Volume 91, Issue 7

Jadeitite, a rare high P/T rock, is associated spatially with blueschist and/or eclogite terranes. Scanning electron microscope (SEM) and cathodoluminescence (CL) petrography of jadeitite samples from several major occurrences [in Burma (Myanmar), Guatemala, Japan, Kazakhstan, and the U.S.A.] show that grains were deposited from fluids. Jadeite grain compositions indicate these fluid compositions changed with time. CL imagery guided the acquisition of oxygen-isotope and trace-element analyses with the ion microprobe. Jadeite grains in each rock grew in cycles that began with red- and/or blue-luminescent and ended with green-luminescent zones. The CL images were used to order the data into crystallization sequences. These data and electron-microprobe, major-element analyses document the association of green CL with increases in Ca, Mg, and Cr: (1) toward grain exteriors; (2) in fine-grained matrix around porphyroblasts; (3) in shear zones that cut grains; (4) in former open spaces now filled with jadeite; or (5) in veins. Abundances of many trace elements are greater in green-CL jadeitite compared with the red- or blue-CL zones. Some of these elements.in particular Li, Rb, Sr, Ti, Hf, Zr, Y, and REE.are unlikely to have been derived from serpentinite. Although crystal-chemical effects may explain some of the trace-element systematics (e.g., preferential incorporation of REE into Ca-richer jadeite), some kinetic control is suggested by sector-zoned, rhythmically zoned grains. The oxygen-isotope data suggest that jadeitite-depositing fluids either had multiple sources or evolved in composition along their flow paths (or both).

The blue pigment and the plaster layer of Michelangelo.s fresco .The Last Judgment. have been characterized by means of powder XRD, FT-IR, and Raman spectroscopy, as well as analytical TEM and SAED. The pigment is composed of a mineralogical assemblage that is consistent with a raw lapis lazuli rock. To try to define the provenance of this pigment, we investigated various lapis lazuli samples from different deposits that are currently considered as possible sources of the material. According to our results, the provenance of the pigment is quite dubious but the Pakistani one seems the most probable.

The energetics of magnetic and cation ordering have been modeled using an atomistic approach based on empirical exchange interaction parameters. The model has been applied to the study of magnetic and cation ordering in the ilmenite-hematite solid solution via Monte Carlo simulations, providing new insight into the effect of nanoscale microstructures on the magnetic properties of this system. The lowest energy state for intermediate compositions is an intergrowth of cation-disordered antiferromagnetic (AF) hematite and cation-ordered paramagnetic (PM) ilmenite, separated by mixed Fe 2+ /Fe 3+ “contact layers”. The intergrowth carries a stable defect moment (“lamellar magnetism”) due to the presence of uncompensated spins. The net magnetization is parallel to the spin alignment direction, i.e., perpendicular to the spin-canted magnetization of the AF hematite. Contact-layer spins are coordinated to fewer magnetic neighbors than bulk spins and thus disorder more rapidly on heating. This leads to a decrease of net moment with increasing temperature. A small net moment is present, however, up to the Néel temperature of the AF hematite phase. The maximum temperature for acquisition of a chemical remanent magnetization due to lamellar magnetism is 800 ± 25 K, corresponding to the temperature for the eutectoid reaction PM hematite → AF hematite + PM ilmenite. If only cation interactions are considered, Fe 3+ and Fe 2+ in the contact layers become ordered so that Fe 3+ shares an octahedral face with Fe 3+ in the neighboring hematite layer and Fe 2+ shares an octahedral face with Ti 4+ in the neighboring ilmenite layer. If both cation and magnetic interactions are considered, however, an alternative ordering scheme is stabilized, whereby Fe 3+ shares an octahedral face with Ti 4+ in the neighboring ilmenite layer and Fe 2+ shares an octahedral face with Fe 3+ in the neighboring hematite layer. The model has been used to investigate the nature of exchange coupling between ordered/antiordered domains and disordered antiphase boundaries, with a view to elucidating the mechanism of self-reversed thermoremanent magnetization. Antiphase boundaries are enriched in Fe relative to the ordered/antiordered domains, with enhanced enrichment observed in simulations performed within the PM hematite + PM ilmenite miscibility gap. Monte Carlo simulations of magnetic ordering show no evidence of self-reversal in systems containing two equally well-ordered ferrimagnetic (FM) domains separated by Fe-enriched AF boundaries. Systems displaying partial long-range order, however, do display self-reversed magnetization. Partial long-range order is characterized by a mixture of highly ordered Ti-rich FM domains and poorly ordered Fe-rich domains with a weak FM moment. The magnetic ordering temperature of the poorly ordered Fe-rich domains is significantly higher than that of the highly ordered Ti-rich domains, and act as the “x-phase”. Evidence in support of the proposed mechanism from neutron diffraction and transmission electron microscopy is presented.

In the Monte Rosa area, Northern Italy, the assemblage garnet + phengite + quartz was formed in polymetamorphic metapelites from pre-existing biotite and plagioclase during an eclogite-facies Alpine metamorphic overprint. While phengite nucleated within plagioclase, garnet formed 10 to 20 μm wide continuous rims along the original biotite-plagioclase interphase boundaries. Garnet formation involved diffusion of Ca-, Fe-, and Mg-bearing species across the growing rims. The garnet rims show an overall asymmetric compositional zoning and distinct nanometer scale chemical patterns across grain boundaries within the garnet polycrystal. The compositional patterns resulted from the interplay of diffusion along grain and interphase boundaries and volume diffusion. Individual garnet crystals are separated by low-angle grain boundaries, which are predominantly oriented perpendicular to the reaction rim giving rise to an overall palisade microstructure. Grain boundaries contain arrays of closely spaced channels about 2 nm wide, which are filled with an amorphous phase. These channels represent direct links between the garnet-biotite and garnet-plagioclase reaction fronts and provide pathways for fast diffusion. From numerical simulation of the observed chemical patterns, diffusion of Ca and Fe is inferred to have been 5.6 orders of magnitude faster within grain boundaries than through the interior of garnet grains. Although the amorphous grain boundary phase only takes up a small fraction (2.5 per mil) of the total volume of the garnet polycrystal, short-circuit diffusion along grain boundaries contributed substantially to material transfer across the growing garnet rim. Despite the presence of fast pathways, bulk diffusion was too slow to allow for chemical equilibration of the phases involved in garnet rim formation, even on a micrometer scale. Relying on published garnet volume diffusion data, D gb (where gb = grain boundary), is estimated to be on the order of 10 -(19.20) m 2 /s at the estimated reaction conditions of 650 °C and 12.5 kbar, where D Ca gb is slower than D Fe gb by a factor of ten.

Single-crystal polarized Raman spectra (80 to 4000 cm -1 at 4 ≤ T ≤ 700 K) and powder IR spectra (1500 to 4000 cm -1 at 50 < T < 300 K) were measured for two microporous zeolites natrolite, Na 16 [Al 16 Si 24 O 80 ]·16H 2 O, and scolecite, Ca 8 [Al 16 Si 24 O 80 ]·24H 2 O to determine the behavior of H 2 O molecules in the channels. Both IR and Raman spectra show intense O-H stretching and H 2 O bending modes derived from the hydrogen-bonded H 2 O molecule(s) in the channels. Using published crystal structural data for natrolite and scolecite, and a consideration of Raman mode intensities that are sensitive to the H 2 O orientation in the framework channels, the internal stretching and bending modes could be assigned. The Raman spectra also show lower energy lattice modes and, in addition, second-order scattering in the wavenumber range where O-H stretching vibrations occur. The stretching vibrations of H 2 O molecules of natrolite and scolecite are located between 3200 and 3700 cm -1 and bending vibrations occur around 1650 cm -1 . In the case of natrolite, two intense O-H stretching modes can be observed and also several weaker combination modes. The latter was used to derive a low energy external H 2 O translational vibration, T(H 2 O), which is also observed directly in single-crystal Raman spectra. In addition, two H 2 O librational modes are located at about 440 and 500 cm -1 . For scolecite, six O-H stretching modes are observed in the Raman spectra recorded at 4 K, but only five are found at room temperature in the IR or Raman. The single-crystal Raman spectra also show several second-order combination modes consisting of external and internal H 2 O vibrations. They permit the wavenumber of several T(H 2 O) modes at low wavenumbers to be determined. These combination bands are analyzed based on their temperature behavior between 0 and 300 K. It is shown that the wavenumber of the H 2 O bending modes decreases with an increase of the H-O-H angle of the H 2 O molecule in natrolite and scolecite. The dehydration behavior of H 2 O in natrolite and scolecite was investigated by Raman measurements of the intensities of the O-H stretching modes at temperatures from 300 K to 570 K and 720 K, respectively. IR and Raman spectra, obtained over a large temperature range, permit one to obtain a better understanding of inner surface H 2 O-molecule behavior in microporous silicates and energetics and the behavior of hydrogen bonding.

The pressure dependence of the sound velocities, single-crystal elastic constants, and shear and adiabatic bulk moduli of a natural gahnite (ZnAl 2 O 4 ) spinel have been determined to ~9 GPa by gigahertz ultrasonic interferometry in a diamond anvil cell. The elastic constants of gahnite are (in GPa): C 11 = 290(3), C 12 = 169(4), and C 44 = 146(2). The elastic constants C 11 and C 12 have similar pressure derivatives of 4.48(10) and 5.0(8), while the pressure derivative of C 44 is 1.47(3). In contrast to magnetite, gahnite does not exhibit C 44 mode softening over the experimental pressure range. The adiabatic bulk modulus K S0 is 209(5) GPa, with pressure derivative K S ’ = 4.8(3), and the shear modulus G 0 = 104(3) GPa, with G’ = 0.5(2). Gahnite, along with chromite (FeCr 2 O 4 ) and hercynite (FeAl 2 O 4 ) are the least compressible of the naturally occurring oxide spinels. Evaluation of Birch's Law for isostructural minerals indicates that spinels containing transition metals on both the [4] A and [6] B sites follow a trend about five times more negative than oxide and silicate-spinel phases without any, or only one transition metal.

Syntheses have been done at 2.5 GPa and 750 °C to examine the effects of varying the water content and composition of the solid starting mixture on the formation of glaucophane [Na 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 ]. Amphiboles of apparently ideal glaucophane composition, confirmed by electron microprobe analyses, were obtained by treating oxide or oxide-hydroxide mixtures with bulk water contents of 4.5 wt% for durations of about 400 h in multiple treatments with intermittent grindings. This water content is necessary to permit growth of amphibole while preventing the intervention of a sheet silicate, which was identified in this study as vermiculite. To maximize amphibole yields, multiple treatments with intermittent grinding are needed on account of the low water contents that reduce the reaction rate of the synthesis process. Calculation of the solubility curves of talc, jadeite, and glaucophane in log[a Mg(OH) + /a H + ] vs. log[(a Na + )(a AlSiO₄ - )] space at 2.5 GPa and 750 °C shows that glaucophane has a very narrow field of stability without coexisting jadeite or talc. This same diagram can be used to illustrate that the growth of vermiculite relative to glaucophane is not an issue of metastability but of the activity of water.

The 14 C-PMMA impregnation of rock samples and mineral staining methods provide the connected porosity map and the spatial distribution of mineral aggregates, respectively. Combined image analysis of mineral and porosity maps allows quantification of the connected porosity distribution in rockforming mineral aggregates. After the two maps have been superimposed numerically, the 14 C-PMMA method provides an indication of the extent of pore connectivity for each pixel in the analyzed area, which can be used to obtain the porosity distribution as a function of modal mineralogy. When applied to undeformed and deformed Kivetty granodiorite samples from Finland, the method allows for a detailed analysis of the rock porosity. Porosity distributions related to the undeformed rock are unimodal and approximately identical to each other. On the other hand, porosity distributions of the deformed granodiorite are multimodal and vary significantly. Constituent porosity sets of the deformed samples were albite crystals free of alkali feldspar patches and alkali feldspar phenocrysts (average porosity, Φ̅ = 0.21%), albite containing alkali feldspar patches (Φ̅ = 0.59%), rapakivi albitic mantles (Φ̅ = 1.15%), quartz (Φ̅ = 0.39%) and mafic minerals (Φ̅ = 5.8%). Moreover, the analysis indicates that the numerous micropores observed under SEM within alkali feldspar phenocrysts and albite crystals free of alkali feldspar patches are unconnected in 3D.

The reaction 2 talc + 2 forsterite = 5 enstatite + 2 H 2 O has been investigated between 1 and 3.5 GPa. Here, we report the results from 2 sets of experiments: (1) “conventional”. static quenching experiments performed several years ago in non-end-loaded and end-loaded piston-cylinder apparatus, and (2) a new set of experiments conducted in a novel type of apparatus, a “rocking” piston cylinder. This new technique was employed for the reinvestigation to overcome segregation problems encountered during static experiments. The run-products of the latter experiments are homogenous throughout the entire capsule. The reaction was bracketed to 680.690 °C at 1 GPa, to 660.680 °C at 1.7 GPa, to 650.660 °C at 2.5 GPa, to 600.640 °C at 3 GPa, and to 550.600 °C at 3.5 GPa. The location of the reaction in P-T-space is consistent with low-pressure experimental data of Chernosky et al. (1985), and is in reasonable agreement with the “conventional” high-pressure experiments of this study. The new experimental results were used to refine the existing thermodynamic properties of talc within the framework of the Holland and Powell (1998) database. The standard state enthalpy and entropy amount to H o f298 = 5912.358 kJ/mol and S o 298 = 239.1 J/(K·mol). The refined standard entropy is about 9% lower than the entropy obtained calorimetrically. When the clinoenstatite-orthoenstatite transition is taken into account, S o 298 of talc improves by ~6 J/K. We assume that a further increase of the standard state entropy of talc can be achieved by varying the activity of H 2 O in the fluid as a function of pressure and temperature.

Single crystals of Ba(NpO 2 )(PO 4 )(H 2 O) were obtained using hydrothermal synthesis techniques. The structure was determined using single-crystal X-ray diffraction data collected using MoKα radiation and an APEX II CCD detector and was refined on the basis of F 2 for all unique data to R 1 = 2.41%. Ba(NpO 2 )(PO 4 )(H 2 O) crystallized in monoclinic space group P2 1 /n with a = 6.905(3), b = 7.108(3), c = 13.321(6) Å, β = 105.02°, and V = 631.4 Å 3 . The structure contains chains of edge-sharing neptunyl pentagonal bipyramids that link through phosphate tetrahedra to form infinite sheets. This sheet-type is identical to the anion topology of the uranophane group, in particular to that of oursinite, Co[(UO 2 )(SiO 3 OH)] 2 (H 2 O) 6 . Similarities between Ba(NpO 2 )(PO 4 )(H 2 O) and the uranophane group of minerals suggests a charge-balancing mechanism for incorporation of Np 5+ into uranyl minerals.

Hydrogen incorporation into orthoenstatite (Pbca), low-clinoenstatite (P2 1 /c), and high-pressure clinoenstatite (C2/c) has been investigated using polarized and unpolarized infrared spectroscopy. Using shifts in OH stretching frequencies between the spectra and data from different crystal models, we test various models for hydrogen incorporation. The only significant differences between orthoenstatite and low-clinoenstatite spectra relate to anisotropy of the higher wavenumber bands, which implies a change in orientation of longer OH dipoles between the two structures. High-pressure clinoenstatite reverts to low-clinoestatite during depressurization, but subtle differences are noted between IR spectra of samples synthesized in the high-pressure clinoenstatite and low-clinoenstatite stability fields. Differences probably relate to the splitting of oxygen sites into two sets of non-equivalent sites during transformation of high-pressure clinoenstatite. The most realistic models for hydrogen incorporation into all three polymorphs involve association of hydrogen with the underbonded O2a and O2b sites. However, changes in OH dipole orientation between the different polymorphs and the effects of phase transitions on water solubility in the system MgSiO 3 mean the effects and implications of hydrogen incorporation into the three polymorphs may differ considerably.

Raman spectra in the range 80 to 4000 cm -1 of wadsleyite II (Fo 90 with 2.0 wt% H 2 O and Fo 88 with 2.7 wt% H 2 O) have been measured in a diamond-anvil cell with solid rare-gas pressure-transmitting media to 51.4 GPa at room temperature. The ambient Raman spectrum of wadsleyite II is closely similar to wadsleyite modified with bands in frequency regions where the SiO 4 tetrahedral and OH stretching vibrations of hydrous ringwoodite occur. The most intense, characteristic wadsleyite II modes at 709 and 911 cm -1 (Si 2 O 7 and SiO 3 symmetric stretching vibrations, respectively) shift continuously to 51.4 GPa showing no evidence for a change in the crystal structure. A striking feature in the high-pressure Raman spectra of wadsleyite II is a significant growth in intensity in the mid-frequency range (300-650 cm -1 at 10.4 GPa and 400-750 cm -1 at 51.4 GPa) under compression accompanied by the appearance of new Raman modes near 40 GPa, perhaps a result of resonance electronic Raman scattering. In the OH stretching frequency range, the Raman spectrum of wadsleyite II exhibits at least six modes and their high-pressure behavior agrees with that of Fo 90 hydrous wadsleyite: OH stretching modes above 3530 cm -1 remain approximately constant up to at least 21.8 GPa whereas OH modes at frequencies <3530 cm -1 decrease with increasing pressure. The OH stretching modes are consistent with protonation of the non-silicate oxygen O 2 and the O atoms surrounding the partially vacant tetrahedral site Si 2 , as suggested from X-ray diffraction data.

Diamond may form in the Earth.s mantle by recrystallization of graphite, by precipitation from a C-bearing fluid, or by reduction of carbonate. The latter mechanism could result from interaction with a reduced fluid or another phase that would accommodate the oxygen produced by the reduction. One possible such phase is a sulfide-bearing melt, given that sulfides are common inclusions in diamond. Experiments at 1300 °C, 6 and 7.5 GPa successfully reduced magnesite in the presence of a eutectic-composition Fe-S-O melt. Although graphite rather than diamond was produced by this reduction, these experiments demonstrate that this mechanism is a viable mechanism for reducing carbonate to carbon in the Earth.s mantle.

The structural behavior of a cancrinite, Na 5.96 Ca 1.52 [Al 6 Si 6 O 24 ](CO 3 ) 1.57 ·1.75H 2 O, was determined by using in situ synchrotron X-ray powder diffraction data [λ = 0.91806(5) Å] at room pressure and from 25 to 982 °C. The sample was heated at a rate of about 9.5 °C/min, and X-ray traces were collected at about 15 °C intervals. The satellite reflections in cancrinite were lost at about 504 °C, where a phase transition occurs. All the unit-cell parameters for cancrinite also show a discontinuity at 504 °C. Initially, the [Ca·CO 3 ] clusters and their vacancies are ordered in the channels, and this ordering is destroyed on heating to give rise to the phase transition. Cancrinite loses water continuously until about 625 °C; thereafter an anhydrous cancrinite phase exists. From 25 to 952 °C, a minimal amount of CO 2 is lost from the structure. Over this temperature range, the average <Al-O-Si> bridging angle, which is an indication of the degree of rotation of the tetrahedra, increases from 143.7(4) to 147.7(5)°. Rotations of the tetrahedra are caused by expansion of the Na1-O2 bond lengths.

The OH-structural characteristics of an iron-rich palygorskite from Western Macedonia, Greece (Gr-1) and an aluminous palygorskite from Florida (PFl-1) were examined by combined Fourier-transform near-infrared reflectance (NIR) and mid-infrared attenuated total reflectance (ATR) spectroscopy. Analyses of samples heated from ambient to 130 °C allowed for the development of a self-consistent set of band assignments for the structural and surface OH and H 2 O species of both the hydrated and dehydrated forms. The inner octahedral sites of both samples are largely accounted for by dioctahedral AlAlOH, AlFe 3+ OH, and Fe 3+ Fe 3+ OH pairs. Band intensities for these pairs are consistent with variations in the concentration of octahedral Fe and Al in the two samples. In addition, both samples display a trace trioctahedral signature in NIR, which may be related to local trioctahedral domains, or the presence of sepiolite in trace amounts, or as intergrowths. A surface H 2 O species typical of the hydrated phase was identified via its NIR combination mode at 5317 cm -1 . The desorption of this species by heating revealed distinct silanol groups with overtone and combination modes at 7255 and 4575 cm -1 , respectively. Mg-coordinated and zeolitic H 2 O species are strongly coupled in the hydrated phase and give rise to NIR combination modes at 5190 and 5240 cm -1 . The removal of zeolitic H 2 O causes the blue shift of the three dioctahedral OH overtones by ca. 20 cm -1 and the rearrangement of the coordinated H 2 O manifested by the growth of sharp combination modes at ca. 5215 and 5120 cm -1 .

The free energy of formation of zircon (ZrSiO 4 ) from its oxides was determined between 1100 and 1300 K by an electrochemical method, in which values of μ O₂ defined by the two assemblages Fe 2 SiO 4 -Fe-SiO 2 (fayalite-iron-quartz) and Fe 2 SiO 4 -Fe-ZrO 2 -ZrSiO 4 were each measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in μ O₂ between these two assemblages corresponds to the reaction ZrO 2 + SiO 2 (qz) = ZrSiO 4 . The results, when analyzed using calorimetric data for the entropies and high-temperature heat capacities of ZrSiO 4 , ZrO 2 , and SiO 2 (quartz), yields Δ f,ox H 0 298K = -24.0 ± 0.2 kJ/mol for ZrSiO 4 , in good agreement with the calorimetric value of Ellison and Navrotsky (1992). ZrSiO4 is predicted to decompose to ZrO 2 plus SiO 2 (cristobalite) at 1938 K, assuming a temperature of 1430 K for the martensitic phase transition between the tetragonal and monoclinic forms of ZrO 2 (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol. The same experimental approach was used also to determine the free energy of formation of hafnon (HfSiO 4 ). The entropy of hafnon (S 0 298K = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δ f,ox H 0 298K = -25.0 ± 0.2 kJ/mol). The cells with either ZrSiO 4 + ZrO 2 or HfSiO 4 + HfO 2 produce an anomalous excursion in EMF when the temperature of the α-γ transition in Fe metal at 1184 K is traversed; this excursion takes >12 hours to decay back to the equilibrium value. This behavior is presumably related to strain caused by the volume change of the α-γ transition. The redetermination of the μ O₂ of the Fe 2 SiO 4 -Fe-SiO 2 (qz) equilibrium (the quartz-fayalite-iron or QFI oxygen buffer) carried out in the course of this study gave results in reasonable agreement with previous work, but with a different slope vs. temperature, implying a slightly higher value of S 0 298K for Fe 2 SiO 4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).

An experimental study on smectite dissolution was carried out using in-situ TMAFM and CMAFM (tapping mode and contact mode atomic force microscopy, respectively) analyses at 25 °C under alkaline conditions. Smectite particles dissolved via the retreat of the edge surfaces without scratching by the AFM tip, except in a series of the dissolution experiment in CMAFM. The retreat of each straightened edge surface appeared to occur with a constant rate. In contrast, the basal surface was unreactive within the experimental duration. The dissolution rates normalized to the edge surface area (ESA) of smectite at a certain pH and temperature condition, therefore, had a constant value independent of the particle size, whereas the dissolution rates normalized to the total surface area (TSA) varied with the particle size. These dissolution rates were consistent with those derived from wet-chemical data in previous studies. The anisotropic dissolution behavior was also observed along the ESA, that is, the retreat of dissolution fronts along the {110} faces was much faster than that along {010} faces. This difference can be explained by smectite dissolution under alkaline conditions being controlled by OH - attack, which is catalyzed by protonated Al-OH groups, on the bridging O atoms of both Al-O-Si sites, which locate only on the {110} surfaces, and Al 2 -O-Si sites on the {010} and {110} surfaces. The bridging O atoms of Al 2 -O-Si sites on the {010} surfaces are buried more deeply in the structure than those of Al-O-Si sites on the {110} surfaces, which would be more difficult to attack on the {010} surfaces.

The X-ray absorption angle-dependent behavior at the potassium K-edge has been determined for two end-members and two intermediate trioctahedral micas, which are representative of the Fe 2+ Mg -1 , Fe 3+ Al -1 , SiAl -1 FeLi -1 , and F -1 OH exchange vectors, using horizontally polarized synchrotron radiation. Experimental spectra are interpreted by the multiple scattering theory following decomposition according to the angular dependence approach of Brouder (1990). The experimental spectra sum up anisotropic effects deriving from in-plane and out-of-plane photoelectron interactions with the potassium near- and next-near neighbors up to the fifth/sixth coordination sphere. For the first time, the absorption edge of a low Z atom is decomposed so as to produce two partial patterns giving, respectively, the full in-plane absorption spectrum (σ || ) and the full out-of-plane spectrum (σ ⊥ ). They fully describe the complete X-ray absorption dichroic behavior of layered compounds, of which mica is a prototype. σ || essentially reflects the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets. Differences among the four micas depend on structural changes induced by the adaptation of their tetrahedral sheets to the increasing sizes of their octahedral sheet due to chemical substitutions (i.e., the Fe 2+ Mg -1 vector, mainly) and to chemical changes at the octahedral anion site (F -1 OH vector). By contrast, σ ⊥ reflects multiple-scattering interactions entering deep into the mica structure, beyond the tetrahedral sheet and well into the octahedral sheet. Such contributions reflect both changes in the anions coordinating the octahedral cations and increasing total amount of heavy atoms (essentially Fe), and differ based on electronic properties such as oxidation states (e.g., Fe 2+ and Fe 3+ ). Therefore, angle-dependent XANES spectroscopy is one of the rare spectroscopic techniques able not only to detect, but also to quantify structural effects in the atoms at the medium to long-range order.

Mössbauer spectroscopy, H 2 O, and microprobe analysis techniques have been used to study upper mantle ferrikaersutite megacrysts from the scoria cones of the Ash Sham alkaline volcanic field, northeastern part of the Arabian plate. Mössbauer spectra, collected at 298 K, indicate that the kaersutites are highly oxidized and all iron occurs as Fe 3+ . Two components were detected within the Fe 3+ quadrupole splitting distribution of the C-type sites and were assigned to M1 and M2-3 sites. The quadrupole splitting (QS) varies between 0.73.0.87 mm/s (Fe 3+ M1) and 1.28.1.45 mm/s (Fe 3+ M2-3). The kaersutite has a large oxy component in the amphibole OH-site (1.49-1.85 O 2- apfu) similar to the mantle-derived kaersutites. The very high ferric concentration in the kaersutites would suggest crystallization from a relatively oxidizing magma, perhaps with f O₂ close to the fayalite-magnetitequartz (FMQ), and may be a function of the high Fe 3+ /Fe tot of the metasomatic fluid that crystallized these amphiboles.

Piergorite-(Ce) is a new mineral found at Tre Croci, Vetralla, Italy with simplified formula Ca 8 Ce 2 (Al 0.5 Fe 3+ 0.5 ) Σ1 (□,Li,Be) 2 Si 6 B 8 O 36 (OH,F) 2 . It occurs as strong intergrowths of small crystals, colorless to pale yellow, associated with sanidine, mica, magnetite, rutile, titanite, and other Th-U-REE bearing minerals, in miarolitic cavities of a syenitic ejectum. Piergorite-(Ce) is biaxial negative, n α = 1.717 (1), n β = 1.728 (1), and n γ = 1.735 (1), 2V meas = 68(2)°, X = b, and Z ^ c = 7(1)°. Crystals show tabular habit and a very good {010} cleavage; twinning along the (301̄) plane produces “L” forms. The three strongest lines in the simulated powder diffraction pattern (d obs , I, hkl) are: 2.65 Å, 100.0, (213, 4̄13); 1.91 Å, 48.3, (223, 4̄23, 821); 2.90 Å, 44.9, (6̄03, 6̄12). The structure was solved by Patterson synthesis from X-ray diffraction data [monoclinic, space group P2/a, a = 28.097(3) Å, b = 4.777(1) Å, c = 10.236(2) Å, β = 96.81(1)°, V = 1364.2(7) Å 3 , Z = 2] and was refined to a final R obs = 0.059 for 6480 F o with I o > 3σ(I o ). The structure shows similarities with the hellandite group because Si and B tetrahedra form chains along c. Hellandite structure is characterized by a single chain of five-membered rings, whereas piergorite-(Ce) shows a double chain of five-membered rings interconnected by a single octahedral site to form a three-dimensional framework containing five independent eightfold-coordinated M sites and a partly occupied T-cavity.

A native gold bleb found in an olivine phenocryst in a picrite lava from the Emeishan Large Igneous Province (ELIP) may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle-derived magma. Four picrite layers have been recognized in the lower part of the volcanic succession in the Lijiang area, in the western part of the ELIP. The native gold bleb was found enclosed in an olivine phenocryst in the second picritic layer of a basalt-picrite succession in the ELIP. The gold bleb is spheroidal, about 30 μm in diameter, consists of pure gold, and does not contain any other elements. In addition, native copper grains were also discovered in the serpentinized olivine phenocrysts, and native zinc and moissanite (SiC) were separated from an ~20 kg sample of picrite. The paragenesis of these minerals suggests that the primary magmas were S-unsaturated. The native gold and native copper grains are considered to be xenocrysts from the mantle, transported to shallow depths by a rising plume, and then captured by the picritic melts. The discovery of native gold and native copper grains provides direct evidence that the gold in the hydrothermal gold deposits and the native copper deposits in the ELIP lavas ultimately may be derived from a mantle plume.

A cordierite sample (42/IA) from Kiranur, South India, and an ilvaite sample (ISX1) from Serifos, Greece, have been characterized and prepared for use as potential electron microprobe reference samples. By combining different high-precision analytical methods, we determined the major-, minor-, and trace-element composition of these samples and demonstrated their near-stoichiometric composition and high homogeneity at the micrometer level. The chemical composition (working values) of ilvaite ISX1 is (in wt%): SiO 2 = 29.68%; Al 2 O 3 = 0.67%; Fe 2 O 3 = 34.72%; FeO = 18.67%; FeO T = 51.52 %; MnO = 0.31%; MgO = 0.22%; CaO = 13.76%; H 2 O = 2.21%; and of cordierite 42/IA is (in wt%): SiO 2 = 49.65; Al 2 O 3 = 33.17%; FeO T = 2.34%; MnO = 0.03%; MgO = 12.44%; Na 2 O = 0.33%; H 2 O = 1.21%; CO 2 = 0.59%. Aliquots of these samples for use as primary or secondary electron microprobe reference sample can be obtained on request.

We have modeled the vibrational spectrum of brucite, the common phase of magnesium hydroxide, at 1 bar (10 5 Pa) by two separate techniques: molecular dynamics simulation and vibrational mode analysis. Molecular dynamics simulation of a model supercell provides information (from the power spectrum of the atomic velocity autocorrelation function) about the frequencies and directions of atomic thermal motions, using a defined energy force field. Vibrational mode analysis gives complementary information about the frequencies, nature, and infrared and Raman activity of the computed modes of the same system. Using both methods we find (in addition to the spectroscopically active modes) inactive modes up to around 1000 cm -1 , corresponding to MOH bending (OH rotational) motions. We invoke these modes to explain the published inelastic neutron scattering data, and suggest that their relatively high frequency is an inevitable consequence of repulsive interactions between neighboring H atoms.

Alunite and jarosite from Goldfield, Nevada, show spectacular relationships between early alunite and later jarosite. In some cases, jarosite overgrows alunite with the same crystallographic orientation and sharp contacts. Electron microprobe analyses of these phases show that they fall in the alunitejarosite quadrilateral defined by alunite, KAl 3 (SO 4 ) 2 (OH) 6 ; natroalunite, NaAl 3 (SO 4 ) 2 (OH) 6 ; jarosite, KFe 3 3+ (SO 4 ) 2 (OH) 6 ; and natrojarosite, NaFe 3 3+ (SO 4 ) 2 (OH) 6 . A large compositional gap occurs between alunite-natroalunite and jarosite-natrojarosite. This gap has no crystal chemical basis because Al and Fe 3+ can readily substitute for each other in octahedral site coordination. We believe the “on-off switch” behavior between early alunite and later jarosite is caused by an oxidant entering the system, oxidizing Fe 2+ in solution to Fe 3+ , raising the Eh and possibly oxidizing H 2 S to lower the pH, and thus stabilizing jarosite relative to alunite. The activity of Fe (as Fe 2+ ) increased in the solution because of prolonged alunite crystallization but could not readily enter the crystal structure until it was oxidized to Fe 3+ . The jarosite overgrowths show striking oscillatory zoning of Na- and K-rich bands. This reflects up to an order of magnitude change in the fluid K/Na ratio. These textures are interpreted to represent rapid growth and kinetic control of delivery of free Na and K to the crystal-fluid interface. This could be due to some combination of Na and K diffusion rates in the solution and complex ion breakdown involving Na and K.