with very low concentrations of Fe. MAS NMR spectra were collected on Fe 2+ -bearing grossular- and pyrope-rich garnets with up to 25 mol% almandine component and on other Fe 3+ -bearing grossular samples with up to 9 mol% andradite component. Despite peak broadening and signal loss, structural information was even obtained from garnet with relatively high Fe contents (25 mol% almandine component). Paramagnetically shifted NMR peaks, related to the presence of Fe2+, were observed in grossular samples and are similar in nature to those reported previously for natural, relatively low-Fe 2+ pyrope garnets by Stebbins and Kelsey (2009). Additional NMR peaks appear as the concentration of Fe 2+ increases, reflecting an increase in the average number of neighboring Fe2+ cations around AlO 6 and SiO 4 groups. These newly observed peaks hold potential to provide information concerning the presence or absence of short-range ordering in certain Fe-bearing silicate garnets. The effect of Fe 3+ on the MAS NMR spectra of garnet appears to be less pronounced, because it does not produce any observable paramagnetically shifted resonances.

Minor and trace elements can substitute into the crystal lattice of galena at various concentrations. In situ LA-ICP-MS analysis and trace element mapping of a range of galena specimens from different deposit types are used to obtain minor/trace element data, aimed at achieving insight into factors that control minor/trace element partitioning. The previously recognized coupled substitution Ag + +(Bi,Sb) 3+ ↔ 2Pb 2+ is confirmed. However, the poorer correlation between Ag and (Bi+Sb) when the latter elements are present at high concentrations (~>2000 ppm), suggests that site vacancies may come into play: [2(Bi,Sb) 3+ +o ↔ 3Pb 2+ ]. Galena is the primary host of Tl in all mapped mineral assemblages. Along with Cu, Tl is likely incorporated into galena via the coupled substitution: (Ag,Cu,Tl) + +(Bi,Sb) 3+ ↔ 2Pb 2+ . Tin can reach significant concentrations in galena (>500 ppm). Cd and minor Hg can be incorporated into galena; the simple isovalent substitution (Cd,Hg) 2+ ↔ Pb 2+ is inferred. This paper shows for the first time, oscillatory and sector compositional zoning of minor/trace elements (Ag, Sb, Bi, Se, Te, Tl) in galena from two epithermal ores. Zoning is attributed to slow crystal growth into open spaces within the vein at relatively low temperatures. The present data show that galena can host a broader range of elements than previously recognized. For many measured elements, the data sets generated display predictable partitioning patterns between galena and coexisting minerals, which may be dependent on temperature or other factors. Trace element concentrations in galena and their grain-scale distributions may also have potential in the identification of spatial and/or temporal trends within individual metallogenic belts, and as markers of ore formation processes in deposits that have undergone superimposed metamorphism and deformation. Galena trace element geochemistry may also display potential to be used as a trace/minor element vector approach in mineral exploration, notably for recognition of proximal-to-distal trends within a given ore system.

In ferruginous soils, nano- to microscale hematite (α-Fe 2 O 3 ) plays a central role in redox processes and contaminant cycling. Hematite is known to incorporate structural OH - and water, and the requisite charge balance is achieved by iron vacancies. Prior researchers have suggested that the defective hematite structures form unique phases called “protohematite” and “hydrohematite.” Infrared and Raman spectroscopic studies have assigned a lower-symmetry space group to “hydrohematite” (R3̄c) relative to that of stoichiometric hematite (R3̄c). However, the existence and structure of these phases have been contentious, largely due to the lack of in situ X-ray diffraction data Here we present a new structure refinement for “hydrohematite” precipitated hydrothermally at 200 °C in a monoclinic space group (I2/a) using time-resolved synchrotron X-ray diffraction (TRXRD) data collected during the in situ hydrothermal precipitation of akaganeite and its transformation to stoichiometric hematite. Distinct peak splitting was observed in the “hydrohematite” diffraction patterns, indicating a violation of the threefold rotational symmetry. A monoclinic unit cell with parameters of a = 7.3951(10), b = 5.0117(5), c = 5.4417(7) Å, β = 95.666(5)° provided a good fit and significant reduction in χ 2 and R wp relative to space group R3̄c. Rietveld analyses revealed that water concentrations in the first-formed crystals of “hydrohematite” were comparable to water contents of akaganeite and goethite. Thus, the hydrothermal transformation of akaganeite to “hydrohematite” is promoted not by dehydration but by reconstruction of the oxygen framework.

A 57 Fe Mössbauer spectroscopic study of natural eosphorite, (Mn,Fe 2+ )AlPO 4 (OH) 2 H 2 O, is reported. The Mössbauer spectra were collected at temperatures between 295 and 4.2 K. At temperatures exceeding 30 K, the Mössbauer spectra consist of a somewhat broadened quadrupole doublet with a splitting of 1.73 mm/s at 295 K. From the spectrum recorded at 250 K in an external magnetic field of 60 kOe, it is derived that the sign of the principal component of the electric field gradient (EFG) is negative and that the EFG’s asymmetry parameter is large (~0.5). From these findings, it is concluded that the octahedral coordination of the ferrous cations exhibits in first order a trigonal compression with a further lowering from axial symmetry due to an additional distortion. The spectrum collected at 4.2 K shows the existence of magnetic ordering. It was analyzed in terms of the full hyperfine interaction Hamiltonian, yielding a magnetic hyperfine field value of 146 kOe and EFG characteristics that are fully in line with those obtained from the external-field spectrum. In the temperature range from 10 to 30 K, the spectra indicate the occurrence of relaxation effects. They could be satisfactorily described in terms of the Blume-Tjon (BT) model for electronic relaxation, assuming a superposition of a slow- and a fast-relaxation component. The observed temperature dependence of the isomer shift is adequately described by the Debye model for the lattice vibrations. The characteristic Mössbauer temperature was found as (360 ± 20) K and the zero-Kelvin intrinsic isomer shift as (1.480 ± 0.005) mm/s. The variation of the quadrupole splitting with temperature is explained by the thermal population of the Fe 2+ electronic states within the T 2g orbital triplet, which is split by the trigonal crystal field in a singlet ground state and an upper doublet state, the latter being further split into two orbital singlets by an additional distortion. The energies of these latter excited states with respect to the ground state are calculated to be 280 ± 20 and 970 ± 50 cm -1 , respectively, based on a point-charge calculation of the 5 D level scheme. This calculation confirms the sign of the V zz component of the electric field gradient being negative as it is determined from the external magnetic field measurement and from the magnetic spectrum acquired at 4.2 K. Iron phosphates are widely spread minerals in the Earth’s crust and are expected to occur in soils and rocks on Mars as well. Mössbauer spectroscopy as complementary source of information to the results of other analytical techniques, could therefore be useful to identify and characterize the environmental Fe-bearing phosphates.

The pressure-volume-temperature (P-V-T) equation of state (EoS) of synthetic uvarovite has been measured at high temperatures up to 900 K and high pressures up to 16.20 GPa, by using in situ angle-dispersive X-ray diffraction and diamond-anvil cell. Analysis of room-temperature P-V data to a third-order Birch-Murnaghan EoS yielded: V 0 = 1736.9 ± 0.5 Å 3 , K 0 = 162 ± 2 GPa, and K′ 0 = 4.5 ± 0.3. With K′ 0 fixed to 4.0, we obtained: V 0 = 1736.5 ± 0.3 Å 3 and K 0 = 164 ± 1 GPa. Fitting of our PV- T data by means of the high-temperature third-order Birch-Murnaghan equations of state, given the thermoelastic parameters: V 0 = 1736.8 ± 0.8 Å 3 , K 0 = 162 ± 3 GPa, K′ 0 = 4.3 ± 0.4, (∂K/∂T)P = -0.021 ± 0.004 GPa/K, and α 0 = (2.72 ± 0.14)×10 -5 K -1 . We compared our elastic parameters to the results from the previous studies for uvarovite. From the comparison of these fittings, we propose to constrain the bulk modulus and its pressure derivative to K 0 = 162 GPa and K′ 0 = 4.0-4.5 for uvarovite. Present results were also compared with previous studies for other ugrandite garnets, grossular and andradite, which indicated that the compression mechanism of uvarovite might be similar with grossular and andradite. Furthermore, a systematic relationship, K 0 (GPa) = 398.1(7)-0.136(8) V 0 (Å 3 ) with a correlation coefficient R 2 of 0.9999, has been established based on these isostructural analogs. Combining these results with previous studies for pyralspite garnets-pyrope, almandine, and spessartine-the compositional dependence of the thermoelastic parameters (bulk modulus, thermal expansion, and the temperature derivative of the bulk modulus) were discussed.

Scanning electron microscope and electron microprobe analyses of 3 to 15 μm diameter grains present within a garnet-quartz granofels from a tungsten skarn reveal the possible existence of at least two new rare earth element (REE)-bearing phosphate phases: Ca(Ce,La,Bi,Nd) 2 [(P,As)O 4 ] 2 (OH) 2 and Ca(La,Ce,Nd,Pr,Bi) 2 [(P,As)O 4 ] 2 (OH) 2 . The analyzed REEs constitute up to 50 wt% of the phases; bismuth oxide contents range from 4.1 to 16.1 wt%. Structural data has proved impossible to obtain from these tiny grains, presumably due to radiation damage by thorium decay. These potentially new phosphate minerals are present within alteration assemblages of REE-rich epidote crystals, as well as along grain boundaries and cracks cross-cutting the quartz-garnet host rock. Association with the zeolite brewsterite-Ba suggests that these hydroxyl phosphates formed during water-rich, low-temperature, retrograde mineralization in the skarn environment.

The extent to which secondary Sb minerals control Sb dispersion in the supergene environment is yet to be fully understood. Stability studies of klebelsbergite have been undertaken to better under- stand its role in controlling Sb mobility and relationships with other secondary Sb minerals. Solubility in aqueous 0.1084 M HNO 3 was determined at 298.15 K and the data obtained used to derive ∆G f θ (klebelsbergite, s, 298.15 K) = -2056.4 ± 5.0 kJ/mol. Solubility data have been used to deduce the conditions under which the mineral can form as a thermodynamically stable phase. The single-crystal X-ray structure of synthetic klebelsbergite has been determined 293 K and is essentially the same as that reported earlier for atoms with Z ≥ 8. Crystal data: orthorhombic, space group Pca2 1 , a = 5.7563(4), b = 11.2538(7), c = 14.8627(9) Å, V = 962.81(11) Å 3 , Z = 4. Refinement converged to R1 = 0.0154 for 2206 unique reflections with I > 2σ(I). The present study has located the hydroxyl H atoms on both O5 and O9. The H-bond arrangements are somewhat different to those proposed earlier with the quasi-linear O9-H···O3 interaction having <(DHA) = 171(6)°. The O5 hydroxyl H atom lies in a different position to that proposed earlier and is involved in a bifurcated H-bond arrangement with O2 and with itself in a symmetry-related position, with <(DHA) = 133(5) and 125(5)°, respectively.

A mica-group mineral characterized by a high V content and free of Al was found in the mangane- siferous beds within the metacherts of the ophiolitic sequences at the Cerchiara mine, Eastern Liguria (Italy), in association with hematite, quartz, and calcite. Chemical and structural characterization supported by Raman data defines this phase as a new mineral species, which is named balestraite after Corrado Balestra, a prominent Italian amateur mineralogist. Balestraite, ideally KLi 2 V 5+ Si 4 O 10 O 2 , is a 1M trioctahedral mica crystallizing in the C2 space group, with a = 5.2024(5), b = 8.9782(7), c = 9.997(2) Å, β = 100.40(2)°, V = 459.3(1) Å 3 , Z = 2. The reduction of symmetry from the “ideal” space group C2/m is related to the ordering of V at only one of the two pseudo-symmetric octahedral sites. Vanadium forms very distorted octahedra with a [2+2+2] geometry characteristic of the valence state +5. The Li,V composition of the octahedral sheet, the pure tetrasilicic character of the tetrahedral sheet, and the anhydrous character produce unusual geometrical features for this mica. The occurrence of 5+ as the dominant valence state of V and the virtually complete O 2- → OH - substitution at the O4 site indicate strongly oxidizing conditions of crystallization, which are consistent with balestraite occurring at the boundary between carbonate-bearing veins and hematite bands. The new mineral and name were approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2013-080).

A method for the prediction of the enthalpies of formation ΔH O f for minerals of hydrous sulfates is proposed and is decomposed in the following two steps: (1) an evaluation of ΔH O f for anhydrous sulfates based on the differences in the empirical electronegativity parameter Δ H O= M z+ (c) characterizing the oxygen affinity of the cation M z+ ; and (2) a prediction of the enthalpy of hydration based on the knowledge of the enthalpy of dissolution for anhydrous sulfates. The enthalpy of formation of sulfate minerals from constituent oxides is correlated to the molar fraction of oxygen atoms bound to each cation and to the difference of the oxygen affinity Δ H O= M z+ (c) between any two consecutive cations. The Δ H O= M z+ (c) value, using a weighing scheme involving the electronegativity of a cation in a given anhydrous sulfate, is assumed to be constant. This value can be calculated by minimizing the difference between the experimental enthalpies and calculated enthalpies of formation of sulfate minerals from constituent oxides. The enthalpy of hydration is closely related to the nature of the cation in the anhydrous salt, to the number of water molecules in the chemical formula and to the enthalpy of dissolution for the anhydrous salt. The results indicate that this prediction method gives an average value within 0.55% of the experimentally measured values for anhydrous sulfates and 0.21% of the enthalpies of hydration or hydrous sulfates. The relationship between Δ H O= M z+ (sulfate), which corresponds to the electronegativity of a cation in a sulfate compound, and known parameter Δ H O= M z+ (aq) was determined. These determinations allowed the prediction of the electronegativity of some anhydrous transition metal double sulfate and contributed to the prediction of the enthalpy of formation for any hydrous double sulfate. With a simplified prediction of the entropy of a hydrous sulfate, calculations of Gibbs free energy of formation can be evaluated and contribute to the knowledge of the stability of some hydrous sulfates in different environmental conditions such as temperature or air moiety. Therefore, to check the reliability of the predictive model, stability fields for some hydrous ferric sulfates such as pentahydrate ferric sulfate, lawsonite, kornelite, coquimbite, and quenstedtite vs. temperature and relative humidity were studied and compared with experimental measurements.

The crystal structure of a novel Mg 2 Fe 2 O 5 oxide synthetized at 15 GPa and 1550 °C has been determined by means of single-crystal X-ray diffraction. This compound is isostructural with Fe 4 O 5 and can be considered as the other end-member of a solid solution between these two oxides involving the substitution of Fe 2+ for Mg. The resulting unit-cell lattice parameters a = 2.8889(4), b = 9.7282(4), and c = 12.5523(7) Å are smaller than those of Fe 4 O 5 . Mg and Fe 3+ cations are found to be disordered among the three crystallographic sites of the Mg 2 Fe 2 O 5 structure, although preference of Mg for the trigonal prism coordination (M3) is observed. Substitution of Mg into the Fe 4 O 5 structure reduces the octahedral distortion of both the M1 and M2 sites. Like Mg, Cr has recently been found to substitute into Fe 4 O 5 , so that Fe 3+ /ΣFe can vary from 0 to 1.0 in the Mg-Cr-Fe oxides system. Substitution of both Mg and Cr in Fe 4 O 5 also makes this phase more relevant for bulk compositions expected in the Earth’s transition zone and deep upper mantle. M4O5 phases having the CaFe 3 O 5 -type structure, therefore, need to be considered as a new addition to the phase relations of several simple oxide systems at pressure and temperature conditions at which the spinel-structured phases become unstable.

Effects of H 2 O on the solution behavior of fluorine and chlorine in peralkaline sodium aluminosilicate glasses quenched from melts at high temperature (1400 °C) and pressure (1.5 GPa) were studied by combining solubility measurements and Raman spectroscopy. With increasing H 2 O content from 0 to ~10 wt%, the fluorine solubility increases from 3.3 to 4.4 mol% in Al-free glasses and from 6.3 to 9.3 mol% in Al-rich glasses (10 mol% Al 2 O 3 ). In contrast, in the same H 2 O concentration range the chlorine solubility decreases from 5.7 to 3.4 mol% in Al-free glasses and from 3.6 to 1.7 mol% in Al-rich glasses. In Al-free glasses, interaction between H 2 O and the silicate to depolymerize the network is Q 4 + H 2 O ↔ Q 2 (H) and Q 3 + H 2 O ↔ Q 2 (H). The effect of water on silicate melt structure is different in halogen-bearing melts because in hydrous melt systems both F and Cl can act to depolymerize the melt further. For fluorine, this is accomplished via formation of Si-F, Al-F, and Na-F bonding in addition to Si-OH, whereas in chlorine-bearing hydrous melts, there is no interaction between Si 4+ and Cl - . The halogen solubility in the magmatic liquid influences mineral/melt partition coefficients of chlorine and fluorine and implies partition coefficients different from unity. Moreover because of the contrasting effects of H 2 O on fluorine and chlorine solubility, the Cl/F ratio in magmas formed in water-rich environments such as subduction zones can be a sensitive indicator of H 2 O content during arc magmas genesis. Transport properties of melts, such as diffusion and viscosity, also vary differently in halogen-bearing hydrous melts compared with halogen-free systems. Moreover, those effects on melt properties are the strongest in F-bearing systems.

The elastic constants C ij of a set of synthetic single crystals belonging to the join MgAl 2 O 4 -MnAl 2 O 4 (spinel sensu stricto-galaxite) were determined by Brillouin spectroscopy at ambient conditions. The C 11 component tends to remain constant for Mg-rich compositions (X Mn < 0.5) and decreases in Mn-rich compositions, whereas C 12 increases and C 44 decreases almost linearly from MgAl 2 O 4 to MnAl 2 O 4 . The bulk modulus K S is weakly dependent upon Mg-Mn substitution within experimental uncertainties, whereas the shear modulus G decreases with increasing Mn 2+ content. For MnAl 2 O 4 , C 11 = 271.3(1.3) GPa, C 12 = 164.8(1.3) GPa, C 44 = 124.9(5) GPa, K S = 200(1) GPa, and G = 88.7(5) GPa. Based on the “polyhedral approach,” we developed a model that describes the crystal bulk moduli of the MgAl 2 O 4 -MnAl 2 O 4 spinels in terms of their cation distribution and the polyhedral bulk moduli of the different cations. We refined a set of values for the effective polyhedral bulk modulus of Mg, Mn 2+ , and Al in tetrahedral (T) and octahedral (M) sites, which span from 153 to 270 GPa ranking as follows: K M Mn < K M Mg < K T Mg ≈ K T Mn < K M Al << K T Al . Crystal bulk modulus was perfectly reproduced within 0.1% for all Mn 2+ -bearing samples. We also found a high linear correlation between the effective polyhedral bulk modulus and the ionic potential, IP, of the coordinating cations: K j i (GPa) = 20(2) IP + 108(10) (where i indicates the cation and j the polyhedral site). We tested this simple correlation by calculating the specific effective polyhedral bulk modulus of several cations in T and M coordination and then predicting the crystal bulk modulus for several spinel compositions. The success of our simple correlation in modeling the bulk modulus of spinels outside the MgAl 2 O 4 -MnAl 2 O 4 system is encouraging, and suggests that the relationships between chemical composition, cation distribution and elastic properties in spinel-structured minerals and materials can indeed be expressed by relatively simple models.

Nickel aluminate is a transition metal oxide with spinel structure with potential applications as catalysts and sensors. Both applications benefit from high specific surface areas as well as chemical stoichiometry control. However, a systematic approach to understand synthetic parameters affecting stoichiometry and agglomeration of nickel aluminate nanoparticles is still lacking. In this work, coprecipitation using direct and reverse strikes and polymeric precursor techniques were comparatively studied to address this problem. While the polymeric method could deliver stoichiometric spinel, the samples were highly agglomerated exhibiting low surface area. Both co-precipitation procedures produced smaller sizes and less agglomerated nanoparticles as compared to the polymeric precursor, but for the reverse-strike, Ni 2+ preferentially formed a soluble complex with ammonia and led to nickel deficient nanoparticles. Stoichiometric (1 mol NiO:1 mol Al 2 O 3 ) nanocrystalline nickel aluminate was only achieved when using controlled excess Ni 2+ . The normal-strike lead to more stoichiometric compositions without need for excess cations, but the obtained nanoparticles were less homogeneous and showed smaller surface areas as compared to the reverse-strike method.