Volume 100, Issue 5-6

Little is known about the physico-chemical processes that lead to the formation of dolomite in nature. Issues requiring further investigation include: (1) the role played by amorphous carbonate precursors, (2) the mechanisms of transformation of such precursors into proto-dolomite and dolomite, and (3) the controlling factors of the kinetics of the reactions that eventually result in the crystallization of highly ordered dolomite. In the article by Rodriguez-Blanco et al. in this issue entitled “A route for the direct crystallization of dolomite,” the authors present experimental evidence of a threestage process that, starting from the precipitation of an amorphous magnesium calcium carbonate, produces proto-dolomite via spherulitic growth, and subsequently ordered dolomite. This article provides new insights into reaction pathways toward the formation of dolomite.

Kohn et al. 2015 (this volume) present models of zircon dissolution and growth in hydrous MORB and metapelite bulk compositions under metamorphic conditions. They evaluate the propensity metamorphic minerals in equilibrium with zircon, to assess what minerals (besides zircon) affect whole rock Zr mass balance. Results indicate that mineral reactions over a range of metamorphic grades could play an important role in the dissolution and growth of metamorphic zircon, with Zr preferentially incorporated into melt, rutile, garnet, and hornblende with increasing temperatures and decreasing pressures.

Linear partitioning refers to a graphical plot of a partition ratio D ≤1.0 against a composition ratio X 2 given as the mole fraction of a refractory component 2. When this plot is linear from D = 1.0, X 2 = 0.0, its intercept at X 2 = 1.0 is a value on the D scale here identified as the value of the exchange coefficient K D . The plot is generated from phase compositions 1 and 2 in states L V or L S or S S depending on whether the system is a boiling mixture, a melting equilibrium, or a solid-solid equilibrium. The linear partitioning equation so generated is a mathematical description of a binary solution loop, and it has the form y = ax + b where y ≡ D, a ≡ K D , x ≡ X 2 , and b = 1 - x ≡ 1 - X 2 . In practice, the linearity is tested by regressing values of D against X 2 to find the intercept K D . If linearity occurs, the system is a binary solution loop; if it does not occur, the system is not a binary loop. Strict linearity is not always observed even in true binary solutions; in such cases the path to K D may be either segmented or moderately curved. Such is the case with the melting equilibria of both plagioclase and olivine, possibly a clue to the non-ideality of solution. Loop width is an inverse function of K D , and can vary with pressure as in the case of plagioclase in troctolites and gabbros. Systems with two loops joined at a common minimum or maximum are called azeotropes and all of them show linear partitioning. Sanidine crystalline solutions form a classic example of such behavior. When the system An-Ab is revisited to repeat the Bowen thermodynamic calculation from the latent heats of fusion with modern data, the array shows a single modest curvature. The monoclinic pyroxene pairs augite and pigeonite form a binary loop; augite-orthopyroxene does not. The olivine compositions of rocks in the Kiglapait intrusion follow a linear partitioning line with K D = 0.26 for data above 50% crystallized (50 PCS). All the rocks below 50 PCS occupy a new trend in the linear partitioning diagram. This trend is anchored at D = 0.0, X 2 S = 1.0 and runs to the calculated liquid composition at its intercept with the D = 1.0 upper bound. The new trend is an artifact of a nearly constant liquid composition and serves only to show low Fo contents in the range 0-50 PCS.

In the past 150 years, with the discovery of petroleum and the invention of the gasoline-powered internal combustion engine, the rate of extraction of minerals from the Earth has made humans a geologic agent for the first time in the history of the planet. Unprecedented changes have resulted in human society as a result of this extraordinary use of minerals, and perhaps no mineral illustrates that linkage more than the mineral apatite; even the exponential growth of the human population to seven billion Earth inhabitants has been allowed by the extraction of sufficient P from apatite ore to provide fertilizer to feed the planet’s population. Apatite is used extensively in various geological applications, including dating techniques and studies of rare earth element variation in rocks, and is also widely used in material science and medical applications. Apatite forms virtually all hard parts of the human body, and the bioengineering of anion exchange in the apatite material of human tooth enamel through fluoridation is considered one of the top 10 public health achievements of the twentieth century by the U.S. Centers for Disease Control. In addition, the carbonate content of apatite calcifications is currently being investigated as a non-invasive tool in distinguishing between benign and malignant breast tumors. In material applications, apatite is the principle raw material in the fluorescent lighting industry, and its unique crystal chemical properties make it useful in the production of lasers with controllable properties. Apatite is increasingly being employed in the environmental remediation industry through the process of phosphate-induced metal stabilization (PIMS), and its properties make it a useful material for the storage of radioactive waste as substituents in the Ca sites. Despite its remarkable utility and its fundamental role in feeding the world’s population, the details of the apatite atomic arrangement are not fully understood. The Ca phosphate apatites are an anion solid solution (F = fluorapatite; OH = hydroxylapatite; Cl = chlorapatite), and the anion positions in binary and ternary members of the solid solution are not predictable from the anion positions in the pure end-members because of steric effects of anion-anion interactions. Recent attention has focused on a more complete understanding of the apatite atomic arrangement and its properties, both in inorganic and biominerals apatites, and knowledge of the atomic arrangement is advancing. Apatite illustrates the role of minerals in the evolution of modern society, and also the importance of research in the mineral sciences in the broadest sense.

We have studied the thermal expansion of 15 synthetic and two natural F-Cl apatite solid solutions through the calculation of unit-cell dimensions at elevated temperatures based on X‑ray powder diffraction data collected from room temperature to ~900 °C at 50 to 100 °C intervals. Coefficients of thermal expansion for the a and c unit-cell axes show sensitivity to composition, with α a increasing by about 50% and α c decreasing by a third from chlorapatite to fluorapatite. Despite the relationships observed for a and c, the thermal expansion coefficient for unit-cell volume shows little sensitivity to composition, which can be explained only by a mutually compensating structural adjustment along the latter axes as temperature rises. Results of this study also imply that the thermodynamically ideal volumes of mixing for F-Cl apatite solid solutions observed at ambient conditions continue to at least 900 °C.

Pieczkaite, ideally Mn 5 (PO 4 ) 3 Cl, is a new apatite-supergroup mineral from Cross Lake, Manitoba, Canada. It occurs as small patches and narrow veins in large crystals of apatite and (Mn,Cl)-bearing apatite in phosphate pods in the quartz core of a granitic pegmatite. Veins of Mn-bearing apatite narrow to ~25 μm where the Mn content becomes high enough to constitute pieczkaite. It is gray with a grayish-white streak, does not fluoresce under ultraviolet light, and has no observable cleavage or parting. Mohs hardness is 4-5, and pieczkaite is brittle with an irregular fracture. The calculated density is 3.783 g/cm 3 . Optical properties were measured using a Bloss spindle stage at a wavelength of 590 nm (using a gel filter). Pieczkaite is uniaxial (-) with indices of refraction ω = 1.696, ε = 1.692, both ±0.002. Pieczkaite is hexagonal, space group P6 3 /m, a = 9.504(4), c = 6.347(3) Å, V = 496.5(1) Å 3 , Z = 2, c:a = 1:0.6678. The six strongest lines in the X‑ray powder diffraction pattern are as follows: d (Å), I, (hkl): 2.794, 100, (2̅31, 1̅31); 2.744, 88, (030); 2.639, 34, (1̅22); 2.514, 25, (031, 022); 1.853, 25, (3̅42, 1̅42); 3.174, 24, (002). Chemical analysis by electron microprobe gave P 2 O 5 37.52, MnO 41.77, FeO 2.45, CaO 13.78, Cl 3.86, H 2 O 0.60, O≡Cl -0.87, sum 99.11 wt% where the H 2 O content was calculated as 1 - Cl apfu. The resulting empirical formula on the basis of 12 O anions is (Mn 3.36 Fe 0.20 Ca 1.40 ) Σ4.96 (P 1.01 O 4 ) 3 (Cl 0.62 OH 0.38 ) 1.00 , and the end-member formula is Mn 5 (PO 4 ) 3 Cl. The crystal structure of pieczkaite was refined to an R 1 index of 4.07% based on 308 observed reflections collected on a three-circle rotating-anode diffractometer with MoKα X-radiation. Pieczkaite is isostructural with apatite, Mn is the dominant cation at both the [9]- and [7]-coordinated-cation sites in the structure, and Cl is the dominant monovalent anion.

Understanding the ability of grain boundaries to accommodate point defects and enhance diffusion rates in mantle materials represents an important but challenging problem. Extant experimental studies and recent computational efforts are mainly limited to the ambient pressure. Here, we investigate this problem for MgO at the atomistic level by performing first-principles simulations, based on density functional theory, of the {310)}/[001] tilt grain boundary in MgO at pressures up to 100 GPa. Our results show that native defects and impurities (Ca, Al, and proton modeled here) favorably segregate to the boundary, with the segregation considerably increasing with pressure. They also imply that grain boundary diffusion is easier, and more anisotropic and complex than bulk (lattice) diffusion: The calculated migration enthalpies for host ions and impurities at the grain boundary are smaller than the bulk values, more so at higher pressures with their values being as low as ~1.5 eV at 100 GPa compared to the bulk values of ~4 eV. Thus demonstrated high-defect activity of grain boundaries in MgO-a major phase of Earth’s lower mantle is expected to be relevant to our understanding of mantle rheology and geochemical process.

Upper mantle carbonates are thought to be iron poor and magnesium rich. However, at lower mantle conditions spin-pairing transitions in iron-bearing phases may trigger iron redistribution between the minerals. Here, using visible and near infrared absorption measurements, we examine the siderite crystal field up to 65 GPa. Optical spectrum of siderite at 1 bar has an absorption band at 10 325 cm -1 corresponding to the crystal field splitting energy (10Dq) of ferrous iron in an octahedral field. This band intensifies and blue-shifts (86 cm -1 /GPa) with pressure, but disappears abruptly at 44 GPa signaling the spin transition. Simultaneously, a new absorption band centered at 15 629 cm -1 (88 cm -1 /GPa) appears in the spectrum. Tanabe-Sugano diagram analysis allowed assigning the observed absorption bands to 5 T 2g → 5 E g and 1 A 1g → 1 T 1g electronic transitions in high- and low-spin siderite, respectively. Similarly, we evaluate the crystal field splitting energy of low-spin siderite 10Dq = 17 600 cm -1 (45 GPa), as well as the Racah parameters B = 747 cm -1 and C = 3080 cm -1 . We find that the crystal field stabilization energy (CFSE) of ferrous iron in low-spin siderite (45 700 cm -1 at 45 GPa) is an order of magnitude higher than that in the high-spin phase (4130 cm -1 at 1 bar). From the derived CFSE values we estimate the iron-partitioning coefficient for the carbonate-perovskite system and show that lowspin carbonates are iron rich and magnesium poor. We also show that the color of siderite is governed by the 1 A g → 1 T 1g absorption band and the Fe-O charge transfer.

Interpretation of lateral variations in upper mantle seismic wave speeds requires constraints on the relationship between elasticity and water concentration at high pressure for all major mantle minerals, including the garnet component. We have calculated the structure and energetics of charge-balanced hydrogen substitution into tetragonal MgSiO 3 majorite up to P = 25 GPa using both classical atomistic simulations and complementary first-principles calculations. At the pressure conditions of Earth’s transition zone, hydroxyl groups are predicted to be bound to Si vacancies (□) as the hydrogarnet defect, [□ Si +4OH O ] X , at the Si2 tetrahedral site or as the [□ Mg +2OH O ] X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [□ Mg +2OH O ] X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al 2 O 3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [□ Mg +2OH O ] X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnvs/dlnvp, in MgSiO 3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H 2 O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnvs/dlnvp for ringwoodite. The incorporation of 3.2 wt% Al 2 O 3 also decreases R(H 2 O) by ~0.2-0.4. Therefore, to accurately estimate transition zone compositional and thermal anomalies, hydrous majorite needs to be considered when interpreting seismic body wave anomalies in the transition zone.

Synthetic sodium saponites, Na x Mg 3 (Si 4-x Al x )O 10 (OH) 2 ·nH 2 O, with 0.33 ≤ x ≤ 1, and trioctahedral sodium micas series, Na(Mg 3-y Al y )(Si 3-y Al 1+y )O 10 (OH) 2 ·nH 2 O, with 0 ≤ y ≤ 1, have been investigated by MAS-NMR spectroscopy. The presence of anhydrous, one-layer and two-layer hydrates, deduced by X‑ray diffraction, has been associated with specific lines detected in 23 Na MAS-NMR spectra. In these phyllosilicates, the location of tetra- and octahedral charge has been analyzed by 27 Al MAS-NMR spectroscopy. The salient result is the major effect of the interlayer charge on 29 Si chemical shift of the four NMR components ascribed to Si 3 , Si 2 Al, SiAl 2 , and Al 3 environments. This effect is much more important than the most commonly accepted contribution of the ditrigonal distortion of tetrahedral sheets. In saponites, 29 Si MAS-NMR spectra change considerably with the sodium hydration. In dehydrated samples, where Na cations are engaged in two pseudo-hexagonal cavities, 29 Si MAS-NMR components split as a consequence of the partial sodium occupancy of three neighboring hexagonal rings that surrounds a particular tetrahedron. In hydrated samples, where Na + cations interact with water, chemical shifts of resolved components are averaged as a consequence of interlayer water and cation mobilities.

Majoritic garnet, characterized by an excess of silicon (>3 Si per formula unit), is considered one of the major phases of the Earth’s transition zone from 410-660 km depth. Quantifying the H 2 O content of nominally anhydrous mantle minerals is necessary to evaluate their water storage capacity from experiments and modeling the Earth’s deep water cycle. We present mineral-specific infrared absorption coefficients for the purpose of quantifying the amount of water incorporated into majorite as hydroxyl point defects. A suite of majoritic garnet samples with varying proportions of Si, Fe, Al, Cr, and H 2 O was synthesized at conditions of 18-19 GPa and 1500-1800 °C. Single-crystals were characterized using X‑ray diffraction, electron microprobe analysis, secondary ion mass spectrometry (SIMS), IR, Raman, and Mössbauer spectroscopy. We utilize SIMS and Raman spectroscopy in combination with IR spectroscopy to provide IR absorption coefficients for water in majoritic garnets with the general mineral formula (Mg,Fe) 3 (Si,Mg,Fe,Al,Cr) 2 [SiO 4 ] 3 . The IR absorption coefficient for majoritic garnet in the OH stretching region is frequency dependent and ranges from 10 470 ± 3100 Lmol -1 cm -2 to 23 400 ± 2300 Lmol -1 cm -2 .

The Special Collection “Building Planets: The dynamics and geochemistry of core formation” aims to combine cutting edge experimental, analytical, and modeling results with review articles defining the state of the science and current challenges to our understanding of the origin, geophysics, and geochemistry of planetary cores. Our goal is to highlight novel and interdisciplinary approaches that address aspects of core formation and evolution at the atomic, grain, and planetary scales.

The last decade has seen general agreement that moderately siderophile elements (MSE) in Earth’s primitive upper mantle (PUM) can be explained by metal-silicate equilibrium at mid-mantle depths in an early Earth magma ocean environment. Despite the agreement, there are some differences in the detailed modeling that has been carried out. This paper will examine siderophile element metal/silicate partitioning with respect to three different topics: (1) an examination of aspects of the modeling that one might suspect leads to differences in outcomes or in comparison between models, but actually are in agreement with experimental data and between models; (2) a discussion of the role of the deep mantle in modeling efforts; and (3) the role and/or fate of volatiles in magma ocean scenarios with an emphasis on where data are lacking.

The hydroxy sulfate jarosite [(K,Na,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 ] has both been discovered on Mars, and is associated with areas of highly acidic runoff on Earth. Because jarosite is extremely sensitive to formation conditions, it is an important target mineral for remote sensing applications. Yet at visible and near infrared (VNIR) wavelengths, where many spacecraft spectrometers collect data, the spectral abundance of a mineral in a mixture is not linearly correlated with the surface abundance of that mineral. Radiative transfer modeling can be used to extract quantitative abundance estimates if the optical constants (the real and imaginary indices of refraction, n and k) for all minerals in the mixture are known. Unfortunately, optical constants for a wide variety of minerals, including sulfates like jarosite, are not available. This is due, in part, to the inherent difficulty in obtaining such data for minerals that tend to crystallize naturally as fine-grained (~10 μm) powders, like many sulfates including jarosite. However, the optical constants of powders can be obtained by inverting the equation of radiative transfer and using it to model laboratory spectra. In this paper, we provide robust n and k data for synthetic potassium, hydronium, and sodium jarosite in the VNIR. We also explicitly describe the calculation procedures (including access to our Matlab code) so that others may obtain optical constants of additional minerals. Expansion of the optical constants library in the VNIR will facilitate the extraction of quantitative mineral abundances, leading to more in-depth evaluations of remote sensing target locations.

A series of natural zircon samples (with U concentrations of 140-2600 ppm and ranging from well crystalline to severely radiation damaged) were investigated by means of REE 3+ photoluminescence spectroscopy. We found systematic changes in REE 3+ emissions depending on the accumulated radiation damage expressed by the effective time-integrated a-dose of zircon samples. Structural reconstitution as caused by dry annealing resulted in intensity gains and decreases of half-widths of REE 3+ emissions. The band half-widths of distinct luminescence Stark’s levels of the 4 F 9/2 → 6 H 13/2 transition of Dy 3+ (~17 250 cm -1 ; ~580 nm wavelength) and the 4 F 3/2 → 4 I 9/2 transition of Nd 3+ (~11 300 cm -1 ; ~885 nm wavelength) were found to correlate sensitively with the degree of radiation damage accumulated. These REE 3+ emissions are proposed as potential measure of the irradiation-induced structural disorder of zircon. The two emissions are considered particularly suitable because (1) they are commonly detected in PL spectra of natural zircon, and (2) they are hardly biased by other emissions or Stark’s levels. Preliminary calibration curves that relate band-width increases to the a dose were established using a suite of well-characterized Sri Lankan zircon. Band broadening upon increasing corpuscular self-irradiation is assigned to increasing structural destruction, i.e., the increasing perturbation of REE 3+ cationic lattice sites. Possible advantages of REE 3+ luminescence spectroscopy, complementary to Raman spectroscopy, as method to quantify structural radiation damage are discussed.

This paper describes the biotite chloritization process with a focus on mass transfer in the Toki granitic pluton, Central Japan, and also depicts the temporal variations in chemical characteristics of hydrothermal fluid associated with chloritization during the sub-solidus cooling of the pluton. Singular value decomposition (SVD) analysis results in chloritization reaction equations for eight mineral assemblages, leading to the quantitative assessment of mass transfer between the reactant and product minerals, and inflow and outflow of components through the hydrothermal fluid. The matrices for SVD analysis consist of arbitrary combinations of molar volume and closure component in the reactant and product minerals. The eight reactions represent the temporal variations of chemical characteristics of the hydrothermal fluid associated with chloritization: the progress of chloritization results in gradual increase of silicon, potassium, and chlorine and gradual decrease of calcium and sodium in the hydrothermal fluid with temperature decrease. The biotite chloritization involves two essential formation mechanisms: chlorite formation (CF) mechanism 1, small volume decrease from biotite to chlorite and large inflow of metallic ions such as Al 3+ , Fe 2+ , Mn 2+ , and Mg 2+ from the hydrothermal fluid, and CF mechanism 2, large volume decrease and large outflow of the metallic ions into hydrothermal fluid. Chlorite produced with CF mechanism 1 dominates over that of CF mechanism 2, resulting in the gradual decrease of the metallic components in the hydrothermal fluid with chloritization progress. The chloritization reactions also give the temporal variations in physicochemical parameter of the hydrothermal fluid: a gradual decrease of pH and a gradual increase of redox potential in the hydrothermal fluid as chloritization proceeds. The combination of continuous reactions based on compositional variations in chlorite together with corresponding continuous Al IV variations gives an indication of the temporal variations in rates of decreasing and increasing concentration of chemical components in the hydrothermal fluid associated with chloritization. The biotite chloritization and resultant temporal variations of chemical and physicochemical characteristics in hydrothermal fluid act as a trigger for the successive dissolution-precipitation process of a granitic rock.

This study investigates the oxidation of fayalite Fe 2 2+ SiO 4 that is present in lithophysae from a rhyolite flow (Obsidian Cliffs, Oregon). Textural, chemical, and structural analyses of the successive oxidation zones are used to constrain: (1) the oxidation processes of olivine, and (2) the role of temperature, chemical diffusion, and meteoric infiltration. Petrologic analyses and thermodynamic modeling show that the rhyolite flow emplaced at 800-950 °C. Fayalite-bearing lithophysae formed only in the core of the lava flow. Variations in the gas composition inside the lithophysae induced the oxidation of fayalite to a laihunite-1M zone Fe 1 2+ Fe 2 3+ □ 1 (SiO 4 ) 2 . This zone is made of nano-lamellae of amorphous silica SiO2 and laihunite-3M Fe 2+ 1.6 Fe 3+ 1.6 □ 0.8 (SiO 4 ) 2 + hematite Fe 2 O 3 . It probably formed by a nucleation and growth process in the fayalite fractures and defects and at fayalite crystal edges. The laihunite-1M zone then oxidized into an “oxyfayalite” zone with the composition Fe 2+ 0.52 Fe 3+ 2.32 □ 1.16 (SiO 4 ) 2 . This second oxidation zone is made of lamellae of amorphous silica SiO 2 and hematite Fe 2 O 3 , with a possible small amount of ferrosilite Fe 2+ SiO 3 . A third and outer zone, composed exclusively of hematite, is also present. The successive oxidation zones suggest that there may be a mineral in the olivine group with higher Fe 3+ content than laihunite-1M. The transformation of laihunite-1M to this “oxyfayalite” phase could occur by a reaction such as This would imply that Fe 3+ can also be incorporated in the M1 site of olivine.

The role of grain size and porosity in the piezoelectric and elastic properties of SiO 2 -based materials was investigated using resonant piezoelectric spectroscopy (RPS) and resonant ultrasound spectroscopy (RUS). RPS performed on agate revealed a piezoelectric effect comparable in magnitude to that in single-crystal quartz. The observed strong piezoelectricity in agate requires preferential orientation of SiO 2 during crystal growth. Similarly, in novaculite and sandstone finite (but weak) RPS signals were evident, suggesting that the expected randomization of the piezoelectric quartz grains is incomplete. On the other hand, Vycor, a silica glass with a porosity of 40%, showed no evidence of the piezoelectric effect. According to temperature-dependent RPS and RUS measurements, the α-β transition temperature in quartz does not change in polycrystalline samples. Finally, the temperature dependence under heating of the elastic constants is reversible in quartz and agate and irreversible in sandstone and Vycor.

The direct crystallization of dolomite from an aqueous solution at temperatures between 60-220 °C was followed in situ through time-resolved synchrotron-based energy-dispersive X‑ray diffraction combined with offline high-resolution imaging, X‑ray diffraction, and infrared spectroscopy. Crystalline CaMg(CO 3 ) 2 phases form through a three-stage process. In the first stage, a nanoparticulate magnesium-deficient, amorphous calcium carbonate (Mg-ACC) with a nominal formula of Ca 0.606 Mg 0.394 CO 3 ·1.37H 2 O forms. After a temperature-dependent induction time, during stage 2 the Mg-ACC partially dehydrates and orders prior to its rapid (<5 min) crystallization to non-stoichiometric proto-dolomite. This occurs via the dissolution of Mg-ACC, followed by the secondary nucleation of proto-dolomite from solution. The proto-dolomite crystallization proceeds via spherulitic growth that follows a growth front nucleation mechanism with a de-nuovo and continuous formation of nanocrystalline proto-dolomite subunits that form spherical aggregates. In stage three of the reaction, the proto-dolomite transforms to highly crystalline and stoichiometric dolomite on a much longer timescale (hours to days), via an Ostwald-ripening mechanism. Such a three-stage crystallization can explain microbially induced proto-dolomites observed in modern hypersaline settings and may also be the route by which the Cryogenian cap dolomite deposits of the Neoproterozoic formed.

Hydrated (with D 2 O and H 2 O) sodium tetrasilicate glasses, quenched from melts at 1400 °C and 1.5 GPa, are studied using 1 H, 2 H, and 29 Si solid-state nuclear magnetic resonance (NMR) spectroscopy. Whereas D 2 O and H 2 O depolymerize the silicate melt to similar degrees, protium and deuterium intramolecular partitioning between different molecular sites within the glasses is very different and exemplified by a strong preferential association of deuterons to sites with short O-D···O distances. This preference is independent of total water content and D/H ratio. Substantially different intramolecular D-H partitioning is also observed in a glass with a model hydrous basalt composition. Such large differences in isotope partitioning cannot result from classic equilibrium fractionation because of the high synthesis temperature. Potential kinetic isotope effects are excluded via a slow quench experiment. The apparent fractionation is likely governed by density/molar volume isotope effects, where deuterium prefers sites with smaller molar volume. Large differences in intramolecular site partitioning in melts could lead to significant differences in D-H partitioning between water-saturated melt and exsolved aqueous fluid (where D/H W,Melt ≠ D/H W,Fluid ) during crystallization of Earth’s magma ocean, potentially controlling the D/H content of the Earth’s oceans.

Single-crystal X-ray diffraction study of CaGeO 3 perovskite has been conducted over the temperature range of 98 to 1048 K. The crystal begins to deteriorate at a temperature above about 900 K and completely amorphizes by 980 K. The diffraction-intensity distribution and the structure refinements indicate that the Pbnm structure is kept until the occurrence of amorphization. The obtained unit-cell parameters, unit-cell volumes, bond lengths, and displacement parameters increase monotonously with increasing temperature. Thus, no evidence for the existence of the Cmcm high-temperature phase, previously suggested above 520 K, is observed. The Ge-O bond lengths show much smaller thermal expansions than the Ca-O bond lengths; the former ranges between 0.42(2) × 10 −5 K −1 and 0.57(2) × 10 −5 K −1 , and the latter between 1.58(4) × 10 −5 K −1 and 3.96(6) × 10 −5 K −1 . The Debye temperatures and static disorder components for each constituent atom were determined by applying the Debye model to the temperature dependence of mean square displacements (MSDs) of the atoms. Consequently, no significant static disorder components can be detected in each atom. The Debye temperatures averaged over all directions, obtained from the Debye model fitting to U eq , yield the harmonic one particle potential coefficients of 4.76(2) eVÅ −2 for Ca, 11.0(1) eVÅ −2 for Ge, 5.02(2) eVÅ −2 for O1, and 5.33(5) eVÅ −2 for O2. These values become larger in order of Ca < O1 < O2 << Ge, which shows that the one particle potential of Ge is much narrower than that of Ca. This relationship between Ca and Ge is consistent reasonably with bonding stiffness expected from the thermal expansion coefficients of the bond lengths. The anisotropies of MSDs are remarkable in O1 and O2 atoms as a consequence of the strong interaction with adjacent Ge atoms, forming the rigid bonds with these O atoms. In comparison of the three Pbnm orthorhombic perovskites of CaGeO 3 , CaTiO 3 , and MgSiO 3 , all of these have the BO 6 octahedra more rigid than the AO 12 polyhedra (A = Ca or Mg; B = Ge, Ti, or Si) and the tilt angles of BO 6 octahedra are the largest in MgSiO 3 perovskite. These observations indicate that if MgSiO 3 perovskite under high pressures undergoes the same sequence of the high-temperature phase transitions as CaTiO 3 perovskite, the phase boundaries have positive Clapeyron slopes and the phase transition temperatures should become further much higher than those (1512 K for the Pbnm to I4/mcm transition and 1635 K for the I4/mcm to Pm3̅m transition) observed in CaTiO 3 perovskite at ambient pressure. This leads to the conclusion that the high-temperature phase transition to a perovskite phase with different symmetry under high pressures previously suggested in MgSiO 3 perovskite is unlikely.

The exchange of carbon dioxide for methane in natural gas hydrates is an attractive approach to harvesting CH 4 for energy production while simultaneously sequestering CO 2 . In addition to the energy and environmental implications, the solid solution of clathrate hydrate (CH 4 ) 1-x (CO 2 ) x ·5.75H 2 O provides a model system to study how the distinct bonding and shapes of CH 4 and CO 2 influence the structure and properties of the compound. High-resolution neutron diffraction was used to examine mixed CO 2 /CH 4 gas hydrates. CO 2 -rich hydrates had smaller lattice parameters, which were attributed to the higher affinity of the CO 2 molecule interacting with H 2 O molecules that form the surrounding cages, and resulted in a reduction in the unit-cell volume. Experimental nuclear scattering densities illustrate how the cage occupants and energy landscape change with composition. These results provide important insights on the impact and mechanisms for the structure of mixed CH 4 /CO 2 gas hydrate.

Recent interest in hydrogen incorporation in feldspars has been driven by the potential of this common mineral species to record magmatic water contents. Accurate measurement of H concentrations in feldspars by Fourier transform infrared (FTIR) spectroscopy is hampered by the need to collect polarized spectra in three mutually perpendicular directions, which can be impractical for crystals characterized by small dimensions, polysynthetic twinning, and/or chemical zoning. SIMS is an attractive alternative to FTIR, offering high spatial resolution, high precision, and the feasibility of attaining low detection limits. In this study we compare FTIR and SIMS data for 19 feldspars, including plagioclase, anorthoclase, sanidine, microcline, and orthoclase. We present adjustments to previously published FTIR data on some of these samples. Our new SIMS and FTIR data are well correlated and we demonstrate the feasibility of quantitatively measuring H concentrations as low as 1-2 ppmw H 2 O using SIMS. Combination of the new data together with re-evaluation of the NMR calibration of Johnson and Rossman (2003) indicates that the IR absorption coefficients for hydrous species in feldspar increase with decreasing frequency of their O-H absorptions, in accord with theory. We derive new molar integral IR absorption coefficients (I) for feldspars with the following hydrous species as defined by Johnson and Rossman (2003): Type I and II H 2 O (microcline and orthoclase): I = 120 470 ± 11 360 L·mol -1 H 2 O cm -2 Type IIb OH (sanidine): I = 150 000 ± 15 000 L·mol -1 H 2 O cm -2 Type IIa OH (plagioclase and anorthoclase): I = 202 600 ± 20 260 L·mol -1 H 2 O cm -2 These absorption coefficients depend on critical assumptions with regards to SIMS matrix effects. If accurate, one important implication is that the H concentrations of plagioclase crystals estimated in the literature are too high by up to a factor of two, requiring revision of previously estimated plagioclasemelt H partitioning coefficients.

The light-induced alteration of realgar and β-As 4 S 4 is a well-known phenomenon but still displays interesting aspects not completely explained. In the transformation from realgar to pararealgar the molecule As 4 S 4 undergoes a structural modification, and ever since the initial studies two important issues have been highlighted: the influence of the oxygen and the reversibility of the process. The previous study on the reversibility of the altered β-As 4 S 4 points out that this polymorph exhibits a dual behavior. When the light-induced alteration occurs with the presence of the air, pararealgar and arsenolite, along with amorphous material, are the products, while if the air is not present β-As 4 S 4 turns completely into pararealgar. Moreover, when annealing the altered material in the realgar stability fields (220 °C), in the first case pararealgar and amorphous material turn into stoichiometric alacranite, while in the second case the alteration is completely reversible. Similarly, the present study focuses the attention on the question if realgar, when altered by means of the light and when annealed, might behave as β-As 4 S 4 does. These results display that the phenomenon is more complex. The alteration of realgar with the presence of the air yields pararealgar along with arsenolite, a small quantity of uzonite and amorphous material, and when the air is not present pararealgar is the only product. In the first case, when annealing the products of alteration at 220 °C, alacranite, β-As 4 S 4 , realgar, and little amorphous material occur along with arsenolite. In the second case at first β-As 4 S 4 crystallizes, then it turns into realgar, but this process yields orpiment and amorphous material as it moves forward, showing that from the products of alteration of realgar it is not possible to obtain the starting material. Examination of the stoichiometry of the products let to infer that the amorphous material occurring in the two cases has very different content of arsenic and sulfur.

Calcite is one of the most ubiquitous minerals in the Earth’s crust and is mostly present as calcite or the slightly denser polymorph aragonite. In addition five different phases of CaCO 3 (calcite II-VI), which display similar structural features as calcite, have been observed with increasing pressure in different experiments by several authors. Experimentally, the CaCO 3 -III and CaCO 3 -IIIb polymorphs have recently been observed by Merlini et al. (2012) applying pressures between 2.5-15 GPa on natural samples of calcite using single-crystal synchrotron X‑ray diffraction. Here we report an occurrence of metastable authigenic CaCO 3 -III and CaCO 3 -IIIb nanocrystals for the first time in nature. Using transmission electron microscopy, idiomorphic, 50-150 nm sized crystals were observed within several meters from the surface in quaternary loess deposits in Central Asia. Nanocrystals contain higher surface energy per volume compared to coarse-grained materials due to their larger surface area. The internal pressure of a solid, PS, is at equilibrium with the surface stress, which increases with decreasing particle size. We estimated internal pressures inside the observed nanocrystals between 2.54-4.06 GPa, assuming spherical crystals with 1 nm diameter and specific surface energies, between 1.27-2.03 J/m 2 (Forbes et al. 2011).

A growing literature is demonstrating that platinum (Pt) is transformed under surface conditions; yet (bio)geochemical processes at the nugget-soil-solution interface are incompletely understood. The reactivity of Pt exposed to Earth-surface weathering conditions, highlighted by this study, may improve our ability to track its movement in natural systems, e.g., focusing on nanoparticles as a strategy for searching for new, undiscovered sources of this precious metal. To study dissolution/re-precipitation processes of Pt and associated elements, grains of Pt-Fe alloy were collected from a soil placer deposit at the Fifield Pt-field, Australia. Optical- and electron-microscopy revealed morphologies indicative of physical transport as well as chemical weathering. Dissolution “pits,” cavities, striations, colloidal nano-particles, and aggregates of secondary Pt platelets as well as acicular, iron (Fe) hydroxide coatings were observed. FIB-SEM-(EBSD) combined with S-μ-XRF of a sectioned grain showed a fine layer of up to 5 μm thick composed of refined, aggregates of 0.2 to 2 μm sized crystalline secondary Pt overlying more coarsely crystalline Pt-Fe-alloy of primary magmatic origin. These results confirm that Pt is affected by geochemical transformations in supergene environments; structural and chemical signatures of grains surfaces, rims, and cores are linked to the grains’ primary and secondary (trans)formational histories; and Pt mobility can occur under Earth surface conditions. Intuitively, this nanophase-Pt can disperse much further from primary sources of ore than previously thought. This considerable mineral reactivity demonstrates that the formation and/or release of Pt nanoparticles needs to be measured and incorporated into exploration geochemistry programs.

Surface characteristics of sedimentary opal-A and -CT were investigated for a large collection of samples from the Monterey Formation, California, based on the bulk rock mineral and chemical analysis, cation exchange capacity (CEC), and water content. Two approaches were used: (1) modeling bulk CEC and adsorbed water content for the entire data set using the contents of opal and clay minerals measured by XRD, and (2) correcting the chemical composition and CEC of the most pure opal samples for the mineral impurities quantified by XRD. Modeling indicates that the bulk rock data can be best explained by mixing an illite-smectite (CEC = 59 meq/100 g, 7-8% H 2 O), consistent with the XRD characteristics of the clay fraction, with opal-A (8 meq/100 g, 3.4% H 2 O), and opal-CT (13 meq/100 g, 3.7%H 2 O). Correcting the chemical composition of the most pure opal samples leaves a large excess of cations (Al, Fe, Na, K, Ca, and Mg). Iron is suspect to form traces of separate (oxy-)hydroxide phases, not detected by XRD, while Al for Si substitution in the opal structure produced local negative charge, which was compensated by Na, K, Ca, and Mg exchange cations. A perfect balance of positive and negative charges is observed if the clay admixture in pure opals has the composition of montmorillonite. The concentration of heterogeneous impurities in silica network in opal leads to smectite formation on or within the diatom frustules. These dispersed smectite particles, perhaps monolayers, can be missed during the bulk rock mineral quantification. The recalculated CEC of the opal, assuming the occurrence of dispersed smectite particles, varies from 3 to 11 meq/100 g, which is slightly less than that evaluated by modeling all the rock samples in the set, and corresponds to ~10-50% of the total opal charge quantified by the degree of Al for Si substitution. The remaining charge of the opal structure represents non-exchangeable cations. As opposed to smectite, opal CEC may depend on the size of cation used for the CEC measurement. For opals in the Monterey Formation the content of water removable at 200 °C can be modeled as a sum of a constant value and a variable value dependent on CEC; the latter component is similar to the H 2 O-CEC relationship that is typical for smectite. The combined system of a constant H 2 O + variable H 2 O in opal can potentially be applied for mineral modeling programs in wireline log formation evaluation in diatomaceous hydrocarbon reservoirs.

Nickel ferrites with different Ni content, Ni x Fe 3-x O 4 , 0 ≤ x ≤ 1, are technologically important materials for microwave, electronic, and magnetic storage devices. They are members of solid-solution series of spinel-type materials (Fe 3 O 4 -NiFe 2 O 4 ) having specific magnetic properties and different degree of electron delocalization. They demonstrate good gas sensing properties and catalytic activity in various catalytic processes, such as complete oxidation of waste gases, oxidative dehydrogenation of hydrocarbons, decomposition of alcohols, etc. The preparation of such materials is still an actual problem due to several difficulties in their synthesis and the use of special techniques. A series of nickel-containing ferrite materials Ni x Fe 3-x O 4 (x = 0.25, 0.5, 1) were prepared by precipitation method using FeCl 3 ·6H 2 O, FeCl 2 ·4H 2 O, and NiCl 2 ·6H 2 O as precursors. The performed analyses show the dependence of the rate of formation of the spinel phase on the chemical composition. To obtain the exact conditions for a single-phase spinel material preparation several investigations have been performed: thermal analysis (thermogravimetry, differential thermogravimetry, and differential thermal analysis) and various studies of the intermediates by powder X‑ray diffraction, Mössbauer spectroscopy (at room and liquid nitrogen temperature), BET method, and SEM. As a result, the appropriate conditions of obtaining monophase nanomaterials of doped magnetite are found. The synthesis involves a precipitation process combined with a low-temperature heat treatment of materials (at 300 °C and in argon atmosphere) or mechanochemical processing. Application of the second procedure leads to two interesting results: (1) synthesis of the target compounds under soft and clean conditions without heating, which is important for industrial technology and environmental protection and (2) the prepared samples have better characteristics (higher dispersion degree, better magnetic properties, and higher activity in photocatalytic purification of wastewater from the textile industry).

In marked contrast to the single, narrow 29 Si MAS NMR resonance for pure forsterite (Mg 2 SiO 4 ), the spectra for synthetic forsterite containing 0.05 to 5% of the Mg 2+ replaced with Ni 2+ , Co 2+ , or Fe 2+ display between 4 and 26 additional, small, paramagnetically shifted peaks that are caused by interactions of the unpaired electron spins on the transition metal cations and the nuclear spins. Analyses of these relative peak areas, their numbers, and comparison of their positions to those in spectra of synthetic monticellites (CaMgSiO 4 ) containing similar levels of transition metals, allows at least partial assignment to the effects of cations in either the M1 octahedral site only or to both M1 and M2 sites. More detailed analyses indicate that in forsterite, Ni 2+ occupies only M1, Fe 2+ occupies M1 and M2 roughly equally, and Co 2+ occupies both M1 and M2 in an approximately 3:1 ratio. These findings for low concentrations agree with expectations from previous studies by other methods (e.g., XRD) of olivines with much higher transition metal cation contents. However, even low concentrations of Mn 2+ (e.g., 0.1%), as well as higher Fe 2+ contents (e.g., in natural San Carlos olivine) can broaden NMR peaks sufficiently to greatly reduce this kind of information content in spectra.

Almost all alkali feldspar crystals contain a rich inventory of exsolution, twin, and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions, sometimes multiple. Microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering. They are central to the retention or exchange of trace elements and of radiogenic and stable isotopes. This review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process. We suggest a systematic approach that employs methods available in most well founded laboratories. The crystallographic relationships of complex feldspar intergrowths were established by the 1970s, mainly using single-crystal X‑ray diffraction, but such methods give limited information on the spatial relationships of the different elements of the microtexture, or of the mode and chronology of their formation, which require the use of microscopy. We suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections. Sections cut parallel to the perfect (001) and (010) cleavages are the easiest to locate and most informative. Techniques described are light microscopy; scanning electron microscopy using both backscattered and secondary electrons, including the use of surfaces etched in the laboratory; electron-probe microanalysis and analysis by energy-dispersive spectrometry in a scanning electron microscope; transmission electron microscopy. We discuss the use of cathodoluminescence as an auxiliary technique, but do not recommend electron-backscattered diffraction for feldspar work. We review recent publications that provide examples of the need for great care and attention to pre-existing work in microtextural studies, and suggest several topics for future work.

Here we report the results of an experimental study aimed at testing the existence of stable, superliquidus immiscibility between silica- and Fe-rich multicomponent melts at temperatures above 1100 °C. Four pairs of the potentially immiscible compositions were tested in a 1-atm gas-mixing furnace (Ar/H 2 -CO 2 gas mixture) at 1150 and 1200 °C and at the oxygen fugacity corresponding to that of the QFM buffer. Pre-synthesized pairs of the silica-rich and Fe-rich starting compositions were loaded in Pt wire loops, fused separately at 1300 °C, then brought in contact and kept at constant experimental temperature for more than 24 h. Three pairs of compositions out of four used in this study did not mix. Some temperature-dependent chemical re-equilibration was observed in the Fe-rich liquid phase but, in the cases of immiscibility, the two liquids remained compositionally distinct and showed sharp compositional gradients at contacts. One pair of liquids crystallized some tridymite, whereas the other compositions were clearly above the liquidus. Overall, the results of the experiments are in good agreement with the earlier centrifuge study and confirm the existence of stable, super-liquidus immiscibility in some Fe-rich basaltic-andesitic compositions at temperatures up to 1200 °C.

High-pressure polymorphs recovered in terrestrial craters are evidence of meteoroid impact events on the Earth’s surface. Despite countless impact craters on the Moon, high-pressure polymorphs have not been reported to date in returned Apollo samples. On the other hand, recent studies report that the high-pressure polymorphs of silica, coesite, and stishovite occur in shocked lunar meteorites. We investigated regolith breccia 15299, which was returned by the Apollo 15 mission, using the combined techniques of focused ion beam (FIB), synchrotron X‑ray diffraction (XRD), and transmission electron microscopy (TEM). The regolith breccia 15299 studied here consists of a mafic impact melt breccia with millimeter-sized, coarse-grained, low-Ti basalt clasts. The mafic melt breccia consists of fragments of minerals (olivine, pyroxene, plagioclase, silica, and ilmenite) and glass. Several quartz, tridymite, and cristobalite grains of 10-100 mm across occur in the mafic impact melt breccia. Vesicular melt veins of less than ~200 mm wide cut across the mafic melt breccia matrix and mineral fragments. Some silica grains are entrained in the melt veins. One of the silica grains entrained in the melt veins consist of stishovite [a = 4.190(1), c = 2.674(1) Å, V = 46.95 Å 3 , space group P4 2 /mnm] along with tridymite and silica glass. This is the first report of high-pressure polymorphs from returned lunar samples. TEM images show that the stishovite is needle-like in habit, and up to ~400 nm in size. Considering the lithologies and shock features of 15299, it is inferred that the stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the nearside of the Moon.