Band 96, Heft 2-3

Paulscherrerite, UO 2 (OH) 2 , occurs as an abundant dehydration product of metaschoepite at the Number 2 Workings at Radium Ridge, Northern Flinders Ranges, South Australia. The mineral name honors the contribution of Swiss physicist Paul Scherrer (1890-1969) to mineralogy and nuclear physics. Individual paulscherrerite crystals are tabular, reaching a maximum of 500 nm in length. Paulscherrerite has a canary yellow color and displays no fluorescence under UV light. Chemically, paulscherrerite is a pure uranyl hydroxide/hydrate, containing only traces of other metals (<1 wt% in total). Bulk (mg) samples always contain admixtures of metaschoepite (purest samples have ~80 wt% paulscherrerite). A thermogravimetric analysis corrected for the presence of metaschoepite contamination leads to the empirical formula UO 3 ·1.02H 2 O, and the simplified structural formula UO 2 (OH) 2 . Powder diffraction shows that the crystal structure of paulscherrerite is closely related to that of synthetic orthorhombic α-UO 2 (OH) 2 . However, splitting of some X-ray diffraction lines suggests a monoclinic symmetry for type paulscherrerite, with a = 4.288(2), b = 10.270(6), c = 6.885(5) Å, β = 90.39(4)°, V = 303.2(2) Å 3 , Z = 4, and possible space groups P2, P21, P2/m, or P2 1 /m. Paulscherrerite-like material was synthesized using various methods, including heating metaschoepite in water at 150 °C and slow hydration of UO 3 (am) in air; material synthesized using hydrothermal techniques displayed peak splitting indicative of monoclinic symmetry. Paulscherrerite has been reported under the name “dehydrated schoepite” as an early weathering product of uraninite/pitchblende in several deposits, such as Shinkolobwe, Zaire; Nopal I deposit, Mexico; and the granitic pegmatites of New Hampshire, U.S.A.

Predicting phase relations and reactions involving phyllosilicates is of critical importance when modeling geological reservoirs and making thermobarometric estimates at temperatures below 350°C. However, it is difficult to perform experimental studies of phase relations of phyllosilicates because of very slow reaction rates. Simple methods of estimations of thermodynamic properties of minerals such as oxide summation techniques are insufficient to constrain activity models for minerals. In this study, we use a recent atomistic technique that allows constraining activity models in minerals independently from kinetics. Among the various solid solutions occurring in phyllosilicates, the magnitude and the thermodynamic significance of the pyrophyllitic substitution has strong implications on the stability of clay minerals at surface conditions. We have applied a lattice energy estimation method combined with Monte Carlo simulations to estimate the energy of mixing along the muscovite-pyrophyllite solid solution at 25°C and 1 bar. The results suggest a strongly positive and asymmetric Gibbs free energy of mixing, concordant at first order with previous thermodynamic models issued from phase relations and with field observations. The calculated solvus implies that pyrophyllite-rich phyllosilicates are unstable, unless another phenomenon such as hydration stabilizes them. Calculated structures at low muscovite contents present large variations of interlayer occupancies due to short- and long-range ordering. The observed ordering suggests that sub-layers in illite/smectite mixed layers minerals are not independent as the alternation of K-rich and K-depleted sublayers minimizes the Gibbs free energy of the mineral

The Paleoproterozoic Khondalite belt in the North China craton preserves evidence for ultrahightemperature (UHT) crustal metamorphism associated with the collision of the Yinshan and Ordos Blocks. Here we apply two-feldspar geothermometry to UHT granulites from two localities newly reported in this study (Tuguishan and Xuwujia) and another two localities from previous studies (Dajing/ Tuguiwula and Dongpo) in the Khondalite belt. The presence of abundant perthite/mesoperthite in these rocks reflects post-peak slow cooling. The minimum estimated peak metamorphic temperatures are 832-998, 819-952, 844-1037, and 966-1019 °C computed at 8 kbar for the Dajing/Tuguiwula, Dongpo, Tuguishan, and Xuwujia areas, respectively. These results confirm the previous report of extreme metamorphism at Dajing/Tuguiwula and Dongpo, and reveal similar conditions in the new localities reported here, suggesting that UHT metamorphism is widespread in the Khondalite belt of the North China craton. Our study demonstrates that UHT metamorphism can be recognized using the two-feldspar geothermometry in rocks that do not possess other key UHT assemblages

The Coeur d’Alene mining district of northern Idaho is one of the major Ag-Pb-Zn producers in the world; it contains Ag-Pb-Zn replacement veins in clastic burial metamorphic rocks of the Middle Proterozoic Belt Supergroup. Electron microprobe analyses and Raman spectroscopy of samples from the Gold Hunter vein of the Lucky Friday mine revealed the presence of the rare iron oxide wüstite (Fe 1-x O) coexisting with stable magnetite. An assemblage of galena, pyrite, and siderite is associated with the two iron oxides although these minerals do not share phase boundaries with wüstite and their coexistence is, therefore, thermodynamically precluded. Wüstite is stable under high-temperature conditions (above 570°C at 1 bar). Increasing pressure can lower this temperature significantly. However, the veins in the Coeur d’Alene district formed well outside the wüstite stability field and represent a low-temperature (250-350 °C), low-pressure (1-3 kbars), and low-f O₂ environment. To form wüstite at temperatures around 250-350°C a pressure increase of at least 70 kbars would be required. This corresponds to burial of around 250 km, which is geologically unrealistic. Similarly, to form wüstite, the temperature must have exceeded 570°C, which exceeds the maximum temperatures of vein formation by at least 200 °C. A temperature increase of that scale is not reflected in the mineral assemblage of the veins or the host rocks, which both contain siderite. We conclude that wüstite formed as a metastable phase at 250-350 °C and pressures below 100 bars because of the low-f O₂ conditions in the Coeur d’Alene district

Gelosaite, BiMo 6+ (2-5x) Mo 5+ 6x O 7 (OH)·H 2 O (0 ≤ x ≤ 0.4), occurs at the type locality in quartz veins hosted by granitic rocks at Su Senargiu, near Sarroch, Sardegna, Italy. The name is in memory of Mario Gelosa (1947-2006) who first found the mineral. The mineral also occurs in the oxidized zones of the Old 25 and Wolfram pipes at Kingsgate, New South Wales, Australia. Both the mineral and its name have been approved by the IMA CNMNC (IMA 2009-022). Gelosaite occurs as yellow, yellowish green, and pale blue, prismatic crystals with a white streak. It is transparent with an adamantine luster, non-fluorescent, brittle, and has a conchoidal fracture. Mohs hardness is ~3. The mineral is monoclinic, space group P2 1 /n, with a = 5.8505(4), b = 9.0421(6), c = 13.917(1) Å, β = 100.42(1)°, V = 724.1(1) Å 3 , Z = 4 (yellow Su Senargiu crystal); a = 5.8570(5), b = 9.0517(8), c = 13.992(1) Å, β = 100.44(1)°, V = 729.5(1) Å 3 , Z = 4 (pale blue Su Senargiu crystal); a = 5.837(3), b = 9.040(5), c = 13.904(7) Å, β = 100.64(1)°, V = 721.0(6) Å 3 , Z = 4 (blue Kingsgate crystal). Strongest lines in the powder X-ray pattern [d (Å)(Irel)] are 4.83(100), 3.41(21), 3.30(25), 3.015(50), 2.755(60), 2.080(50), 1.688(20), and 1.509(30). The single-crystal X-ray structure of gelosaite was determined for three separate crystals, two from Su Senargiu and one from Kingsgate. The structure consists of layers of distorted MoO 6 octahedra, plus minor amounts of interstitial Mo ions, and layers made up of eight-coordinate Bi 3+ ions, plus further small amounts of interstitial Mo ions. The theoretical Mo(VI) end-member has the stoichiometry BiMo 6+ 2 O 7 (OH)·H 2 O and excess Mo in the interstices requires increasing amounts of Mo(V) to be present. The theoretical Mo(V) end-member has the stoichiometry BiMo 5+ 2.4 O 7 (OH)·H 2 O

Quartz occurs in a wide range of geologic environments throughout the Earth’s crust. The concentration and distribution of trace elements in quartz provide information such as temperature and other physical conditions of formation. Trace element analyses with modern electron-probe microanalysis (EPMA) instruments can achieve 99% confidence detection of ~100 ppm with fairly minimal effort for many elements in samples of low to moderate average atomic number such as many common oxides and silicates. However, trace element measurements below 100 ppm in many materials are limited, not only by the precision of the background measurement, but also by the accuracy with which background levels are determined. A new “blank” correction algorithm has been developed and tested on both Cameca and JEOL instruments, which applies a quantitative correction to the emitted X-ray intensities during the iteration of the sample matrix correction based on a zero level (or known trace) abundance calibration standard. This iterated blank correction, when combined with improved background fit models, and an “aggregate” intensity calculation utilizing multiple spectrometer intensities in software for greater geometric efficiency, yields a detection limit of 2 to 3 ppm for Ti and 6 to 7 ppm for Al in quartz at 99% t-test confidence with similar levels for absolute accuracy

Indentation testing of natural single-crystal α-quartz parallel to the crystallographic c axis, using a triangular pyramidal diamond indenter at a maximum load of 500 mN, produced a very small residual volume of less than 1 μm3 in which α-quartz is highly stressed. Laser Raman microspectroscopy across the indentation at room temperature and pressure revealed a shift in the Raman bands, interpreted to reflect the residual stress field generated within the α-quartz. Based on the observed Raman shift, we identified a steep gradient in the residual non-hydrostatic stress field after complete unloading in quartz near the impression formed by indentation. At the center of the indentation, the maximum compressive stress and tensile stress were inferred to be higher than 2.2 and 0.1 GPa, respectively

We present a transmission electron microscopy (TEM) investigation of lillianite (Pb3Bi2S6) and heyrovskýite (Pb 6 Bi 2 S 9 ), from Vulcano, Aeolian Islands, Italy. The minerals investigated represent the only naturally occurring Ag- and Cu-free sulfosalts in the lillianite homologous series (LHS). Three methods (crushing, ion-milling, and ultramicrotomy) were used to prepare TEM specimens. Selected area electron diffraction (SAED) patterns and high-resolution TEM (HRTEM) images indicate wellordered crystals with only minor stacking faults and, more rarely, nanoscale intergrowths of lillianite and heyrovskýite. The latter were sometimes found to form an incommensurate structural modulation with an angle of ~29° relative to b* in the (hk0) plane and a wavelength of ~75 Å. This represents the first observation of such incommensurate modulations in heyrovskýite. Although considerable evidence points toward an artifact induced by the sample preparation technique (i.e., ion-milling), the possibility that the incommensurate modulation could be a primary feature of heyrovskýite itself cannot be completely ruled out. The modulation could derive from an ordering process of Pb and Bi cations over Me4 and Me5 sites within the PbS-like layer or from ordering of vacancies, naturally present or induced by Bi 2 S 3 sublimation during ion-milling

Inelastic neutron scattering (INS) was used to study dynamics of the hydrogen atoms in natural 2M 1 muscovite in the 150-1200 cm -1 energy range. The resultant INS spectra are interpreted by means of solid-state density functional theory calculations covering both normal mode analysis and molecular dynamics. While signatures of the Al-O-H bending modes were found over the whole energy transfer range, the dominant contributions were observed between 800-1000 cm -1 . The modes assigned to the in-plane movements of the respective hydrogen atoms are well defined and always appear at high energies. In contrast, the modes corresponding to the out-of-plane movements are spread over large energy transfer ranges, extending down to the region of external (lattice) modes. The positions of the high-energy modes contributing to the INS band at -907 cm -1 depend on the distance of respective hydrogen atoms to the nearest oxygen atom of the basal net and its polarity

Petrologic analysis is used to determine the intensity and extent of the thermal effects of a postkinematic granite on its surrounding rocks to understand the thermal evolution of an accretionary prism in Central Kyushu, Japan. The relatively young (14 Ma) Okueyama granite represents a single magma chamber, which solidified as a zoned pluton, and is therefore suitable for the study of the thermal effects by a single heat source. The Lower Shimanto Group country rock, an accretionary prism of Cretaceous age, has undergone contact metamorphism, to form an orthopyroxene zone, cordierite zone, and biotite zone in order of decreasing metamorphic grade. Singular value decomposition (SVD) analysis of composition matrices of relevant mineral assemblages results in the following isograd reactions 0.475Bt Crd = 0.538Opx + 0.179Bt Opx + 0.665Kfs + 0.075Crd + 0.067Ilm, for the orthopyroxene isograd, and 0.612Bt Bt + 0.218Ms Bt + 0.063Ilm = 0.292Kfs Crd + 0.686Bt Crd + 0.137Crd Crd , for the cordierite isograd. Peak metamorphic temperatures for each zone were determined using ternary feldspar and garnetbiotite thermometers, yielding a peak thermal gradient of 0.071°C/m. This thermal gradient yields isograd temperature conditions of 740°C for the orthopyroxene isograd and 550°C for the cordierite isograd, respectively

Crosscutting dissolution surfaces (DSs) are rare features observed in magmatic plagioclase crystals. In this study, crosscutting DSs of dacitic plagioclase crystals from the Shirahama Group, Izu Peninsula, Japan, were investigated using a Nomarski differential interference contrast imaging microscope and chemical compositional data. Crosscutting DS slopes were examined by tracing digitized images. The following features of crosscutting DSs are reported for the first time: (1) traces of the crosscutting DSs showed parabolic-like shapes; (2) crosscutting DSs were found on select (adjacent or opposite) faces of the crystals suggesting a preferential dissolution of a limited area of the plagioclase; (3) projecting points were observed with directional characteristics; and (4) square or angular patterns were found in the less dissolved parts of the crystals. The formation of crosscutting features is controlled by dissolution and crystal settling into hotter magma. Process velocities depend on the physical properties of the magma, such that a description of crosscutting features can reveal valuable information about the magmatic environment and the dynamic behavior of crystals

The Néel temperature (T N ) of (Mg 1-X ,Fe X )O and its compositional dependence have been studied with a superconducting quantum interference device (SQUID) in the temperature range 5-300 K at constant magnetic field of 1000 Oe. We determined the magnetic phase diagram at ambient pressure. As the T N constantly increased with iron content, we found a kink in the paramagnetic-antiferromagnetic boundary at around X = 0.5, which suggests a possible change of the magnetic structure in the antiferromagnetic state. We propose that the magnetic boundary ends at X = 0.12(1) and 0 K. Our results show that the pressure of the rhombohedral distortions of (Mg,Fe)O can be very sensitive to Fe content if the structure distortion is correlated to magnetic ordering

Geochemical profiles across chemically altered zones (selvages) surrounding amphibolite facies quartz+kyanite veins were investigated to determine major, minor, and trace element mass transfer and volume strain due to channelized fluid flow. The three profiles are perpendicular to two veins cutting the Wepawaug Schist, Connecticut, U.S.A., which underwent Barrovian-style metamorphism during the Acadian orogeny. Selvages are highly aluminous with considerably more kyanite, staurolite, and garnet, and less quartz, plagioclase, and mica, than surrounding wallrocks. Kyanite crystals increase in size toward veins, and reach several centimeters in length within veins. Mass balance analysis indicates 50% silica loss and 34% volume loss from selvages, on average. Silica, transferred locally from selvages to veins, accounts for 40 to 80% of the vein silica; the remainder must have been precipitated from fluids that flowed through the veins during regional devolatilization. Fluid flow also transported elements into the rock mass, which were concentrated in the selvages. Gains of Fe, Mn, Y, and HREE were due to the growth of selvage garnet; Fe, Zn, and Li were sequestered into staurolite. Kyanite, staurolite, and garnet growth resulted in Al mass gains. Destruction of mica (particularly muscovite) and of plagioclase in the selvages resulted in losses of K, Na, Ba, Pb, Sn, and volatiles and losses of Na, Sr, and Eu, respectively. Thin Na and Sr enrichment zones associated with increased modal plagioclase are found along the margins between selvages and less altered wallrock and may represent either chemical self-organization produced during diffusive mass transfer and reaction, or are relics from a possible earlier period of Na enrichment in the selvages. Simple, two-dimensional numerical modeling of flow in a fractured porous medium indicates that fluxes vary significantly over short distances (<1 m) adjacent to veins. Fluids, channelized into the high-permeability fracture conduits, carry the bulk of the fluid flow; mass transfer to and from selvages adjacent to the conduits occurred by some combination of diffusion and flow. In contrast, areas distal to the conduits are impoverished in fluid and undergo much more limited infiltration. As a consequence, different workers can come to vastly different conclusions about the magnitude of fluid fluxes and element transfer depending on the part of a fractured outcrop studied. This extreme spatial variability due to channelization can largely explain contrasting views that have arisen in the literature regarding the nature and intensity of non-volatile element mass transfer during Barrovian metamorphism. Determining “average” time-integrated fluid fluxes and levels of element transport across outcrops remains as important research challenges due to the spatial variability of flow.

The thermal behavior of three amphiboles along the join “Mg-richterite” [MRIC: ANaB(NaMg) CMg5 TSi8O22 W(OH)2]-richterite [RIC: A Na B (Na C a)CMg 5 T Si 8 O 22 W (OH) 2 ] was investigated by in situ synchrotron radiation powder diffraction between 90 and 873 K. The studied samples have Bsite compositions Na1Mg1 (sample RN1), Na 0.97 Mg 0.8 Ca 0.24 (sample RN2), and Na 0.97 Mg 0.58 Ca 0.45 (sample RN6). The evolution of cell parameters as a function of T shows a discontinuity in the two Mg-richer samples (RN1 and RN2), which is interpreted as evidence of a P2 1 /m → C2/m phase transition, whereas the Ca-richer sample (RN6) shows no evidence of a phase transition. The transition in samples RN1 and RN2 follows a different thermodynamic behavior, being tricritical in end-member “Mg-richterite” (RN1) and second order in the BCa-bearing amphibole RN2. A thermodynamic analysis done according to the Landau formalism and allowing for order parameter saturation, gives T c = 462(3) and 378(1) K, and saturation temperature θ s =116(21) and 141(7) for RN1 and RN2, respectively. Comparison with data from literature shows that the thermal strain of C-centered amphiboles with constant A-, C-, T-, and W-site occupancy equal to Na, Mg5, Si8, and (OH) 2 , respectively, and a B-site occupied by variable amounts of Li, Na, Mg, and/or Ca, mainly expands about 70° from c toward the a cell-edge onto the 010 plane. Conversely, the spontaneous strain accompanying the thermal transition shows that the maximum expansion is oriented about 25° from c and is coupled with a contraction close to the a cell direction. On the other side, transition induced by solid solution at room-T follows an almost opposite deformation pattern. The present data confirm the hypothesis of a first-order character of the transition induced by the increase of the B-site dimension for increasing B Ca contents, similarly to the closely related P2 1 /c → C2/c transition in pyroxenes

The structure of barite, or baryte, BaSO 4 , has been refined by single-crystal methods at room temperature to a maximum of 21.5 GPa in He medium. Lattice parameters have been obtained at select pressures by powder diffraction and complemented by the collection of Raman specta at the highest attained pressure. The results of all analyses correspond to BaSO 4 remaining in the baritetype structure to the highest investigated pressure. Therefore, the search for transitions leading to a post-barite structure, in BaSO 4 , must be conducted at pressures higher than those investigated here, or, in “lower pressure” analogs

Brushite, CaHPO 4 ∙2H 2 O, is a layered structure in which the layers are held together by water molecules via hydrogen bonds. Brushite loses its water molecules in two steps when heated, to form monetite CaHPO 4 . The loss of water results in the formation of an amorphous phase along with monetite. We investigated the dehydration process primarily by means of X-ray diffraction, and quantified the amount of monetite and the amorphous phase as a function of heating rate and temperature via Rietveld refinements. Water loss was recorded by thermogravimetric analysis and the nature of the products was investigated by Fourier transform infrared spectroscopy. Recrystallization, water loss, and the formation of the amorphous phase depend on the heating rate and/or time of exposure at temperature. The conversion temperature is about 220°C, although brushite can convert to monetite at lower temperatures during prolonged exposure to heat. More amorphous phase forms during slower dehydration at slower heating rates. The amorphous phase forms due to non-linear dehydration of brushite. The amorphous phase is hydrated and can be regarded as a highly disordered monetite with some free water trapped in the structure

Transient (non-equilibrium) microstructures in crystals may arise in an order-disorder phase transition that generates lattice strain. A two-phase field can develop if fluctuations of the order parameter lead to nucleation of an ordered phase in a disordered matrix, as we describe here for columbite. Synchrotron X-ray diffraction and transmission electron microscopy show that ordering in columbite involves two discrete phases with different degree of order but the same composition. A highly unusual distribution of ordered rhombic-shaped domains within a disordered matrix establishes on a nanometer scale and remains relatively stable over a prolonged period of annealing. Progressive ordering takes place within the ordered domains and the disordered matrix but the domains maintain more or less constant shape and distribution. We speculate that a new family of such microstructures could develop in other oxide phases with cation ordering transitions that are strongly first order in character. Long-term stability of such microstructures and their dependence on strain could open up the possibility of engineering the properties of crystals containing a percolating disordered matrix with ordered nanodomains of controlled dimensions

The volume and unit-cell parameters of analbite (i.e., NaAlSi 3 O 8 with complete Al,Si disorder) have been determined by single-crystal X-ray diffraction to a maximum pressure of ~8.71 GPa. The volume variation with pressure is described by a fourth-order Birch-Murnaghan equation of state with K 0T = 50.3(5) GPa, K 0 ′ = 8.9(5), and K 0 ″ = -2.4(3) GPa-1. The value of the room-pressure bulk modulus is ~4% lower than that of low albite, and the onset of volume softening in analbite is at ~6.7 GPa, some 1.7 GPa higher than the onset in albite. The anisotropy of compression of analbite is less than that in albite. Single-crystal structure determinations of analbite to ~9.4 GPa show that there is no significant detectable compression of the T-O bonds within the structure, and the compression of the framework of tetrahedra is therefore accommodated by changes in the T-O-T angles, which result in significant compression of the “crankshaft chains” within the framework. No significant shear of the tetrahedral rings of analbite was detected, in contrast to the structural compression of albite. Overall, the structural changes that occur in analbite from 0.0001 to 9.4 GPa resemble those seen in ordered albite over the pressure range 0.0001-4 GPa. Therefore analbite shows a significantly greater structural rigidity than low albite up to pressures of 9.4 GPa

In-situ synchrotron X-ray powder diffraction studies of K-, Rb-, and Cs-exchanged natrolites between room temperature and 425 °C revealed that the dehydrated phases with collapsed frameworks start to form at 175, 150, and 100°C, respectively. The degree of the framework collapse indicated by the unit-cell volume contraction depends on the size of the non-framework cation: K-exchanged natrolite undergoes an 18.8% unit-cell volume contraction when dehydrated at 175 °C, whereas Rband Cs-exchanged natrolites show unit-cell volume contractions of 18.5 and 15.2% at 150 and 100°C, respectively. In the hydrated phases, the dehydration-induced unit-cell volume reduction diminishes as the cation size increases and reveals increasingly a negative slope as smaller cations are substituted into the pores of the natrolite structure. The thermal expansion of the unit-cell volumes of the dehydrated K-, Rb-, and Cs-phases have positive thermal expansion coefficients of 8.80 × 10 −5 K −1 , 1.03 × 10 −4 K −1 , and 5.06 × 10 −5 K −1 , respectively. Rietveld structure refinements of the dehydrated phases at 400 °C reveal that the framework collapses are due to an increase of the chain rotation angles, ψ, which narrow the channels to a more elliptical shape. Compared to their respective hydrated structures at ambient conditions, the dehydrated K-exchanged natrolite at 400°C shows a 2.2-fold increase in ψ, whereas the dehydrated Rb- and Cs-natrolites at 400°C reveal increases of ψ by ca. 3.7 and 7.3 times, respectively. The elliptical channel openings of the dehydrated K-, Rb-, to Cs-phases become larger as the cation size increases. The disordered non-framework cations in the hydrated K-, Rb-, and Csnatrolite order during dehydration and the subsequent framework collapse. The dehydrated phases of Rb- and Cs-natrolite can be stabilized at ambient conditions

The original structure determination for aravaipaite, Pb 3 AlF 9 (H 2 O), indicated it to be monoclinic, P21/n, with a = 25.048(4), b = 5.8459(8), c = 5.6505(7) Å, β = 94.013(3)°, V = 829.7(2) Å 3 , and Z = 4. Examination of additional crystal fragments from the same specimen revealed that some have a triclinic cell, P1̅, with a = 5.6637(1), b = 5.8659(1), c = 12.7041(9) Å, α = 98.725(7)°, β = 94.020(7)°, γ = 90.683(6)°, V = 416.04(3) Å 3 , and Z = 2. The topology of the structure is the same as that reported previously, but the structure refinement is significantly improved, with R1 = 0.0263 for 1695 observed reflections [F o > 4σF] and 0.0306 for all 1903 reflections, and with the H atoms located. Twinning may be responsible for the original monoclinic cell or the two structures could be order-disorder (OD) polytypes. New X-ray diffraction data collected on a crystal of calcioaravaipaite, PbCa 2 Al(F,OH)9, showed it to be triclinic, P1, with a = 5.3815(3), b = 5.3846(3), c = 12.2034(6) Å, α = 91.364(2)°, β = 101.110(3)°, γ = 91.525(3)°, V = 346.72(3) Å 3 , and Z = 2. This cell is essentially identical to the reduced cell reported in conjunction with an earlier structure solution on a twinned crystal using the OD approach. Our study confirms the findings of the earlier study and significantly improves upon the earlier structure refinement, yielding R1 = 0.0195 for 2257 observed reflections [F o > 4σF] and 0.0227 for all 2427 reflections. The structures of aravaipaite and calcioaravaipaite are based upon square-packed layers of F atoms on either side of which are bonded Pb atoms (in aravaipaite) or Ca atoms (in calcioaravaipaite) in fluorite-type configurations. These layers parallel to (001) serve as templates to which on both sides are attached AlF6 octahedra and PbF 6 (H 2 O) 2 polyhedra (in aravaipaite) or PbF 12 polyhedra (in calcioaravaipaite). The Pb 2+ cations in these structures have stereoactive 6s 2 lone-electron-pairs, manifest in off-center coordinations. The very different sizes of the Pb 2+ and Ca 2+ cations yield fluorite-type layers with very different metrics, reflected in the a and b cell dimensions of the two structures; but more significantly, the lone-pair effect results in a very irregular template of F atoms peripheral to the fluorite-type layer in aravaipaite, while the F atoms peripheral to the fluorite-type layer in calcioaravaipaite are in a more regular, nearly planar array. As a result, the interlayer AlF 6 octahedra and PbF 6 (H 2 O) 2 polyhedra in aravaipaite form a relatively open configuration, while the AlF6 octahedra and PbF 12 polyhedra in calcioaravaipaite form a more tightly packed configuration containing no H 2 O molecules. The Raman spectra of aravaipaite and calcioaravaipaite are consistent with the results of the structure studies, except that the spectrum of calcioaravaipaite exhibits the strong bands typically associated with OH stretching vibrations, while the structure refinement is most consistent with full occupancy of all anion sites by only F

The ferrous Mössbauer parameters are determined for synthetic triphylite (LiFePO 4 ) over a wide temperature range between 8 and 900 K. A sharp magnetic order-disorder transition is observed at (52.5 ± 0.5) K. In the paramagnetic state, the Mössbauer spectra are adequately analyzed by one Fe 2+ quadrupole doublet with a narrow line width of 0.285 mm/s at room temperature. In the magnetic state, the spectra show up to eight absorption lines, which is typical for Fe 2+ . These spectra are analyzed through diagonalization of the full nuclear-interaction hamiltonian. The hyperfine field B hf is found to be collinear to the local principal axis of the electric field gradient (EFG) tensor. The saturation value for B hf is 125.5 kOe. For temperatures below 52 K, the adjusted value for the asymmetry parameter of the EFG is η ≈ 0.8 and for the quadrupole splitting ΔE Q = 3.06 mm/s. This latter value is perfectly in line with the value at 80 K (paramagnetic regime). The temperature dependence of the quadrupole splitting is interpreted within the 5 D orbital energy level scheme of Fe 2+ by a crystal field calculation based on the point symmetry of the Fe 2+ site in triphylite. The variation of the hyperfine field with temperature is interpreted within the molecular field approximation, however, taking exchange magnetostriction into account. From the temperature dependence of the isomer shift the characteristic Mössbauer temperature Θ M is determined to be 418 K and the intrinsic isomer shift δ I = 1.46 mm/s. From these, the Mössbauer fraction at room temperature is calculated to be 0.785

In this report, synthesis and characterization data for françoisite-(Nd), Nd[(UO 2 ) 3 O(OH) (PO 4 ) 2 ]·6H 2 O are presented. Experimental studies including chemical digestion, thermogravimetric analysis (TG), attenuated total reflection infrared spectroscopy (ATR-IR), Raman spectroscopy, and X-ray powder diffraction (XRD) were conducted. The local bonding environments of uranyl and water were investigated with ATR-IR and Raman spectroscopy. The average axial uranium-oxygen bond lengths [R(U=O)] and the oxygen-oxygen hydrogen bond distances [R(O···O)] in water were calculated from the vibrational frequencies of the stretching modes of uranyl and water, respectively. The IR spectra of protiated and deuterated françoisite-(Nd) appear to confirm a coordinated hydroxyl group bridging vicinal uranyl groups in the structural unit. These studies provide the basis for further thermodynamic (e.g., calorimetry, solubility, and sorption) and geochemical modeling investigations of hydrated uranyl phosphate compounds

The new mineral fluorphosphohedyphane, Ca 2 Pb 3 (PO 4 ) 3 F, the F-analog of phosphohedyphane, is hexagonal with space group P6 3 /m and cell parameters a = 9.6402(12), c = 7.0121(8) Å, V = 564.4(1) Å 3 , and Z = 2. It occurs in the oxidation zone of a small Pb-Cu-Zn-Ag deposit, the Blue Bell claims, about 11 km west of Baker, San Bernardino County, California. It forms as sub-parallel intergrowths and irregular clusters of transparent, colorless, highly lustrous, hexagonal prisms with pyramidal terminations. It is found in cracks and narrow veins in a highly siliceous hornfels in association with cerussite, chrysocolla, fluorite, fluorapatite, goethite, gypsum, mimetite, opal, phosphohedyphane, plumbogummite, plumbophyllite, plumbotsumite, pyromorphite, quartz, and wulfenite. The streak of the new mineral is white, the luster is subadamantine, and the Mohs hardness is about 4. The mineral is brittle with subconcoidal fracture and no cleavage. The calculated density is 5.445 g/cm 3 based upon the empirical formula. Optical properties (589 nm): uniaxial (-), ω = 1.836(5), ε = 1.824(5), nonpleochroic. SEM-EDS analyses yielded the averages and ranges in wt%: O 21.28 (20.31-22.14), F 1.59 (1.38-1.91), P 10.33 (9.81-10.83), Ca 9.66 (8.97-10.67), Pb 60.08 (57.67-61.21), total 102.95 (102.02-103.88), providing the empirical formula (based on P = 3): Ca 2.00 (Pb 2.61 Ca 0.17 ) Σ2.78 P 3O11.91 F 0.75 . Infrared spectroscopy showed no evidence of OH or carbonate. The strongest powder X-ray diffraction lines are [d(hkl)I]: 8.38(100)22, 3.974(111)28, 3.506(002)25, 2.877(121,211)100, 1.878(213,123)26. Fluorphosphohedyphane has the apatite structure (R 1 = 1.75% for 444 reflections with F o > 4σF) with ordering of Ca and Pb in two cation sites, as in hedyphane and phosphohedyphane. The Pb 2+ cation exhibits a stereoactive 6s 2 lone-electron-pair. The X anion site at (0, 0, ½) is fully occupied by F forming six bonds of 2.867 Å to Pb atoms, in contrast to the six Pb-Cl bonds of 3.068 Å in phosphohedyphane

We report a new mineral that occurs in shock-induced melt veins of the Tenham L6 chondrite. The new mineral, identified by transmission electron microscopy (TEM), occurs as acicular nanocrystals in a glassy matrix at the edge of shock-induced melt veins that crystallized during rapid quench at high pressure. The elongate crystals have aspect ratios up to 25. Widths range from -5 to -40 nm and lengths are up to 500 nm. Energy-dispersive X-ray spectroscopy (EDS) analyses provide the relative cation abundances that are consistent with a pyroxene-like stoichiometry: Na 0.06 Ca 0.02 Mg 0.71 Fe 0.20 Al 0.11 Si 0.94 O 3 . Selected area electron diffraction (SAED) patterns from single-crystal and polycrystalline aggregates indicate an olivine structure with refined cell parameters: a = 4.78, b = 10.11, and c = 5.94 Å and a calculated density of 3.32 g/cm 3 . Synchrotron X-ray microdiffraction data are consistent with an olivine structure and provide similar cell parameters: a = 4.778, b = 10.267, c = 5.937 Å. The pyroxene composition represents a large deviation from olivine stoichiometry, (Na 0.08 Ca 0.03 Mg 0.95 Fe 0.26 Al 0.15 Si 0.25□0.28 ) 2 Si 1 O 4 , with 0.28 formula units of vacancies (□), 0.11 of Na + plus Ca 2+ , and 0.25 of Si 4+ , in octahedral sites. Our observations indicate that a metastable and nonstoichiometric olivine structure can crystallize from a silicate melt during rapid quench. Trace amounts of such defects may be present in stable olivines in the deep upper mantle

The WURM project is a database of computed Raman and infrared spectra and other physical properties for minerals. The calculations are performed within the framework of the density-functional theory and the density-functional perturbation theory. The database is freely available for teaching and research purposes and is presented in a web-based format, hosted on the http://www.wurm.info web site. It provides the crystal structure, the parameters of the calculations, the dielectric properties, the Raman spectra with both peak positions and intensities and the infrared spectra with peak positions for minerals. It shows the atomic displacement patterns for all the zone-center vibrational modes and the associated Raman tensors. The web presentation is user friendly and highly oriented toward the end user, with a strong educational component in mind. A set of visualization tools ensures the observation of the crystal structure, the vibrational pattern, and the different spectra. Further developments include elastic and optical properties of minerals

Dissolution of natural and commercial melanterite generates a significant decrease in pH that is not thermodynamically explained by Fe 2+ hydrolysis. It has been recently proposed that the production of acidity is actually caused by hydrolysis of Fe 3+ occurring in trace amounts in melanterite. Following this finding, the experiments of melanterite dissolution previously conducted by the author have been reviewed and modeled with PHREEQC. Without invoking oxidation of Fe 2+ to Fe 3+ , modeling results indicate that the amount of Fe 3+ needed to significantly decrease solution pH is low (0.16-0.20 wt%) and already contained in melanterite in the form of contaminant

The crystal structure and chemical composition of a crystal of Na 2 MgSi 5 O 12 garnet synthesized in the model system Mg 3 Al 2 Si 3 O 12 -Na 2 MgSi 5 O 12 at 17.5 GPa and 1700 °C have been investigated. Quantitative analysis leads to the following formula: Na 1.98 Mg 1.00 Si 5.01 O 12 . Na 2 MgSi 5 O 12 garnet was found to be tetragonal, space group I41/acd, with lattice parameters a = 11.3966(6), c = 11.3369(5) Å, V = 1472.5(1) Å 3 . The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 Å for both the sites), whereas Si is ordered at both the Y (mean: 1.793 Å) and Z sites (means: 1.630 and 1.624 Å). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na 2 MgSi 5 O 12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages

The infrared spectra of hydrated San Carlos olivine were measured from room temperature to 1100°C at 1 bar using a heating stage. The spectra show that even at the highest temperatures studied, there are still two well-separated groups of OH bands centered between 3200 and 3300 cm -1 and between 3500 and 3600 cm -1 , respectively. The distinction between “group I” (ν > 3450 cm -1 ) and “group II” bands (ν < 3450 cm -1 ) is therefore still meaningful at upper mantle temperatures. However, a prominent band at 3612 cm -1 already loses intensity by 100°C and nearly disappears by 300°C, suggesting that it corresponds to a particular H environment that only formed during cooling of the sample and that it is not stable at high temperature. A similar band is prominent in many olivines from mantle xenoliths and previously it has sometimes been assigned to OH groups surrounding Si vacancies. We show that structural models of water dissolution in olivine derived from infrared spectra at ambient conditions may not be fully applicable for the upper mantle