Band 99, Heft 7

Hydrogen-rich and water-rich fluids exert different control on dissolution mechanism of oxide and silicate minerals in the Earth’s interior. With Mgsilicate- H 2 fluids, dissolution tends to be incongruent with the (Mg/Si) fluid < (Mg/Si) Mg-silicate with formation of SiOH and SiH 4 complexes in the fluid (Shinozaki et al. 2013, 2014). In contrast, in Mg-silicate-H 2 O systems, Mg-silicate minerals in the mantle (pyroxene and forsterite) dissolve stoichiometrically (congruently) in aqueous fluids to at least 10 GPa pressure. Metasomatic alteration by H 2 -rich fluids enriches, therefore, the mantle in SiO 2 compared with alteration by H2O fluid. This difference becomes increasingly important with mantle depth because the environment becomes more reducing, which results in an increase of H 2 /H 2 O fluids (Shinozaki et al. 2014, this issue). Chemical gradients with depth of the Earth could be affected by increased H 2 /H 2 O of mantle fluids whereby Mg/Si ratios, for example, will become variable. Silicate-H 2 alteration processes likely also played major roles during the early, core-forming stages of the Earth. Such a process could be responsible for Mg/Si changes in the early silicate Earth.

The paper describing the new phosphate mineral correianevesite, recently published by Chukanov et al. (2014), clearly demonstrates the importance of pegmatite phosphates. For material scientists, these minerals are a source of new structure types; for petrologists, they constitute valuable tools necessary to accurately determine the temperature conditions prevailing during pegmatite differentiation processes.

The paper by Proyer et al. (2014; this issue) demonstrates that Ti- and Zr-bearing accessory minerals (rutile, titanite, zircon) record through their reaction textures part of the metamorphic history of a UHP marble. Calculation of relevant petrogenetic grids has the power to constrain the retrograde P-T path based on phase stability fields and the geothermobarometric evaluation of H2O-independent mineral reactions involving these Ti- and Zr-bearing minerals. They calculated simple petrogenetic grids in the system TiO 2 -ZrO 2 -CaO-MgO-Al 2 O 2 - SiO 2 -CO 2 -H 2 O (TZCMASCH) for calcite-dolomite marbles with forsterite/antigorite in excess and including those Ti- and Zr-bearing minerals for which thermodynamic data are known (rutile, titanite, geikielite, zircon, baddeleyite) and delineated their stability fields as well as the succession of their stability regions. This approach allowed the authors to infer the shape of the retrograde P-T path. Thus the combination of very careful petrography, calculated simple petrogenetic grids and the application of geothermometry involving these Ti- and Zr-bearing accessory minerals becomes an indispensible tool when reconstructing a metamorphic rock’s evolution.

We have characterized complex iron- and sulfate-bearing samples from Rio Tinto (Spain) using X‑ray diffraction (XRD), visible-near infrared reflectance (VNIR) spectroscopy, and laser Raman spectroscopy (LRS). Samples were collected for this study from the Peña de Hierro region of Rio Tinto because this site represents a natural acidic environment that is a potential analog for such environments on Mars. We report an evaluation of the capabilities of these three techniques in performing detailed mineralogical characterization of potential Mars-like samples from a natural acidic terrestrial environment. Sulfate minerals found in these samples include gypsum, jarosite, and copiapite, and iron hydroxide bearing minerals found include goethite and ferrihydrite. These sulfate and iron hydroxide/ oxyhydroxide minerals were detected by XRD, VNIR, and LRS. Minor quartz was identified in some samples by XRD as well, but was not identified using VNIR spectroscopy. Coordinating the results from these three techniques provides a complete picture of the mineralogical composition of the samples. Field instruments were used for this study to mimic the kinds of analyses that could be performed in the field or on martian rovers.

Phosphate is an essential nutrient for life on Earth, and therefore if life exists or ever existed on Mars it may have required phosphate. Amorphous Al- and Fe-phosphates rapidly precipitate from acidic solutions and amorphous Al-phosphates likely control phosphate concentrations in some natural waters on Earth. The amorphous fraction of martian soils has also been shown to be enriched in P, and amorphous phosphates are therefore also likely important in the phosphate cycle on Mars. Despite this importance, however, few dissolution rates exist for amorphous Al- and Fe-phosphates. In this study, dissolution rates of amorphous Al- and Fe-phosphates were measured in flow-through reactors from steady state concentrations of Al, Fe, and P. A pH-dependent rate law, log R = log k - npH was determined from the dissolution rates, where R is the dissolution rate, k is the intrinsic rate constant, and n is the reaction order with respect to H+. For amorphous Al-phosphate, log k = -6.539 ± 1.529 and n = 2.391 ± 0.493. For amorphous Fe-phosphate, log k = -13.031 ± 0.558 and n = 1.376 ± 0.221. The amorphous Al-phosphate dissolves stoichiometrically under all experimental conditions measured, and the amorphous Fe-phosphate dissolves non-stoichiometrically, approaching stoichiometric dissolution as pH decreases, due potentially to Fe oxyhydroxides precipitating and armoring grain surfaces. Perhaps due to these effects, amorphous Al-phosphate dissolution rates are approximately three orders of magnitude faster than the amorphous Fe-phosphate dissolution rates measured under these experimental conditions. Amorphous Al-phosphate dissolution rates measured in this study are also faster than published dissolution rates for the crystalline Al-phosphate variscite. The rapid dissolution rates measured in this study therefore suggest that, if these phases are present on Mars, phosphate would be rapidly released into acidic environments.

We study hydrated magnesium sulfate minerals from first-principles calculations based on densityfunctional theory. We determine the heat of hydration for MgSO 4 ·nH 2 O, compute the Raman and infrared spectra for several phases and calculate the S isotope partitioning as a function of hydration. We find that epsomite and meridianiite with, respectively, n = 7 and n = 11 water molecules per MgSO 4 unit are particularly stable with respect to other individual or combinations of hydration states. The Raman spectra of all phases present clear SO4 features that are easily identifiable. We use this to show one can use the vibrational spectroscopic information as an identification tool in a remote environment, like the martian surface. We discuss the character and atomic displacement pattern of all vibration modes and compute the 34S/32S partitioning; this work shows that hydration favors enrichment in the lighter S isotope 32S with respect to the heavier 34S, which is accumulated in the less hydrous structures. We show for the first time that the signature of 34S/32S partitioning could be observed by in situ spectroscopy on the surface of Mars. Moreover this can be related to the diurnal cycle of hydration and dehydration and hence it can improve the modeling of the water circulation on Mars.

Merrillite [Ca 9 NaMg(PO 4 ) 7 ] occurs as a dominant primary Ca-phosphate mineral in martian meteorites and therefore presumably also on Mars. The mineral is an important phase in exploring differences in geologic processes between Earth and Mars, and also has astrobiological implications due to its potential role as a significant source of the bio-essential nutrient phosphate. Merrillite does not occur terrestrially as a discrete mineral phase, making it difficult to obtain for Mars-relevant studies. It can, however, be synthesized from a similar terrestrial mineral, whitlockite (natural or synthetic), through dehydrogenation. Here we present methods for synthesizing relatively large quantities (0.5 g or greater per batch) of coarse crystalline (75 μm+) Mg-whitlockite, Fe-whitlockite, mixed Fe/Mg-whitlockites, and from these synthesized minerals produce Mg-merrillite, ferrous and ferric Fe-merrillite, and ferrous and ferric mixed Fe/Mg-merrillite. Chemistry and atomic structures of synthesized Fe- and mixed Fe/ Mg-whitlockite and ferrous and ferric Fe- and mixed Fe/Mg- merrillite resulting from single-crystal X-ray diffraction, infrared spectroscopy, and electron microprobe analyses are presented. We also present a mechanism for maintaining charge balance during the formation of merrillite from whitlockite. Our results shed light on these mineral structures for future martian studies, and provide methods for creating coarse crystalline merrillite for use in Mars-relevant thermodynamic, kinetic, soil/dust simulant, crystallographic, astrobiological, and other studies.

For the first time, this work reports concentration maps of asbestos minerals in contaminated mine tailings drawn using the results of Rietveld quantitative phase analysis (QPA). The investigated sites are located in the Valle d’Aosta region (Italy): Crètaz, the most important Italian magnetite mine, active until 1979 and Emarèse, one the most important chrysotile asbestos mines in Italy, active until 1968. The results of the study permit to draw the spatial distribution of the asbestos (chrysotile and tremolite in this specific case) concentration, useful to plan reclamation of the sites, with priority given to the areas with the highest asbestos concentration. Because of the complexity of the mineral assemblage, which includes, among the others, antigorite, chlorite, talc, and tremolite, the concentration of chrysotile was cross-checked using different experimental techniques such as X-ray powder diffraction (XRPD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), polarized light optical microscopy (PCOM), and differential thermal analysis (DTA). The accuracy of the results was validated by analyzing standard samples with known concentrations of chrysotile and tremolite. The comparison allowed to point out the advantages and disadvantages of each experimental method. At Crètaz, chrysotile ranges from 4.4 to 22.8 wt% and tremolite from 1.0 to 10.3 wt%, whereas at Emarèse the concentration of chrysotile varies from 3.3 to 39.5 wt% and tremolite from 5.9 to 12.4 wt%. Antigorite and chlorite are the major accompanying phases with variable amounts of other accessory minerals including magnetite, carbonates, talc, olivine, pyroxene, talc, and brucite. The results of our study are of key importance for the local environmental policies as the knowledge of the spatial distribution of the asbestos concentration allows to plan a detailed reclamation agenda of the contaminated sites. The spots with the highest surface contamination of both chrysotile and tremolite were identified and classified as priority areas in the reclamation plan.

The pressure-volume equation of state for the two spinel end-member compositions chromite Fe- Cr 2 O 4 and magnesiochromite MgCr 2 O 4 was determined for flux-grown synthetic single crystals at room temperature up to 8.2 and 9.2 GPa, respectively, by single-crystal X-ray diffraction using a diamondanvil cell. The pressure-volume data show that the linear volume compressibility (here used only for purpose of comparison), calculated as β V = |[(ΔV/V 0 )/ΔP]|, is 0.00468 and 0.00470 GPa -1 , for chromite and magnesiochromite, respectively, with a negligible difference below 0.5%. The experimental data were fitted to a third-order Birch-Murnaghan equation of state (BM3) allowing a simultaneous refining of the following coefficients: V 0 = 588.47(4) Å 3 , K T0 = 184.8(1.7) GPa, and K′ = 6.1(5) for chromite and V 0 = 579.30(4) Å 3 , K T0 = 182.5(1.4) GPa, and K′ = 5.8(4) for magnesiochromite. The difference in K T0 is reduced to <1.5% going from Fe to Mg end-member composition, whereas the first pressure derivative seems not to be affected by the chemical variability. The limited difference in the equation of state coefficients recorded for FeCr 2 O 4 and MgCr 2 O 4 allowed us to fit the pressurevolume data of both to a single BM3 equation, resulting in a K T0 = 184.4(2.2) GPa and K′ = 5.7(6).

Compounds with a spinel-type structure include mineral species with the general formula AB 2 φ 4 , where φ can be O 2- , S 2- , or Se 2- . Space group symmetry is Fd3̄m, even if lower symmetries are reported owing to the off-center displacement of metal ions. In oxide spinels (φ = O 2- ), A and B cations can be divalent and trivalent (“2-3 spinels”) or, more rarely, tetravalent and divalent (“4-2 spinels”). From a chemical point of view, oxide spinels belong to the chemical classes of oxides, germanates, and silicates. Up to now, 24 mineral species have been approved: ahrensite, brunogeierite, chromite, cochromite, coulsonite, cuprospinel, filipstadite, franklinite, gahnite, galaxite, hercynite, jacobsite, magnesiochromite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, qandilite, ringwoodite, spinel, trevorite, ülvospinel, vuorelainenite, and zincochromite. Sulfospinels (φ = S 2- ) and selenospinels (φ = Se 2- ) are isostructural with oxide spinels. Twenty-one different mineral species have been approved so far; of them, three are selenospinels (bornhardtite, trüstedtite, and tyrrellite), whereas 18 are sulfospinels: cadmoindite, carrollite, cuproiridsite, cuprokalininite, cuprorhodsite, daubréelite, ferrorhodsite, fletcherite, florensovite, greigite, indite, kalininite, linnaeite, malanite, polydymite, siegenite, violarite, and xingzhongite. The known mineral species with spinel-type structure are briefly reviewed, indicating for each of them the type locality, the origin of the name, and a few more miscellaneous data. This review aims at giving the state-of-the-art about the currently valid mineral species, considering the outstanding importance that these compounds cover in a wide range of scientific disciplines.

Species dissolved in H 2 fluid were investigated in a SiO 2 -H 2 system. Raman and infrared (IR) spectra were measured at high pressure and room temperature after heating experiments were conducted at two pressure and temperature conditions: 2.0 GPa, 1700 K and 3.0 GPa, 1500 K. With the dissolution of quartz, a SiH vibration mode assignable to SiH 4 was detected from Raman spectra of the fluid phase. Furthermore, an OH vibration mode was observed at 3260 cm -1 from the IR spectra at 3.0 GPa. With decreasing pressure, the OH vibration frequencies observed between 3.0 and 2.1 GPa correspond to that of ice VII, and those observed at 1.4 and 1.1 GPa correspond to that of ice VI. These results indicate that the chemical reaction between dissolved SiO2 components and H2 fluid caused the formation of H 2 O and SiH 4 , which was contrastive to that observed in SiO 2 -H 2 O fluid. Results imply that a part of H 2 is oxidized to form H 2 O when SiO 2 components of mantle minerals dissolve in H 2 fluid, even in an iron-free system.

Based on numerical results from density-functional perturbation theory calculations, we show that the magnetic spin transition in Fe 2+ -rich MgSiO 3 perovskite can be identified as changes in the powder X-ray diffraction (XRD) pattern and the vibrational spectra. In particular theory predicts how the symmetry breaking and the volume reduction associated with the spin transition affects both structural and vibrational properties. The XRD measurements of (Mg 0.5 Fe 0.5 )SiO 3 perovskite indeed demonstrated that the new diffraction peaks and the peak broadening formed during the spin transition can be explained by the associated symmetry breaking. We also show computationally that certain vibrational peaks exhibit a shift at the transition; the Grüneisen parameters of certain modes are affected by the transition, thus bearing on the thermodynamical properties. Raman and/or infrared measurements before and after the spin transition could identify these changes.

Guyanaite, naturally occurring β-CrOOH, has been identified in a xenolith of Cr-rich omphacitite from the Moses Rock diatreme in the Navajo Volcanic Field of the southwestern United States. It occurs as the dominant phase in small clusters of accessory minerals, intergrown with kosmochlor-rich omphacite, zincian chromite, eskolaite, and carmichaelite. The assemblage is interpreted as the result of metasomatism of chromite-bearing serpentinite by slab-derived fluids during subduction of the Farallon Plate in Laramide time. At the time of entrainment of the xenolith, the rock was undergoing prograde metamorphism, with guyanaite dehydrating to eskolaite plus water. This reaction, and the coeval dehydration of the inferred accompanying host serpentinites (which would have been much more volumetrically significant), provided water for hydration of the subcontinental upper mantle, contributing to uplift of the Colorado Plateau. Recognition of guyanaite as a component of a subducted slab supports recent proposals, based on laboratory experiments, that high-pressure polymorphs of other common crustal oxy-hydroxide minerals such as boehmite and goethite (i.e., high-pressure δ-AlOOH and ε-FeOOH) can also transport and store water in the mantle.

The ~151 Ma Guadalupe Igneous Complex (GIC) is a tilted, bi-modal intrusion that provides a rare view into the deeper, mantle-derived portions of a granitic pluton. Major oxide relationships show that GIC granitic rocks formed by in situ differentiation. Assimilation of sedimentary country rock is precluded, as GIC alumina saturation indices (ASI) are too low by comparison, while TiO 2 and P 2 O 5 contents disallow partial melting of metavolcanic lower/middle crust. In contrast, Rb-Sr systematics support in situ magmatic differentiation, as unaltered GIC whole rock samples fall on a single 151 Ma isochron (initial 87 Sr/ 86 Sr = 0.7036) matching zircon age dates (Saleeby et al. 1989). Crystal/liquid segregation, though, was not continuous: mafic and felsic samples form discordant compositional trends, with a gap between 60-66% SiO 2 . We posit that crystal/liquid segregation is continuous between 50-60% SiO 2 , and leads to the genesis of intermediate composition liquids that are then too viscous to allow further continuous liquid segregation. Further crystal/liquid separation thereafter occurs discontinuously (at F ≈ 45-50%), to yield a mafic crystalline (52-59% SiO 2 ) residue and a silicic (70-75% SiO 2 ) liquid (Bachmann and Bergantz 2004), which are, respectively, preserved in the Meladiorite and Granite/Granophyre units of the GIC. Outcrops in the gabbroic section support this view, where mafic crystalline layers feed directly into granitic dikes, and intermediate compositions are absent; mass balance calculations at the outcrop scale also support this model. It is unclear, though, to what extent this model applies to larger Sierran plutons; the smaller GIC may represent an end-member process, where rapid cooling limits mixing, due to rapid increases in mafic/felsic melt viscosity contrasts.

We report a first-principles molecular dynamics study of the equation of state, structural, and elastic properties of MgSiO 3 glass at 300 K as a function of pressure up to 170 GPa. We explore two different compression paths: cold compression, in which the zero pressure quenched glass is compressed at 300 K, and hot compression, in which the liquid is quenched in situ at high pressure to 300 K. We also study decompression and associated irreversible densification. Our simulations show that the glass at the zero pressure is composed of primarily Si-O tetrahedra, partially linked with each other via the bridging O atoms (present in 35%; the remaining being the non-bridging O atoms). With increasing pressure, the mean Si-O coordination number gradually increases to 6, with fivefold and subsequently sixfold replacing tetrahedra as the most abundant coordination environment. The Mg-O coordination comprising of a mixture of four-, five-, and sixfold species at zero pressure picks up more high-coordination (seven- to ninefold) species on compression and its mean value increases from 4.5 to 8 over the entire pressure range studied. Consistently, the anion-cation coordination numbers increase on compression with appearance of oxygen tri-clusters (three silicon coordinated O atoms) and mean O-Si coordination eventually reaching 2. Hot compression produces greater densities and higher coordination numbers at all pressures as compared with cold compression, reflecting kinetic hindrances to structural changes. On decompression from 6 GPa, the glass regains its initial uncompressed structure with almost no residual density. Decompression from 27 GPa produces significant irreversible compaction, and the peak-pressure of decompression significantly influences the degree of density retention with as high as 15% residual density on decompression from 170 GPa. Irreversibility arises from the survival of high coordination species to zero pressure on decompression. With increasing pressure, the calculated compressional and shear wave velocities (about 5 and 3 km/s at the ambient conditions) of MgSiO 3 glass increase initially rapidly and then more gradually at high pressures. Our results suggest that hot-compressed glasses perhaps provide closer analog to high-pressure silicate melts than the glass on cold compression.

The Nagssugtoqidian Orogen is a ca. 1.8 Ga belt of east-west trending, highly deformed rocks that bisects central Greenland. Although a variety of data have suggested this belt marks the location of a continent-continent collision zone, evidence of subduction has been lacking. We report here mineralogical evidence from four samples within a well-defined lithologic unit of metabasic and metasedimentary ocean floor rocks of a previously unrecognized UHP metamorphic episode. The UHP episode is recorded by remnants of orthopyroxene exsolved from majoritic garnet, graphitized diamond, exsolution of rutile from garnet and pyroxenes, exsolution of magnetite from olivine, and complex exsolution textures in ortho- and clinopyroxenes (including omphacite). Associated with these mineralogical features is an unusual occurrence of quartz needles in Mn-rich fayalite. From textural characteristics, we infer that the quartz needles exsolved from the fayalite. To our knowledge, olivine with exsolved silica has not been reported. We note, however, that experimental studies have shown that b-spinel can incorporate excess silica. We therefore speculate these quartz needles may be silica that exsolved from Mn-rich ahrensite, the Fe analog of ringwoodite, upon decompression and inversion to fayalite. If correct, this occurrence would be the first reported sample of naturally occurring olivine (fayalite) that inverted from ahrensite. Corroborating an early UHP history are reaction relationships that delineate a path through high-pressure and high-temperature conditions during decompression. P-T conditions inferred for the UHP episode are ~7 GPa at ~975 °C. The unusually low T for this UHP system at ~1.8 Ga may reflect either very rapid subduction rates at that time, or unexpectedly cool mantle conditions. Preservation of the UHP assemblages probably is due, in large part, to the exceptionally low a H2O during decompression and cooling. These UHP rocks establish that the location of the subduction and suture zones that must have existed prior to and during the collision of continents was along what is now the northern edge of the Nordre Strømfjord shear zone.

An understanding of the mineralogy and petrogenesis of rare earth element deposits has significant implications for their economic viability. Lanthanide-bearing compounds are known to produce sharp absorption features in the visible to short-wave infrared region (VIS-SWIR), however, a significant knowledge gap exists between the fields of hyperspectral reflectance spectroscopy and rare earth element mineralogy. Reflectance spectra were collected from four bastnäsite samples, two parisite samples, and one synchysite sample from the visible into the short-wave infrared. These REE fluorocarbonate mineral samples were characterized via scanning electron microscopy and electron probe microanalysis. Sharp absorptions of REE-bearing minerals are mostly the result of 4f-4f intraconfigurational electron transitions and for the light REE-enriched fluorocarbonates, the bulk of the features can be ascribed to Nd 3+ , Pr 3+ , Sm 3+ , and Eu 3+ . The lanthanide-related spectral responses of the REE fluorocarbonates are consistent across the group, supporting the notion that the REE cation site is very similar in each of these minerals. Carbonate-related spectral responses differed between these minerals, supporting the notion that the crystallographic sites for the carbonate radical differ between bastnäsite, synchysite, and parisite. Exploitable spectral differences include a distinct absorption band at 2243 nm that separates bastnäsite from synchysite and parisite. Similarly, for bastnäsite a dominantly Pr 3+ -related absorption band located is at 1968 nm, while in synchysite and parisite it occurs at 1961 nm.

Whitlockite and merrillite are two Ca-phosphate minerals found in terrestrial and planetary igneous rocks, sometimes coexisting with apatite. Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen. Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite, despite evidence for whitlockite-merrillite solid solution in terrestrial systems. The observation of merrillite in meteoritic and lunar samples has led many to conclude that the magmas from which the merrillite formed were “very dry.” However, the Shergotty martian meteorite has been reported to contain both apatite and merrillite, and recently the apatite has been shown to contain substantial OH abundances, up to the equivalent of 8600 ppm H 2 O. In the present study, we determined the abundances of F, Cl, H 2 O, and S in merrillite from Shergotty using secondary ion mass spectrometry (SIMS). We determined that the merrillite in Shergotty was properly identified (i.e., no discernible whitlockite component), and it coexists with OH-rich apatite. The absence of a whitlockite component in Shergotty merrillite and other planetary merrillites may be a consequence of the limited thermal stability of H in whitlockite (stable only at T <1050 °C), which would prohibit merrillite-whitlockite solid-solution at high temperatures. Consequently, the presence of merrillite should not be used as evidence of dry magmatism without a corresponding estimate of the T of crystallization. In fact, if a whitlockite component in extraterrestrial merrillite is discovered, it may indicate formation by or equilibration with hydrothermal or aqueous fluids.

A sample of Sb-rich segnitite from the Black Pine mine, Montana, U.S.A., has been studied by microprobe analyses, single-crystal X-ray diffraction, and μ-EXAFS and XANES spectroscopy. Linear combination fitting of the spectroscopic data provided Sb 5+ :Sb 3+ = 85(2):15(2), where Sb 5+ is in octahedral coordination substituting for Fe 3+ and Sb 3+ is in tetrahedral coordination substituting for As5+. Based upon this Sb 5+ :Sb 3+ ratio, the microprobe analyses yielded the empirical formula Pb 1.02 H 1.02 (Fe 3+ 2.36 Sb 5+ 0.41 Cu 2+ 0.27 ) Σ3.04 (As 5+ 1.78 Sb 3+ 0.07 S 6+ 0.02 ) Σ1.88 O 8 (OH) 6.00 . The crystal structure refinement and bond valence analysis are consistent with these cation site assignments. The formation of Sb-rich segnitite opens new possibilities for Sb sinks within the supergene zone. Segnitite may, in fact, be an ideal host for the sequestering of several toxic elements for pH < 2. At higher pH values, As is more likely to be incorporated into schwertmannite and ferrihydrite.

The elasticity of monocarboaluminate hydrates, 3CaO·Al 2 O 3 ·CaCO 3 ·xH 2 O (x = 11 or 8), has been investigated by first-principles calculations. Previous experimental study revealed that the fully hydrated monocarboaluminate (x = 11) exhibits exceptionally low compressibility compared to other reported calcium aluminate hydrates. This stiff hydration product can contribute to the strength of concrete made with Portland cements containing calcium carbonates. In this study, full elastic tensors and mechanical properties of the crystal structures with different water contents (x = 11 or 8) are computed by firstprinciples methods based on density functional theory. The results indicate that the compressibility of monocarboaluminate is highly dependent on the water content in the interlayer region. The structure also becomes more isotropic with the addition of water molecules in this region. Since the monocarboaluminate is a key hydration product of limestone added cement, elasticity of the crystal is important to understand its mechanical impact on concrete. Besides, it is put forth that this theoretical calculation will be useful in predicting the elastic properties of other complex cementitous materials and the influence of ion exchange on compressibility.

The structures of eight synthetic samples of hibonite, with variable Ti oxidation state and Ti concentration (2.4-15.9 wt% TiO 2 ) that span the range reported for natural hibonite found in meteorites, were determined by Rietveld refinements of neutron powder diffraction data. Ti 3+ was found to exclusively occupy the octahedral face-sharing M4 site irrespective of the presence or absence of Ti 4+ . Ti 4+ partitions between the trigonal bipyramidal M2 site and the M4 site. The ratio (Ti 4+ on M2):(Ti 4+ on M4) appears to be constant for all the samples, with an average of 0.18(2) irrespective of the concentrations of Ti 3+ and Ti 4+ . These substitutional sites were shown to be the most stable configurations for Ti in hibonite from calculations using density functional theory, although the predicted preference of Ti 4+ for M4 over M2 is not as strong as is observed. This is attributed to the different Ti contents of the experimental and calculated structures and suggests that the Ti site occupancies might change between these concentrations. Furthermore, it is shown that Ti has a preference to occupy neighboring M4 sites such that Ti-Ti interactions occur with stabilization energies of 83 kJ/mol for Ti 3+ -Ti 3+ and at least 15 kJ/mol for Ti 4+ -Ti 4+ . Features in optical spectroscopy and electron spin resonance data from meteoritic and synthetic hibonites that have been used to infer Ti 3+ /Ti 4+ are shown to actually derive from these Ti-Ti interactions. The amount of Ti 4+ in hibonite can be determined from the unit-cell parameters if STi is determined independently. Ti 3+ /Ti 4+ in hibonite may record the oxygen fugacity (f O2 ) of the early solar nebula, however, the existence of Ti 3+ -Ti 3+ and Ti 4+ -Ti 4+ interactions and the potential for Ti 4+ -Ti 3+ interactions need to be considered when interpreting spectroscopic data in terms of Ti valence state and f O2 . Hibonite as a single-mineral oxybarometer must be used with caution due to the potential role of crystal chemistry (including Ti-Ti interactions) to stabilize Ti oxidation states independently of f O2 .

The effect of lithium (Li) on the development of pegmatitic textures was evaluated experimentally at H 2 O-saturated and H 2 O-undersaturated conditions at 500 MPa pressure and temperatures ranging from 400 to 800 °C. The addition of ~3700 ppm Li to a starting material of common granitic composition (Lake County obsidian) reduces the crystallization and melting temperatures by approximately 50-70 and 200 °C, respectively, and promotes the formation of quartz-feldspar graphic and granophyric intergrowths. The presence of Li in the hydrous granitic system lowers significantly the degree of undercooling needed for the development of graphic and granophyric intergrowths to 85-110 °C in comparison with the Li-free systems where approximately 200 °C of undercooling is necessary. The feldspar crystals developed in Li-bearing samples can incorporate from 150 to 250 ppm Li, assume skeletal and spherulitic morphologies, and exhibit one order of magnitude faster growth rates than crystals in Li-free samples. The results of this study attest to the effectiveness of Li as a fluxing agent and highlight the crucial role it plays in the development of pegmatitic textures, leading to important implications for the conditions of formation of Li-bearing granitic pegmatites.

In this work we present a series of experiments that examine the relationship between oxygen fugacity and Cr valence ratio in olivine grown from a basaltic liquid. These experiments are specifically targeted for an olivine-rich martian basalt composition that was modeled after the bulk chemistry of the meteorite Yamato 980459 (i.e., Y-98). The chromium valence ratio in the olivine crystals was measured with X‑ray absorption near edge spectroscopy (XANES) at the Advanced Photon Source, Argonne National Laboratory. Results from the XANES measurements indicate that the ratio of divalent to trivalent Cr in the olivine is not only systematically correlated with f O2 , but is also reflective of the molar Cr 3+ /Cr 2+ in the silicate liquid from which it grew. In this way, measurements of Cr valence in olivine phenocrysts can yield important information about the oxygen fugacity and molar Cr 3+ /Cr 2+ of its parental liquid in the absence of a quenched melt phase. Although the results from the experiments presented in this work specifically apply to the Y-98 parental melt, the concepts and XANES analytical techniques discussed within the text present a novel, generalized methodology that may be applicable to any olivine-bearing basalt. Furthermore, the XANES-based measurements are made on a micrometer-scale, thus potential changes of the Cr 3+ /Cr 2+ in the melt during crystallization could be examined with a great deal of spatial detail.

Nano-precipitates with tripled periodicity along the c-axis are observed in a Ca-rich dolomite sample from Proterozoic carbonate rocks with “molar tooth” structure. This observation is consistent with previous description of d reflections. High-angle annular dark-field STEM imaging (or Z-contrast imaging) that avoids dynamic diffraction as seen in electron diffraction and high-resolution TEM imaging modes, confirms that d reflections correspond to nanoscale precipitates aligned parallel to (001) of the host dolomite. The lamellae precipitates have a cation ordering sequence of Ca-Ca-Mg-Ca-Ca- Mg along the c direction resulting in a chemical composition of Ca 0.67 Mg 0.33 CO 3 . This superstructure is attributed to the extra or d reflections, thus is referred to as the d superstructure in this study. The structure can be simply described as interstratified calcite/dolomite. The crystal structure of the d superstructure calculated from density functional theory (DFT) has a space group of P31c and has a and c unit-cell parameters of 4.879 and 16.260 Å, respectively, values between those of dolomite and calcite. The detailed structural characteristics and parameters obtained from ab initio calculations are also reported in this paper. The method of combining Z-contrast imaging and ab initio calculations can be used for solving structures of other nano-precipitates and nano-phases.

The electrical conductivities of granites with different chemical compositions [XA = (Na 2 O + K 2 O + CaO)/SiO 2 = 0.10, 0.13, 0.14, and 0.16 in weight percent] were measured at 623-1173 K and 0.5 GPa in a multi-anvil high-pressure apparatus using a Solartron-1260 Impedance/Gain Phase analyzer within a frequency range of 10 -1 -10 6 Hz. The conductivity of the granite sample with X A = 0.13 was also measured at 0.5-1.5 GPa. The results indicate that pressure has a very weak influence on the electrical conductivity in the stability field of granite, whereas increases in temperature and the value of X A produce dramatic increases in the electrical conductivity. For the granite samples with X A = 0.16 and 0.13, the activation enthalpies are 1.0 eV above 773 K and 0.5 eV below 773 K, suggesting that impurity conduction is the dominant conduction mechanism in the lower-temperature region. For the granites with X A = 0.14 and 0.10, the activation enthalpy is 1.0 eV over the whole temperature range, suggesting that only one conduction mechanism dominates the conductivity. Based on the value of activation enthalpy (~1.0 eV) and the dependence of electrical conductivity and activation enthalpy on X A at high temperatures, we propose that intrinsic conduction is the dominant conduction mechanism in all samples, and that K + , Na + , and Ca 2+ in feldspar are the probable charge carriers controlling the conductivity. All conductivity data at high temperatures can be fitted to the general formula where σ 0 is the pre-exponential factor; α, β, and γ are constants; ΔH 0 is the activation enthalpy at very small values of X A ; k is the Boltzmann constant; and T is the temperature. The present results suggest that the granite with various chemical compositions is unable to account for the high conductivity anomalies under stable mid- to lower-crust and southern Tibet.

Rutile, titanite, and zircon formed as relatively coarse-grained accessory minerals in several samples of high-grade calcite-dolomite marble with an early ultrahigh-pressure history. These minerals decomposed to a texturally complex set of secondary minerals during subsequent stages of retrograde metamorphism. The reactions involve several generations of geikielite-ilmenite as well as zirconolite [(Ca,Th,U)Zr(Ti,Fe,Nb,Ta) 2 O 7 ], kassite/cafetite [CaTi 2 O 4 (OH) 2 /CaTi 2 O 5 ·H 2 O], Ti-bearing humite group minerals, thorianite, and sometimes euxenite [(Ca,U,Th,REE)(Nb,Ta,Ti) 2 (O,OH) 6 ]. Stable coexistence of zircon and olivine is observed and stably coexisting titanite with olivine and/or humite-group minerals is reported here for the first time outside of carbonatites, kimberlites, or lamprophyres. Petrogenetic grids constructed for Ti- and Zr-bearing olivine/antigorite-saturated calcite-dolomite marbles show that geikielite is stable at highest pressures, followed by titanite and rutile, and that baddeleyite + diopside replaces zircon + calcite to higher pressures. The observed reaction textures are consistent with an earlier derived P-T path for the Kimi Complex. They corroborate a period of heating during decompression from 25 to 20 kbar and ca. 800 °C, where the assemblage olivine-diopside-spinelrutile- zircon formed. This assemblage partially re-equilibrated during subsequent decompression and cooling, thus forming the observed reaction textures. Even though no memory of the UHP path is preserved in the accessory minerals, their reaction relationships turn out to be potentially very useful for geothermobarometry over a large range of metamorphic conditions.

Using the hybrid B3LYP density functional method, we computed the ab initio thermodynamic and thermophysical properties of two sapphirine end-members, Mg 4 Al 8 Si 2 O 20 (sapphirine-442) and Mg 3 Al 10 SiO 20 (sapphirine-351), in the join Mg3(Mg 1-X Al X )Al 8 (Al X Si 1-X )SiO 20 with X = 0-1. Static and vibrational calculations performed in the framework of the quasi-harmonic approximation allowed to define the equation of state (EOS), elastic constant tensor, seismic velocities, IR spectra, mode Grüneisen parameters, and thermodynamic properties of both sapphirine end-members. A modified Kieffer’s model was adopted to evaluate the optic and acoustic mode contributions to thermodynamic functions stemming from ab initio phonon frequencies and directionally averaged seismic velocities, respectively. The extrinsic stability and liquidus phase relations of sapphirine were investigated in the model system MgO-Al 2 O 3 -SiO 2 (MAS) at different pressure conditions by coupling first-principles calculations with the hybrid polymeric approach (HPA) for multicomponent liquids and minimizing the Gibbs free energy of liquid and solid phases through the convex-hull analysis of equipotential surfaces. According to our thermodynamic modeling, sapphirine turns out to have a small field of primary crystallization in the MAS ternary diagram at 1-bar pressure, which becomes larger due to pressure effects up to 10 kbar, then progressively shrinks and disappears above 21 kbar.

Isomorphic nambulite, natronambulite, marsturite, and lithiomarsturite belong to the p-p (pectolitepyroxene) series of pyroxenoid group minerals with five-periodic single chains of tetrahedra and the common simplified composition (Li,Na)(Mn,Ca) 4 [Si 5 O 14 (OH)] (Z = 2, space group P1̄). New crystal structure refinements including localization of H positions of four samples (two nambulite, one natronambulite, and one marsturite) with varying Li and Na concentrations and major to trace element compositional data from different localities are presented. Na occupies a strongly distorted eightfoldcoordinated site (M5). Li replacing Na has a substantially smaller ionic radius and occupies a pocket of the large M5 coordination polyhedron and is only fivefold coordinated by oxygen. Thus, the Li ↔ Na substitution has a significant influence on the bond-valence sums of oxygen sites forming the large cage around M5. Two of the cage-building oxygen sites (O1 and O11) are involved in hydrogen bonding. If M5 is occupied by Na or empty as in the closely related babingtonite, Ca 2 Fe 2 [Si 5 O 14 (OH)], the OH-group is at O1 and exhibits a strong hydrogen bond to O11. If a pocket of M5 is occupied by Li, the hydrogen bond system is reversed with OH at O11 and a strong hydrogen bond to O1. This study emphasizes that short hydrogen bonds with O-H···O separations of ca. 2.46 Å may be modified by homovalent substitution, which contributes to the understanding of strong hydrogen bonds and their role in the stability of hydrous pyroxenoids with strongly curled silicate chains.

Ye’elimite-larnite rocks in the Hatrurim formation of the Negev Desert, Israel, are natural analogs of belite sulfoaluminate (BSA) cement clinkers. They have been produced by ultrahigh-temperature combustion metamorphism at ambient pressure of a calcareous sedimentary precursor. Their mineralogy consists of 35-50 vol% β-Ca 2 SiO 4 , 15-20 vol% ye’elimite, 7-15 vol% ferrites, and 15-20 vol% fluorapatite and/or fluorellestadite. A few grains of hatrurite (Ca 3 SiO 5 ) and α′-Ca 2 SiO 4 have been observed as well. The composition of α′- and β-Ca 2 SiO 4 polymorphs by EPMA are near Ca 1.96-1.98 Na 0.01-0.02 Si 0.96 P 0.03 Al 0.01 O 4 , whereas ye’elimite has an approximate composition by EPMA of Ca 3.99 Mg 0.02 Ba 0.01 Na 0.02 K 0.02 Al 5.73 Fe 3+ 0.16 Si 0.10 S 0.97 P 0.02 O 16 . The Al content of brownmillerite Ca 2 (Fe 1-x Al x ) 2 O 5 ranges from x = 0.20-0.27. Fe-analog of shulamitite (Ca 3 Fe 2 TiO 8 ) contains up to 15.1 wt% TiO 2 . Ye’elimite-larnite rocks were derived from chalky sediments by burning of combustible gas with a T max at 1200-1350 °C. The mineral content, microstructure, and texture/fabric of the ye’elimite-larnite rocks imply that chalky and/or marly sediments with randomly distributed clay, phosphorite, and gypsum may be utilized as cheap naturally homogenized and pulverized mixtures for industrial production of BSA cement clinker, as an environment-friendly alternative to ordinary Portland cement (OPC).

The vast majority of precious opal on the world market comes from opal fields in the Great Artesian Basin of Australia pointing to very special prerequisites for amorphous silica to consolidate in a way that leads to the famous play-of-color. We analyzed 20 opal-A samples from the Andamooka (South Australia) and Yowah (Queensland) precious opal fields, using petrographic microscopy, XRPD, SEM, and EPMA to identify and characterize opaline silica, the mineral assemblage, and the host rock. Opal-A consists of submicrometer-sized silica spheres with an average diameter of 140-320 nm. The average diameter of monodisperse spheres is 140-290 nm with a relative standard deviation (RSD) of <6%. Polydisperse spheres show an average diameter of 160-320 nm with a RSD larger than 10%. This dichotomy in size is reflected by the Na/K ratio at both localities. Monodisperse spheres show values below 1.2 while polydisperse ones show a ratio larger than 3.0, whereas other contaminations with higher valence show no correlations at all. We therefore suggest that the jump in Na/K signals a fundamental change of pH and salinity of the silica-bearing mineralizing fluids. Judging from the pH stability of the host rock minerals with predominating alunite, kaolinite, illite and gypsum, and omnipresent barite and anatase we conclude that the dominant late-stage mineralization leading to precious opal happened at acidic pH. Our findings indicate that the host rocks and associated minerals are the key to unravel the complex history of opal-forming solutions. A quantitative opal classification based on sphere diameters and their variability, decoupled from gemological properties, is to be established.

The mineral sabieite, NH 4 Fe 3+ (SO 4 ) 2 , was found in 2011 along the banks of the Huron River near Milan, Ohio, where it formed as the result of a natural fire in an oil-bearing shale. The mineral is directly associated with pyracmonite, tschermigite, and voltaite and occurs as colorless, pale pink, tan, and yellow hexagonal tablets. The streak is white. Crystals are transparent with vitreous luster. Mohs hardness is 2½, tenacity is brittle, fracture is irregular, and cleavage is perfect on {001}. The measured density is 2.65(2) g/cm 3 . The mineral is optically uniaxial (-) with indices of refraction ω = 1.657(3) and ε = 1.621(5) (white light). The empirical formula (based on 2 S apfu) is [(NH 4 ) 0.73 (H 3 O) 0.22 K 0.04 Na 0.01 ] Σ1.00 (Fe 3+ 0.95 Al 0.02 Mg 0.01 ) Σ0.98 (SO 4 ) 2 . Powder diffraction showed crystals to be combinations of the 2H and 3R polytypes. The structure of the 2H polytype was solved and refined from single-crystal data yielding R 1 = 0.0694 for 509 F o > 4σ(F) reflections. The 2H polytype has space group P6 3 and cell parameters a = 4.83380(17), c = 16.4362(9) Å, V = 332.59(2) Å 3 , and Z = 2 and the 3R polytype has space group R3̄ and cell parameters a = 4.835(2), c = 24.496(15) Å, V = 495.9(5) Å 3 , and Z = 3. The sabieite polytypes (including the original sabieite from Sabie, South Africa, which is the 1T polytype) have glaserite-like structures with layers consisting of Fe3+O6 octahedra that share each of their vertices with SO4 tetrahedra. NH4 groups occupy 12-coordinated sites in the interlayer region, bonding to 6 O atoms in each of the adjacent layers. In the 1T polytype, successive layers have identical configuration and orientation, providing a one-layer repeat sequence. In the 2H polytype, alternate layers are flipped (or rotated), in a two-layer repeat sequence. In the 3R polytype, successive layers are shifted relative to one another, in a three-layer repeat sequence. The different orientations of adjacent layers in the structures of the 2H and 3R polytypes result in significant changes in the linkages between the (NH 4 )O 12 and Fe 3+ O 6 polyhedra.

Te-rich raspite, Pb(W 0.56 Te 0.44 )O 4 , from the Grand Central mine, Tombstone, Arizona, U.S.A., was studied with single-crystal X-ray diffraction, Raman spectroscopy, and electron microprobe analysis. The mineral represents the first natural example of Te 6+ substitution for W 6+ . It displays monoclinic symmetry with space group P2 1 /a and unit-cell parameters a = 13.621(3), b = 5.019(1), c = 5.586(1) Å, b = 107.979(5)°, and V = 363.2(2) Å 3 . Its structure consists of distorted MO 6 (M = W + Te) octahedra sharing edges to form zigzag chains running parallel to [010]. These octahedral chains are linked together by seven-coordinated Pb 2+ cations. In addition, a refinement of the regular raspite structure with measured chemistry Pb 1.00 W 1.00 O 4 , P2 1 /a symmetry, and unit-cell parameters a = 13.5773(8), b = 4.9806(3), c = 5.5670(3) Å, b = 107.658(3)°, and V = 358.72(4) Å 3 is presented. Compared with regular raspite (PbWO 4 ), the partial substitution of the small radius Te 6+ for larger W 6+ results in a decrease in the MO 6 octahedral distortion, with a concomitant increase in the MO 6 octahedral volume and the average Pb-O bond length. In addition, as should be expected for mixed occupancy compounds, most Raman bands for the mixed Te-rich raspite are broader than the corresponding ones for the end-member regular raspite. High-temperature annealing experiments reveal that Te-rich raspite transforms irreversibly to the stolzite structure at 590(10) °C, which is considerably higher than the reported transformation temperature of 395(5) °C for regular raspite.