Volume 96, Issue 8-9

We report in situ Raman spectroscopic studies of ulvöspinel in a diamond-anvil cell under hydrostatic conditions up to 57 GPa at room temperature. Two modes near 493 and 681 cm -1 are observed clearly at 1 GPa. In the cubic spinel structure, the lower frequency peak can be assigned to a mode of F 2g symmetry and the higher frequency peak can be assigned to a mode with A 1g symmetry. The remaining three modes could not be observed unambiguously in the measurements, although there are five Raman-active modes (A 1g +E g +3F 2g ) in the Fd3̅m space group of the spinel structure according to factor group analysis. The peak positions and shapes in the Raman spectra agree well with those measured under ambient conditions. With increasing pressure, the frequencies of the A 1g and F 2g modes increase continuously up to 9 GPa with pressure derivatives of 2.5 and 2.1 cm -1 /GPa, respectively. There is no obvious degradation of crystal symmetry or structural change observed within this pressure range. Upon increasing pressure to ~20 GPa, the F 2g mode splits into B 1g +Eg modes, and then into B 1g +B 2g +B 3g modes. The intensities of the Raman bands gradually decrease due to the tetragonalorthorhombic phase transition. This mode completely disappears at a pressure of 29 GPa. The most striking characteristic of the Raman spectrum of ulvöspinel is that compression leads to the extinction of the Raman-active mode derived from F 2g symmetry. Only one peak resulting from the A 1g mode can be observed continuously up to 57 GPa. The peak shift derived from the A 1g mode and its full-width at half maximum (FWHM) values suggests another phase transition occurring around 30 GPa. The Raman spectrum of ulvöspinel is in good agreement with the spectra of ZnCr 2 O 4 and ZnFe 2 O 4 spinels.

Brearleyite (IMA 2010-062, Ca 12 Al 14 O 32 Cl 2 ) is a Cl-bearing mayenite, occurring as fine-grained aggregates coexisting with hercynite, gehlenite, and perovskite in a rare krotite (CaAl 2 O 4 ) dominant refractory inclusion from the Northwest Africa 1934 CV3 carbonaceous chondrite. The phase was characterized by SEM, TEM-SAED, micro-Raman, and EPMA. The mean chemical composition of the brearleyite is (wt%) Al 2 O 3 48.48, CaO 45.73, Cl 5.12, FeO 0.80, Na 2 O 0.12, TiO 2 0.03, -O 1.16, sum 99.12. The corresponding empirical formula calculated on the basis of 34 O+Cl atoms is (Ca 11.91 Na 0.06 ) Σ11.97 (Al 13.89 Fe 0.16 Ti 0.01 ) Σ14.06 O 31.89 Cl 2.11 . The Raman spectrum of brealryeite indicates very close structural similarity to synthetic Ca 12 Al 14 O 32 Cl 2 . Rietveld refinement of an integrated TEM-SAED ring pattern from a FIB section quantifies this structural relationship and indicates that brearleyite is cubic, I4̅3d; a = 11.98(8) Å, V = 1719.1(2) Å 3 , and Z = 2. It has a framework structure in which AlO 4 tetrahedra share corners to form eight-membered rings. Within this framework, the Cl atom is located at a special position (3/8,0,1/4) with 0.4(2) occupancy and Ca appears to be disordered on two partially occupied sites similar to synthetic Cl-mayenite. Brearleyite has a light olive color under diffuse reflected light and a calculated density of 2.797 g/cm 3 . Brearleyite is not only a new meteoritic Ca-,Al-phase, but also a new meteoritic Cl-rich phase. It likely formed by the reaction of krotite with Cl-bearing hot gases or fluids.

The 3.65 Å phase, a member of the family of dense hydrous magnesium silicates (DHMS) in the system MgO-SiO 2 -H 2 O, was synthesized in a 77 h multi-anvil press experiment at conditions of 10 GPa and 425 °C by using a gel of composition MgSiO 3 plus water in excess as starting materials. From our multi-methodical study including SEM, TEM, EMP, IR, and Raman analysis, we determined the composition of the 3.65 Å phase to be MgSi(OH) 6 . Powder XRD combined with Rietveld refinement revealed the 3.65 Å phase to be isostructural with δ-Al(OH) 3 . The 3.65 Å phase can be described as a hydrous A-site vacant perovskite with probably long-range random distribution of Si and Mg at octahedral sites. Locally, some ordering of Mg and Si might exist, as indicated from the spectroscopic measurements. The 3.65 Å phase represents the second DHMS with exclusively sixfold-coordinated Si, the other being phase D. The 3.65 Å phase is stable at pressures above about 9.0 GPa and temperatures below 500 °C. This limited P-T stability together with its high water content makes the 3.65 Å phase an unrealistic mantle component. If at all, it might only occur under hydrous conditions in the coldest parts of deeply and extremely fast subducted old oceanic slabs.

Water content, mineral chemistry, and oxidation state of clino- and orthopyroxene xenocrysts and megacrysts were investigated by Fourier transform infrared (FTIR) spectroscopy-including focal plane array (FPA) detector mapping, Mössbauer spectroscopy, and electron microprobe. Various tectonic settings, ages, and modes of emplacement are represented by 23 samples from 6 areas (Massif Central, France; Letseng, Lesotho; Colorado, U.S.A.; Kakanui, New Zealand; Oahu, Hawaii; and New South Wales, Australia). The xenocrysts are from both garnet and spinel peridotites-including lherzolite and harzburgite varieties-and one sample of clinopyroxenite. Water contents vary between ~10 and 600 wt ppm. Samples from Massif Central, Colorado, Kakanui, and Hawaii have high water contents: 180-600 wt ppm. The samples from Lesotho and New South Wales have considerably lower amounts: ~10-300 wt ppm. Water contents of xenocrysts and megacrysts from New South Wales vary within a narrow range (clinopyroxene: ~50 wt ppm, orthopyroxene: 15-20 wt ppm), whereas the water content of the Lesothian samples scatter considerably. No significant correlations are observed between water content, mineral chemistry, or oxidation state of the samples. FPA mapping reveals homogenous distribution of water in the pyroxene lattice. The results are compared to available literature and research on water diffusion in natural mantle pyroxene. Altogether, the data suggest that water in pyroxene found in fresh peridotite xenoliths partly reflects the water content of the mantle source region. On the other hand, variable mineral chemistry and water contents of megacryst pyroxenes indicate processes such as magmatic equilibration, magma mixing, and contamination.

Mesocrystals attract increasing interest in the fields of physics, chemistry, materials, and Earth sciences. Yet, structural properties of this new class of solid materials are not fully described. Biominerals often display complex hierarchical mesocrystalline organizations. We report on the crystallography of sclerites, which are small (-80 μm), beautifully shaped calcitic biomineral structures found in the living tissues of octocorals. Despite a layered concentric structure, the submicrometer crystalline units constituting the sclerites display a remarkably simple crystallographic organization of similarly oriented units with only a low degree of misorientation between them. Some sclerites display crystallographic sectors, leading to the concept of “mesotwin.” A mesotwin is to a twin what a “mesocrystal” is to a crystal: an analog with defects. On the basis of EBSD data, we propose a simple conceptual crystallographic model that accounts for the observed features. This model involves different rhombohedral unit blocks, with identical shapes and volumes, but different crystallographic faces. EBSD data show that quite unexpectedly slight misorientations of crystallites in the sclerites are not at random but organized around the three equivalent a axes of the hexagonal unit cell of calcite. In a subtle way, the overall organization and morphologies of the red coral sclerites are connected to the calcite crystallography.

He + ion implantation on albite (Minas Gerais, Brazil) at 4.0 MeV, corresponding to the energy of α particle from 238 U fission, has been conducted to clarify the radiation effects of α particles from radioactive minerals on cathodoluminescence (CL) of albite. CL of albite results in various emission bands at ~380, ~560, and ~740 nm, and in the UV range. Red emission at 700-750 nm is detected in the CL spectra of the implanted samples. Total CL intensities of these UV, blue, yellow, red, and IR emissions vary among the samples. High-resolution CL imaging of the cross-section samples shows a CL halo on the implanted surface of approximately 14 μm thickness, which is consistent with a theoretical range of α particles of 4.0 MeV. It was first confirmed experimentally that the CL halo is created by α particles. The deconvolution of CL spectra in the red emission range by Gaussian fitting provides the component at 1.861 eV that is attributed to a radiation-induced defect center produced by He+ ion implantation. The intensity of the component at 1.861 eV linearly correlates with the dose density of He+ implantation on albite as a function of the population of the radiation-induced defect center, regardless of other factors such as concentration and distribution of other emission centers, existence of microstructures and textures, and crystallographic orientation. The CL spectral deconvolution has a high potential for quantitative evaluation of the radiation dose of α particles from natural radionuclides on albite for a geodosimetry.

Two hexagonal aluminous phases, which could serve as potential Na- and K-host minerals in the lower mantle, with compositions K 1.00 Mg 2.00 Al 4.80 Si 1.15 O 12 and Na 1.04 Mg 1.88 Al 4.64 Si 1.32 O 12 were synthesized at 22-25 GPa and 1500 °C. The K-rich hexagonal aluminous phase was synthesized for the first time. Crystal structures of both hexagonal aluminous phases were refined using the Rietveld method. Obtained interatomic distances and bond angles were compared to published data on the hexagonal aluminous phase CaMg 2 Al 6 O 12 . The general chemical formula of the hexagonal aluminous phase is represented as [M 3 ][M 2 ] 2 [M 1 ] 6 O 12 , where the small-, middle-, and large-sized cations occupy the M1, M2, and M3 sites, respectively. Changes of size and shape of M1O 6 octahedra by the substitution of Si 4+ for Al 3+ in the M1 site make it possible to adjust the size of the M2 and the M3 sites to accommodate Na + and Mg 2+ in the M2 sites and Na + and K + in the M3 sites, respectively. The stability of hexagonal aluminous phases in a relatively wide compositional range of 30-50 mol% in NaAlSiO 4 component along the NaAlSiO 4 -MgAl 2 O 4 join can be explained by possible replacement of Mg 2+ by Na + in the M2 site and by shrinkage and deformation of M1O 6 octahedra with the coupled substitution: M2 Mg 2+ + M1 Al 3+ → M2 Na + + M1 Si 4+ .

We have measured the elastic response of radiation-damaged titanite, CaTiSiO 5 , as a function of thermal annealing. We estimate the bulk modulus of the damaged samples (-24% amorphous) to be 85 GPa, which is much softer than for undamaged crystalline titanite [131.4 GPa; Angel et al. (1999)]. Conversely, the lowest shear modulus of the radiation-damaged material is 52-58 GPa, which is harder that of the undamaged titanite, 46-52 GPa. The bulk and shear moduli of the radiation-damaged materials are close to those of thermal titanite glass, B glass ≈ 75 GPa and G glass ≈ 47 GPa, and are much smaller than expected based on other radiation-damaged materials such as zircon (ZrSiO 4 ). Surprisingly, annealing of the damaged titanite in the range 600 < T < 1000 K leads to additional massive softening of the shear moduli. During annealing the shear modulus of titanite sample 1 softened from 58 to 29 GPa, and sample 2 softened from 52 to 19 GPa. The temperature range for the softening coincides with that found for crystallization of the amorphous regions, as measured previously by diffraction and spectroscopic methods. In contrast to the huge softening of the ultrasonically measured shear modulus, the calorimetrically measured Debye temperature θ D increases by -5%, suggesting a small intrinsic hardening of the acoustic shear modes. Additional heating to 1473 K leads, in one titanite sample, to a steep increase of the shear modulus to values much larger than that of the initial, radiation-damaged material. Theoretical models are discussed to rationalize the massive softening due to both radiation damage and subsequent anneal.

The diffusivity of C and O in calcite in mixed CO 2 -H 2 O fluid was determined over the range in x CO ₂ from 1.0 to about 0.2 at 700 °C, 100 MPa, with selected experiments conducted at pressures to 250 MPa and temperatures of 600 and 800 °C. The diffusivity of C, D C , varies little with x CO ₂, although there is some evidence for a slight increase in DC from -5 × 10 −18 to -5 × 10 −17 cm 2 /s with decreasing x CO ₂. Our data and those of others are consistent with a model for DC ∝ 1/f CO₂ . Despite the large uncertainty, we observed that the diffusivity of O, DO, increases from ~2 × 10 −16 to ~5 × 10 −14 cm 2 /s with x CO ₂ decreasing from 1.0 to 0. There is a good correlation at 700 °C between log D O and log f H ₂ O regardless of the total pressure, matching the observations of previous workers. The data are consistent with a simple two-component model for the diffusion of O in calcite, one component for diffusion in the presence of CO 2 and one in the presence of H 2 O: D O = D O CO₂ + D O H₂O a H₂O . The activity of H 2 O is relative to the fugacity at 100 MPa, 700 °C. D O CO ₂ is 3.45 × 10 −16 , and D O H₂O is 3.8 × 10 −14 cm 2 /s. The data indicate that the rate of diffusion of C and O in calcite is controlled by reactions at the surface of calcite. Adsorption of H 2 O and the creation of vacancies at the surface account for the dependence of the diffusivity on the fugacity of the fluid components. There is little evidence that H itself diffuses into calcite. With this model and the values of D O in pure CO 2 (Labotka et al. 2000) and in pure H 2 O (Farver 1994), the value of D O is predicted over the temperature range 600-800 °C and p H₂O up to 300 MPa, the range of the data. Calculated closure temperatures for diffusive exchange of O between calcite and fluid are reduced by about 150 °C in the presence of an aqueous fluid.

Acid mine drainage remediation is an unresolved matter in abandoned mining districts around the world. Development and implementation of passive treatment systems in these areas are commonly focused on engineering and water quality aspects. Neoformed mineral phases precipitated within the reactive material of these passive treatments account for the removal of pollutants but also can cause clogging and passivation of the reactive substrate. After 20 months of operation and monitoring, the limestone-based passive treatment system implemented in Mina Esperanza (SW Spain) was sampled to study the relationship between water chemistry, mineral composition of the neoformed precipitates, and treatment performance. Water chemical profiles show the existence of three precipitation zones controlled by Fe, Al, and Zn hydrochemistry and also a migration with time of precipitation zones downward into the reactive material. These precipitation zones were also confirmed by a mineral study performed on the solid samples where either schwertmannite and goethite or hydrobasaluminite and Zn-rich green rust were the mineral phases that controlled the metal removal in the three precipation (Fe, Al, or Zn) zones. Iron and Al precipitates were observed to play a critical role in the time evolution of the reactive material hydraulic conductivity. Furthermore, Al precipitates passivated to some extent the limestone grains by armoring, although migration of the Fe precipitation zone and Al redissolution later activated the limestone grains. A higher proportion of limestone in the reactive mixture and the addition of new reagents to the bottom section of the reactive material (to enhance the reducing environment and to promote divalent metal removal) are proposed on the basis of this hydrochemical and mineralogical study for a future design for the Mina Esperanza passive treatment system.

Chromium and vanadium are stable in multiple valence states in natural systems, and their distribution between garnet and silicate melt is not well understood. Here, the partitioning and valence state of V and Cr in experimental garnet/melt pairs have been studied at 1.8-3.0 GPa, with variable oxygen fugacity between IW-1.66 and the Ru-RuO 2 (IW+9.36) buffer. In addition, the valence state of V and Cr has been measured in several high-pressure (majoritic garnet up to 20 GPa) experimental garnets, some natural megacrystic garnets from the western United States, and a suite of mantle garnets from South Africa. The results show that Cr remains in trivalent in garnet across a wide range of oxygen fugacities. Vanadium, on the other hand, exhibits variable valence state from 2.5 to 3.7 in the garnets and from 3.0 to 4.0 in the glasses. The valence state of V is always greater in the glass than in the garnet. Moreover, the garnet/melt partition coefficient, D(V), is highest when V is trivalent, at the most reduced conditions investigated (IW-1.66 to FMQ). The V 2.5+ measured in high P-T experimental garnets is consistent with the reduced nature of those metal-bearing systems. The low V valence state measured in natural megacrystic garnets is consistent with f O₂ close to the IW buffer, overlapping the range of fo₂ measured independently by Fe 2+ /Fe 3+ techniques on similar samples. However, the valence state of V measured in a suite of mantle garnets from South Africa is constant across a 3 log fo₂ unit range (FMQ-1.8 to FMQ-4.5), suggesting that the valence state of V is controlled by the crystal chemistry of the garnets rather than fo₂ variations. The compatibility of V and Cr in garnets and other deep mantle silicates indicates that the depletion of these elements in the Earth’s primitive upper mantle could be due to partitioning into lower mantle phases as well as into metal.

The solubility of OH in pure synthetic rutile was experimentally constrained at 0.5-2.0 GPa and 500-900 °C, in equilibrium with four oxygen fugacity (ƒ O₂ ) buffering mineral assemblages: hematite-magnetite (HM), nickel-nickel oxide (NNO), cobalt-cobalt oxide (CCO), and iron-wüstite (IW). The hydroxyl concentration ([OH], in parts per million H 2 O by weight) of equilibrated rutile crystals was characterized by FTIR spectroscopy. Measurements at 1 GPa at individual ƒ O₂ buffers demonstrate that [OH] in rutile depends strongly on temperature: at HM, [OH] increases from 48 to 267 ppm as temperature rises from 500 to 900 °C, whereas at NNO, [OH] increases from 108 to 956 ppm over the same temperature range. The [OH] in rutile also increases strongly with decreasing ƒ O₂ at any pressure and temperature, and exhibits a slight, linear, positive dependence on pressure at a given temperature and ƒ O₂ . The observed systematic dependences on pressure, temperature, and ƒ O₂ indicate that hydrogen substitutes into rutile as hydroxyl, (OH), via forward progress of the reaction Ti 4+ O 2 + ½H2O = Ti 3+ O(OH) + ¼O 2 . Our measured [OH] values are significantly greater than those determined in previous studies on finer-grained, polycrystalline rutile, which likely suffered diffusive loss of H during quenching. This is supported by our observation of narrow, OH-depleted rims on otherwise high-OH run products, pointing to minor but important diffusive H loss from crystal rims during quenching. Fitting of isothermal variations in composition with ƒ O₂ at 1 GPa and temperature indicates nearly ideal, multi-site mixing of the TiO 2 -TiOOH solid solution. A fit to the entire data set suggests standard volume, enthalpy, and entropy of the hydration reaction of, respectively, 1.90 ± 0.48 cm 3 /mol, 219.3 ± 1.3 kJ/mol, and 19.9 ± 1.4 J/(mol∙K) (1σ uncertainty). These constraints form the basis for use of [OH] in rutile as a thermobarometer and oxybarometer in experimental and natural systems. The moderate to high [OH] in nominally anhydrous rutile at all investigated temperatures, pressures, and ƒ O₂ values imply that Ti 3+ may be higher than previously suspected in some terrestrial geologic settings.

Zircon inclusions, extracted from garnet from an ultrahigh-pressure eclogite in the eastern part of the Dabie orogen, were investigated with high spatial resolution U-Pb geochronology. The unzoned zircon rims have garnet and omphacite inclusions and show weak cathodoluminescence, low-Th/U ratios (0.02-0.18), flat high rare-earth elements patterns, and an absence of Eu anomalies, which suggest metamorphic growth in the presence of garnet and in the absence of feldspar. Dispersed concordia U-Pb ages, ranging from 205.1 ± 3.3 to 255.3 ± 6.1 Ma, defined a weighted mean 206 Pb/ 238 U age of 223 ± 8 Ma (mean square of weighted deviation, MSWD = 38), which is significantly younger than the Lu-Hf garnet-whole rock age of 242.2 ± 2.6 Ma. The U-Pb age probability distribution indicates two age populations at ca. 241 and ca. 216 Ma. The zircon rims, with younger ages, are distinguished from those with older ages by the occurrence of garnet and omphacite inclusions. Well-preserved prograde major- and trace-element zoning in garnet and the significant contribution of older garnet cores in bulk isotopic digestion suggest that the Lu-Hf age mostly reflects an early phase of garnet growth likely at amphibolite facies conditions. The new Lu-Hf age combined with the U-Pb ages of the zircon inclusions in garnet suggest that garnet growth was sustained over a time interval of ~25 m.y.

We report here for the first time that fully and partially NH4-exchanged natrolites can be prepared in hydrated states using the solution exchange method with potassium-natrolite. The structural models of the as-prepared hydrated phases and their dehydrated forms at elevated temperature were refined in space group Fdd2 using in situ synchrotron X‑ray powder diffraction data and Rietveld methods. The unit-cell volumes of the hydrated NH 4 -exchanged natrolites at ambient conditions, (NH 4 ) 16(2) Al 16 Si 24 O 80 ·14.1(9)H 2 O and (NH 4 ) 5.1(1) K 10.9(1) Al 16 Si 24 O 80 ·15.7(3)H 2 O, are found to be larger than that the original sodium-natrolite by ca. 15.6% and 12.8%, respectively. Upon temperature increase, the fully NH 4 -exchanged natrolite undergoes dehydration at ca. 150 °C with ca. 16.4% contraction in the unit-cell volume. The dehydrated phase of the fully NH 4 -exchanged natrolite exhibits marginal volume expansion up to 425 °C and then becomes amorphized during temperature decrease and exposure to atmospheric condition. In the case of the partially NH 4 -exchanged natrolite, the dehydration starts from ca. 175 °C with ~15.1% volume contraction and leads to a partial phase separation to show a phase related to the dehydrated K-natrolite. The degree of the phase separation decreases with temperature increase up to 475 °C, concomitant to the gradual volume contraction occurring in the partially NH 4 -exchanged natrolite in the dehydrared state. Upon temperature decrease and exposure to atmospheric condition, only the dehydrated K-natrolite is recovered as a crystalline phase from the partially NH 4 -exchanged natrolite. In the hydrated model of the fully NH 4 -exchanged natrolite, the ammonium cations and water molecules are statistically distributed along the elliptical channels, similar to the disordered pattern observed in natrolites exchanged with larger alkali metal cations such as the K-, Rb-, and Cs-forms. The dehydrated model of the fully NH 4 -exchanged natrolite at 400 °C is essentially same as the one reported previously from the sample prepared by direct melt exchange method using sodium-natrolite. Both the hydrated and dehydrated structures of the partially NH 4 - exchanged natrolite at RT and at 400 °C, respectively, are characterized by having two separate sites for the ammonium and potassium cations. Comparing the structural models of the monovalent cation forms studied so far, we find that the rotation angle of the natrolite chain is inversely proportional to the cation radius both in the hydrated and dehydrated phases. The distribution pattern of the nonframework species along the natrolite channel also seems to be related to the non-framework cation radius and hence to the chain rotation angle.

It is well known that oriented iron and titanium oxide inclusions occur in pyroxenes and plagioclase of anorthosites and granulites, and they are attributed to exsolution at subsolidus conditions. The oxides occur as needles or platelets. In this study, we determine the morphology of oxide needles as well as their orientation in plagioclase (An 30-35) in anorthosite gneiss from the Adirondack mountains (New York). The investigation was done with electron backscatter diffraction (EBSD) in a scanning electron microscope, as well as Laue diffraction with a microfocus synchrotron X‑ray beam at the Advanced Light Source in Berkeley. It was observed that the needle direction is [110] in magnetite and [101̅0] in ilmenite. The needle direction is consistently parallel to [001] of plagioclase. Furthermore, (111) of magnetite and (0001) of ilmenite are sub-parallel to (1̅20) and (120) of plagioclase. We note that for directions [110] in the magnetite structure and [101̅0] in ilmenite, O atoms are close-packed, and (111) and (0001) are close-packed planes, correspondingly. In plagioclase, [001] is a direction with open channels as well as approximate alignment of Si tetrahedral edges, thus providing nucleation sites with a coincidence lattice relationship. (120) and (1̅20) in this triclinic mineral are planes with approximate tetrahedral sides so that the relationship is structurally plausible. From Laue diffraction, we can determine that the magnetite needle axis is subject to an extensional stress, most likely attained during cooling of the inclusions within the plagioclase host.

The low-temperature isobaric heat capacity (C P ) of synthetic stishovite was measured between 2 and 311 K by the thermal relaxation method. The measured C P is considerably smaller than that of Holm et al. (1967) over the whole temperature range studied. The standard entropy, S o 298.15 , of stishovite obtained is 24.0 J/(mol∙K), which is 3.8 J/(mol∙K) lower than that of Holm et al. (1967). Using the measured C P , the Debye temperature and thermal Grüneisen parameter at 298.15 K were calculated to be 1109 K and 1.68, respectively. The equilibrium coesite-stishovite transition boundary was calculated using the S o 298.15 of stishovite from published thermodynamic data. The calculated boundary has a slope of 3.2 ± 0.1 MPa/K at 1200-1600 K, which is larger than the slope determined by the high-pressure in situ X-ray diffraction study by Zhang et al. (1996).

Synthetic stishovite before and after electron- and γ-ray irradiation has been investigated by singlecrystal electron paramagnetic resonance (EPR) spectroscopy for the first time. Room-temperature single-crystal EPR spectra of as-is stishovite reveal two high-spin (S = 3/2) Cr 3+ centers: one with D 2h symmetry and another of triclinic symmetry. Quantitatively determined spin Hamiltonian parameters, including matrices g, D, and A( 53 Cr) and high-spin Zeeman term BS 3 , suggest that the D 2h center represents a substitutional Cr 3+ ion at the Si site without an immediate charge compensator. The triclinic center, which is characterized by a well-resolved 1 H superhyperfine structure, also arises from a substitutional Cr 3+ ion at the Si site but has an H + charge compensator bonded to one of the four equatorial oxygen atoms. The magnitude and orientation of the 1 H superhyperfine structure yield the location of the H atom at (0.46, 0.12, 0). These structural models for Cr and H in stishovite also have been corroborated by periodic density functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP), with 2 × 2 × 4 supercells, plane-wave basis sets and the projector augmented wave (PAW) potentials. In addition, 85 K EPR spectra of irradiated stishovite show that the two Cr 3+ centers are both converted to an S = 1/2 Cr 5+ center characterized by two 29 Si superhyperfine structures arising from interactions with two nearest and eight second-nearest Si atoms, respectively. The spin Hamiltonian parameters of this Cr 5+ center provide further support for the location of the two Cr 3+ centers at the Si site.

We present the results of a variable-temperature (VT) 31 P magic angle spinning NMR (MASNMR) study of a series of solid solutions between different synthetic rare earth (RE = Y, La, Ce, Pr, Nd, Eu, Dy) orthophosphates (REPO 4 ) taking either the monoclinic monazite or tetragonal xenotime (zircon) crystal structure. Solid solutions were formed by mixing a small amount of a paramagnetic REPO 4 material (RE = Ce, Pr, Nd, Eu, Dy) with either diamagnetic LaPO 4 or YPO 4 , which take the monoclinic and tetragonal crystal structures, respectively. Mixtures were made with up to 10 mol% (nominal content) of the paramagnetic component. 31 P spectra of these materials contained several paramagnetically shifted resonances indicating some dissolution of the paramagnetic rare earth into the host LaPO 4 or YPO 4 phase; however, it is clear that none of the samples studied here reached a state of complete solid solution. The use of multiple paramagnetic species in dilute solid solution with two diamagnetic materials taking different crystal structures enabled an investigation of the probable mechanisms of paramagnetic interactions in the 31 P NMR experiments. A peak assignment model is introduced for the 31P spectra. Our analysis indicates that the paramagnetic interactions are dominated by the Fermi contact shift with a secondary contribution from the so-called “pseudocontact” shift.

Metamorphic garnet with sector zoning in the cores and oscillatory zoning in the rims grew during rapid heating of pelitic rocks in the Chiwaukum Schist. These types of compositional zoning are exemplified by sharp, but low amplitude, boundaries between broad petal-shaped sectors and between narrow concentric zones, respectively. Zoning is most obvious in calcium, which is inversely correlated with iron and magnesium content. Garnet habit inferred from the oscillatory calcium zoning and external morphology indicates a growth transition from early crystal forms with both trapezohedral and dodecahedral forms to later crystal forms dominated by trapezohedral faces. This transition is accompanied by changes in compositional sector zoning and may reflect the roles of local growth dynamics and external forcing mechanisms affecting growth of garnet. Subsequent modification of these textures produced patchy zoning in calcium. Electron backscatter diffraction confirms the inferred crystal growth habits and that compositional zoning occurs within single garnet crystals. Exchange thermobarometry and the peak mineral assemblages indicate that metamorphic conditions reached 640-670 °C at 6.9 kbar. These temperatures are sufficient to allow significant volume diffusion; therefore, preservation of the finely banded compositional zoning in garnet requires rapid heating and cooling. Garnet Sm-Nd ages and indistinguishable zircon U-Pb ages from adjacent orthogneiss bodies confirm that garnet grew rapidly during localized and short-lived heating adjacent to sill-like intrusions of tonalite.

The phase stability, elastic behavior, and pressure-induced structural evolution of a natural metamorphic kalsilite (ideal formula KAlSiO 4 ) from Punalur (Kerala district in southern India), with P31c symmetry and a K/Na molar ratio of ~350, has been investigated by in situ X‑ray single-crystal diffraction up to ~7 GPa with a diamond-anvil cell under hydrostatic conditions. At high-pressure, a previously unreported iso-symmetric first-order phase transition occurs at ~3.5 GPa. The volume compression of the two phases is described by third-order Birch-Murnaghan equations-of-state: V 0 = 201.02(1) Å 3 , K T0 = 59.7(5) GPa, K′ = 3.5(3) for the low-P polymorph, and V 0 = 200.1(13) Å 3 , K T0 = 44(8) GPa, K′ = 6.4(20) for the high-P polymorph. The pressure-induced structural evolution in kalsilite up to 7 GPa appears to be completely reversible. The compression of both phases involves tetrahedral rotations around [0001], which close up the channels within the framework. In addition, compression of the low-pressure phase involves tilting of the tetrahedra. The major structural change at the phase transition is an increase in the tilting of the tetrahedra, but with a reversion of the tetrahedral rotations to the value found at ambient conditions. This behavior is in distinct contrast to that of nepheline, which has a tetrahedral framework of the same topology.

A novel type of high-pressure low-flow metering valve has been designed for experiments at continuous decompression in internally heated pressure vessels (IHPV) at pressures up to 500 MPa and temperatures up to 1500 °C. It consists of a modified high-pressure valve coupled with a piezoelectric nanopositioning system. The piezoelectric actuator (1) positions the needle of the valve statically with very high precision on a nanometer scale for infinitesimally slow decompression and, alternatively, (2) opens and closes the valve completely, within milliseconds, to achieve a fast pressure drop only limited by the diameter of the high-pressure tubing and the entire volume of the autoclave system. The valve connected to an IHPV has been successfully tested to simulate continuous decompression of volatile-bearing magma. Water saturated rhyodacitic melt was synthesized at 1050 °C and 300 MPa and subsequently decompressed to 50 MPa at an integrated decompression rate of 0.28 MPa/s using continuous decompression as well as single-step and multi-step decompression techniques. The experimental results of the three methods show significant differences, having important implications for the interpretation of textures observed in natural volcanic rocks. Bubble number density (BND) values increase from continuous to multi-step and to single-step decompression by two orders of magnitude. Bubble size distribution (BSD) also differs significantly. The BSD curve of the singlestep decompression shows largest variation both in size and in population number density, whereas the BSD trend for the sample of multi-step decompression shows the smallest variation between size and population density of bubbles. The BSD of the continuously decompressed sample indicates similar proportions of bubbles with different size. Channel formation in the sample of single-step decompression may indicate fluid escape from over-pressurized bubbles through the melt. Singlestep and continuous decompression style represent two extreme cases of magma degassing in which bubble nucleation and bubble growth are predominant processes, respectively, whereas multi-step decompression resembles the intermediate case.

The crystal chemistry of Ti-rich phlogopite from deposits of the oldest activity (~740 ka) of Mt. Vulture, Potenza, Italy, was investigated to identify the substitution mechanisms in Ti-rich phlogopite and to determine its potential as a geohygrometer. Substitution mechanisms were determined by electron probe microanalysis (EPMA), single-crystal X‑ray diffraction (SCXRD), Mössbauer spectroscopy, and micro-Fourier transform infrared (FTIR) spectroscopy. Magnetite and sanidine from the mica-rich host rocks were also analyzed. Use of the biotite-magnetite-sanidine geohygrometer in these volcanics (essentially trachytic-phonolitic ignimbrites) is exploited. All investigated phlogopite samples consist of the 1M polytype, with 5.3275(2) ≤ a ≤ 5.3635(4) Å, 9.2211(4) ≤ b ≤ 9.2958(8) Å, 10.1211(5) ≤ c ≤ 10.281(1) Å, and 99.980(3) ≤ β ≤ 100.097(2)°. Structure refinements in space group C2/m converged to 2.42 ≤ R 1 ≤ 4.00% and 2.04 ≤ wR 2 ≤ 4.50%. VI Fe 3+ /Fe tot from Mössbauer analyses ranged from 34(1)-89(1)%. The main bands in the OH-stretching region are 3709, 3682, and 3658 cm -1 , and were assigned to 3Mg-OH - -K-OH - , 3Mg-OH - -K-O 2- , and 2MgFe 3+ - OH - -K-O 2- local configurations, respectively. The overall crystal chemical features are compatible with the M3 + -Tschermak substitutions ( VI M 2+ + IV Si 4+ ↔ VI M 3+ + IV Al, with M 3+ = Al,Fe 3+ ), M 3+ -oxy [ VI M 2+ + (OH)- ↔ M 3+ + (O) 2- + ½H 2 ↑] and Ti-oxy substitutions [ VI M 2+ + 2(OH) - → VI Ti 4+ + 2(O) 2- + H 2 ↑]. The magnetite composition varies between Mt 82 Uv 18 and Mt 93 Uv 07 , whereas the proportion of orthoclase with respect to albite plus anorthite is 0.77 ≤ X Or ≤ 0.82. A partly ionic model was used to estimate annite activity, starting from experimentally determined substitutions in phlogopite. The resulting water fugacity was an order of magnitude lower than that calculated using stoichiometry to determine the phlogopite formula. The results are consistent with the occurrence of a well-developed hydrothermal system that altered the magmatic oxygen fugacity conditions at Mt. Vulture.

In this work, we have calibrated the infrared (IR) method for determining OH concentrations in apatite with absolute concentrations obtained through elastic recoil detection (ERD) analysis. IR spectra were collected on oriented, single-crystal apatite samples using polarized transmission infrared spectroscopy. The weight percent H 2 O is 0.001199 ± 0.000029 (the error is given at 1σ level hereafter) times A/d, where A is the linear absorbance peak height measured using polarized IR when the light vector E is parallel to the c-axis of the apatite crystal, and d is the sample thickness in centimeters. This corresponds to a linear molar absorptivity, ε = 470 ± 11 L/mol/cm -1 . The calibration using linear absorbance can be applied when there is only one dominant peak at 3540 cm-1. If other peaks are significant, then the integrated molar absorptivity, ε = (2.31 ± 0.06) ×10 4 L/mol/cm 2 , should be used. The detection limit of H 2 O concentration in apatite by IR approaches parts per million level for wafers of 0.1 mm thickness. The accuracy based on our calibration is 5-10% relative.

Natural colorless calcite crystals are colored electrolytically by using a pointed cathode and a flat anode at various temperatures and voltages. Ca + and CO 3 − color centers, Ca 2+ vacancies, and CO 3 2- units are produced in colored crystals. Absorption spectral bands of the Ca + and CO 3 − color centers, Ca 2+ vacancies, and CO 3 2- units are reported for the colored crystals at room temperature. Pb 2+ spectral bands are observed in absorption and fluorescence spectra of uncolored and colored crystals. The currenttime curve for electrolytic coloration of natural calcite crystal and its interpretation with respect to the electrolytic coloration process are given. Creation and conversion of color centers are explained.

The sound velocities and elastic properties of a series of hydrous rhyolite, andesite, and basalt glasses have been determined by Brillouin scattering spectroscopy at ambient conditions to elucidate the effect of glass composition on the compressibility of dissolved water. Both the adiabatic bulk (K S ) and shear modulus (μ) of the dry glasses decrease with increasing silica content (K S,basalt > K S,andesite > K S,rhyolite and μ basalt > μ andesite > μ rhyolite ). For each composition, the shear modulus systematically decreases with increasing water content. Although the addition of up to 14 mol% water decreases the K S of andesite and basalt glasses by up to 6%, there is no discernable effect of water on the K S of the rhyolite glasses. The partial molar KS of dissolved water (K S ) in rhyolite, andesite, and basalt glasses are 37 ± 5, 19 ± 7, and 40 ± 3 GPa, corresponding to partial molar isothermal compressibilities (β̅ T ) of 0.029 ± 0.005, 0.042 ± 0.004, and 0.026 ± 0.002 GPa −1 , respectively. These results indicate that the compressibility of dissolved water strongly depends on the bulk composition of the glass; hence, the partial molar volume of water cannot be independent of the bulk composition at elevated pressure. If the compressibility of dissolved water also depends on composition in the analog melts at high temperature and pressure, these observations will have important consequences for magmatic processes such as magma mixing/unmixing and fractional crystallization.

The miniaturized Mössbauer spectrometer (MIMOS II) was used to monitor in situ the mineralogical transformation of lepidocrocite (γ-FeOOH) in a Shewanella putrefaciens culture under anaerobic conditions using methanoate as the electron source. Magnetite was the only biogenic mineral formed during the course of the incubation. The analysis of the biogenic mineral by transmission electron microscopy (TEM) revealed cubic-shaped crystals with a relatively homogeneous grain size of about 50 nm. After one day of incubation, the departure from stoichiometry, δ, of the biogenerated magnetite was very low (δ ~ 0.025) and rapidly reached values close to zero. Such low values of δ were not obtained for magnetite synthesized inorganically when Fe 3+ in the form of γ-FeOOH was reacted with stoichiometric quantities of soluble Fe 2+ and OH - . The experimental setup used in this study could be replicated in field experiments when assessing the formation of magnetite in modern geological settings as its formation is suspected to be caused by a strong bacterial activity.

The relative proportion of divalent and trivalent Eu has proven to be a useful tool for estimating fo 2 in various magmatic systems. However, in most cases, direct determination of the Eu valence state has not been made. In this study, direct determination of Eu valence by XANES and REE abundance in merrillite provide insights into the crystal chemistry of these phosphates and their ability to record conditions of magmatism. Merrillite strongly prefers Eu 3+ to Eu 2+ , with the average valence state of Eu ranging between 2.9 and 3 over approximately six orders of magnitude in fo 2 . The dramatic shift in the REE patterns of merrillite in martian basaltic magmas, from highly LREE-depleted to LREE-enriched, parallels many other trace element and isotopic variations and reflects the sources for these magmas. The behavior of REE in the merrillite directly reflects the relationship between the eightfold-coordinated Ca1 site and adjacent sixfold Na and tetrahedral P sites that enables charge balancing through coupled substitutions.