Showing papers in "American Mineralogist in 2010"

Abbreviations for names of rock-forming minerals

Abstract: Nearly 30 years have elapsed since Kretz (1983) provided the mineralogical community with a systematized list of abbreviations for rock-forming minerals and mineral components. Its logic and simplicity have led to broad acceptance among authors and editors who were eager to adopt a widely recognized set of mineral symbols to save space in text, tables, and figures. Few of the nearly 5000 known mineral species occur in nature with a frequency sufficient to earn repeated mention in the geoscience literature and thus qualify for the designation “rock-forming mineral,” but a reasonable selection of the most common and useful rock-forming minerals likely numbers in the several hundreds. The original list by Kretz (1983) contained abbreviations for 193 of these. We propose an expansion to the list initiated by Kretz (1983) (see next page). Modest expansions and revisions were made by Spear …

The computation of equilibrium assemblage diagrams with Theriak/Domino software

Abstract: In this paper, the term “equilibrium assemblage diagrams” refers to diagrams strictly based on assemblages predicted by Gibbs free energy minimization. The presented Theriak/Domino software uses a unique algorithm of scanning and bookkeeping, which allows to compute completely and automatically a great variety of diagrams: phase diagrams, pseudo-binary, pseudo-ternary, isopleths, modal amounts, molar properties of single phases or bulk-rock properties like total Δ G , volume of solids, etc. The speed and easiness of use makes thermodynamic modeling accessible to any student of Earth sciences and offers a powerful tool to check the consistency of thermodynamic databases, develop new solution models, plan experimental work, and to understand natural systems. The examples described in this paper demonstrate the capacity of the software, but also to show the usefulness and limitations of computed equilibrium assemblage diagrams. For most illustrations, a metapelite (TN205) from the eastern Lepontine Alps is used. The applications include the interpretation of complex diagrams, mineral reactions, the effect of Al content on the equilibrium assemblages, the interpretation of Si per formula unit in white mica, understanding some features of garnet growth, dehydration and isothermal compressibility, a broadening of the concept of AFM diagrams, combining equilibrium assemblage diagram information with thermobarometry, and comparing the results produced with different databases. Equilibrium assemblage diagrams do not always provide straightforward answers, but mostly stimulate further thought.

Determination of nanoparticulate magnetite stoichiometry by Mössbauer spectroscopy, acidic dissolution, and powder X-ray diffraction: A critical review

Abstract: A solid solution can exist of magnetite (Fe3O4) and maghemite (γ-Fe2O3), which is commonly referred to as nonstoichiometric or partially oxidized magnetite. The degree of stoichiometry in magnetite is quantitatively measured by determining the ratio of Fe2+ to Fe3+. Magnetite stoichiometry ( x = Fe2+/Fe3+) strongly influences several physical properties, including the coercitivity, sorption capacity, reduction potential, and crystalline structure. Magnetite stoichiometry has been extensively studied, although very little work exists examining the stoichiometry of nanoparticulate samples (<<100 nm); when the stoichiometry was measured for nanoparticulate samples, it was not validated with a secondary technique. Here, we review the three most common techniques to determine magnetite stoichiometry: (1) acidic dissolution; (2) Mossbauer spectroscopy; and (3) powder X-ray diffraction (pXRD), specifically with nanoparticulate samples in mind. Eight samples of nonstoichiometric magnetite were synthesized with x ranging from 0 to 0.50 and with the particle size kept as similar as possible (BET specific surface area = 63 ± 7 m2/g; particle size ≈ 20 nm). Our measurements indicate excellent agreement between stoichiometries determined from Mossbauer spectra and by acidic dissolution, suggesting that Mossbauer spectroscopy may be a useful means for estimating magnetite stoichiometry in nanoparticulate, multi-phases samples, such as those found in the environment. A significant linear correlation was also observed between the unit-cell length ( a ) of magnetite measured by pXRD and magnetite stoichiometry, indicating that pXRD may also be useful for determining particle stoichiometry, especially for mixed phased samples.

Structure of nanocrystalline phyllomanganates produced by freshwater fungi

Abstract: The crystal structures of biogenic Mn oxides produced by three fungal strains isolated from stream pebbles were determined using chemical analyses, XANES and EXAFS spectroscopy, and powder X-ray diffraction. The fungi-mediated oxidation of aqueous Mn 2+ produces layered Mn oxides analogous to vernadite, a natural nanostructured and turbostratic variety of birnessite. The crystallites have domain dimensions of ~10 nm in the layer plane (equivalent to ~35 MnO 6 octahedra), and ~1.5–2.2 nm perpendicularly (equivalent to ~2–3 layers), on average. The layers have hexagonal symmetry and from 22 to 30% vacant octahedral sites. This proportion likely includes edge sites, given the extremely small lateral size of the layers. The layer charge deficit, resulting from the missing layer Mn 4+ cations, is balanced mainly by interlayer Mn 3+ cations in triple-corner sharing position above and/or below vacant layer octahedra. The high surface area, defective crystal structure, and mixed Mn valence confer to these bio-minerals an extremely high chemical reactivity. They serve in the environment as sorption substrate for trace elements and possess catalytic redox properties.

Detection of structurally bound hydroxyl in fluorapatite from Apollo Mare basalt 15058,128 using TOF-SIMS

Abstract: Fluorapatite grains from Apollo 15 Mare basalt 15058,128 were analyzed by Raman spectroscopy, Raman spectral imaging, time-of-flight secondary ion mass spectrometry (TOF-SIMS), field emission scanning electron microscopy (FE-SEM), and electron probe microanalysis (EPMA) in an attempt to detect structurally bound OH− in the fluorapatite. Although OH− could not be definitively detected by Raman spectroscopy because of REE-induced photoluminescence, hydroxyl was detected in the fluorapatite by TOF-SIMS. The TOF-SIMS technique is qualitative but capable of detecting the presence of hydroxyl even at trace levels. Electron microprobe data indicate that on average, F and Cl (F+Cl) fill the monovalent anion site in these fluorapatite grains within the uncertainties of the analyses (about 0.07 ± 0.01 atoms per formula unit). However, some individual spot analyses have F+Cl deficiencies greater than analytical uncertainties that could represent structural OH−. On the basis of EPMA data, the fluorapatite grain with the largest F+Cl deficiency constrains the upper limit of the OH− content to be no more than 4600 ± 2000 ppm by weight (the equivalent of ~2400 ± 1100 ppm water). The TOF-SIMS detection of OH− in fluorapatite from Apollo sample 15058,128 represents the first direct confirmation of structurally bound hydroxyl in a lunar magmatic mineral. This result provides justification for attributing at least some of the missing structural component in the monovalent anion site of other lunar fluorapatite grains to the presence of OH−. Moreover, this finding supports the presence of dissolved water in lunar magmas and the presence of at least some water within the lunar interior.

Partial high-grade alteration of monazite using alkali-bearing fluids: Experiment and nature

Abstract: Recent advances in the dating of monazite using the electron microprobe indicate that ThSiO 4 - and/ or CaTh(PO 4 ) 2 -enriched intergrowths in monazite can give ages younger than the original monazite. The morphology of the intergrowths suggests that the original monazite grain has been partly altered by a fluid in which Th, Si, and Ca are mobile. This hypothesis has been tested in the piston-cylinder apparatus at 1000 MPa and 900 °C utilizing a natural, unzoned, Th- and Pb-bearing monazite-(Ce) and Na 2 Si 2 O 5 +H 2 O. During the experiment, a subset of the monazite grains acquired ThSi0 4 -enriched areas with sharp compositional boundaries devoid of Pb and show evidence of being a pseudomorphic partial replacement of the monazite rather than an overgrowth of the monazite. These experiments support the hypothesis that similar Th-enriched or Th-depleted patches with sharp compositional boundaries observed in natural monazite could be the result of fluid-induced alteration via coupled dissolution-reprecipitation. If so, such altered regions would yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of single or multiple metasomatic events assuming that all pre-existing radiogenic Pb is removed during alteration.

A relationship between d104 value and composition in the calcite-disordered dolomite solid-solution series

Abstract: X-ray diffraction has been widely used in analyzing Ca-Mg carbonates. Compositions of biogenic and inorganic (Ca,Mg)CO3 crystals are often calculated by comparing their d 104 values with published empirical curves. However, previous studies suggested that these curves do not apply to very high-Mg calcite and disordered dolomite. Based on synthesized high-Mg calcite and disordered dolomite, a new empirical curve between values of magnesian calcite d 104 and MgCO3 content in the calcite-disordered dolomite solid-solution series is constructed. This new curve is consistent with the significant cell parameter changes accompanying the Mg-Ca cation disorder in dolomite, and it can help the characterization of the MgCO3 content of both natural and synthetic magnesian calcite and disordered dolomite, especially for the mineral mixtures that are not suitable for other analysis methods.

Site-specific infrared O-H absorption coefficients for water substitution into olivine

Abstract: There are four ways by which OH - commonly substitutes into olivine, namely those associated with (1) Si vacancies, (2) Mg vacancies, (3) Ti, or (4) trivalent cations. The four mechanisms, which we label [Si], [Mg], [Ti], and [triv], respectively, may each be fingerprinted by their characteristic O-H stretching modes in the infrared spectrum. We show by comparing the integrated intensities of these characteristic infrared peaks against total water content analyzed by secondary ion mass spectrometry, obtained for a suite of synthetic olivines plus one natural olivine, that the different substitution mechanisms require different absorption coefficients (k). For [Ti], we find k = 0.18 ± 0.07, identical to the value previously obtained from natural olivines in which the water was mainly associated with [Ti] defects. Values of k for [Si] and [triv] are 0.57 ± 0.04 and 0.18 ± 0.05, respectively; that for [Mg] is too small to be accurately determined (0.03 ± 0.03). The values of k for [Ti] and [Si] defects differ by a factor of three even though their average wavenumbers are virtually the same. The [Ti] and [triv] defects, on the other hand, have very similar absorption coefficients at significantly different wavenumbers. This highlights the inadequacy of using wavenumber-dependent calibrations for olivine and presumably for NAMs in general. Different substitution mechanisms have their own crystallographic environments that determine their absorption coefficients. The large variation in absorption coefficients within a single mineral emphasizes the importance of distinguishing the substitution mechanism if meaningful quantitative results are to be obtained from infrared spectroscopy.

Serpentine minerals discrimination by thermal analysis

Abstract: This paper reports a complete set of TG, DTG, and DTA data, coupled with emitted gas analysis, for well-constrained, almost pure serpentine samples Serpentine dehydroxylation takes place between 550 and 800 °C, with DTG and DTA peak temperatures progressively decreasing from antigorite (720 and 715 °C, respectively) to lizardite (708 and 714 °C), polygonal serpentine (685 and 691 °C), and chrysotile (650 and 654 °C) Antigorite has an additional diagnostic signal at ~740–760 °C, always absent in the other serpentines, and dependent on antigorite superperiodicity ( T shift of ~20 °C from 36 to 49 A modulation wavelength) A sharp exothermic peak occurs at extremely constant temperatures (~820 °C), independently from the starting serpentine structure The high- T mineral assemblage is always represented by forsterite and enstatite Based on the observed relationships between serpentine structures and DTG/DTA dehydroxylation temperatures, thermal analysis may represent a useful tool for serpentine identification, particularly in the case of natural massive samples where different varieties are mutually intermixed The accurate definition of serpentine mineralogy would have obvious implications in both geological-petrological and health-related issues

The structure of schwertmannite, a nanocrystalline iron oxyhydroxysulfate

Abstract: Schwertmannite is a poorly crystalline mineral that forms ochre rusts and precipitates in acid mine environments. Despite its ubiquity and its role as scavenger of important contaminants such as arsenic or selenium, its structure has not been yet determined. Here, a structure for schwertmannite is presented based on pair distribution function (PDF) data, X-ray diffraction (XRD) analyses, and density functional theory (DFT) calculations. We propose a structure formed by a deformed frame of iron octahedra similar to that of akaganeite. Simulations of X-ray diffraction patterns unveil the presence of long-range order associated with the position of the sulfate molecules, providing a useful way to discern two types of sulfate complexes in the structure. The simulations suggest that two sulfate molecules per unit cell are present in the structure forming one outer sphere and one inner sphere complex inside the channels formed by iron octahedra. Knowledge of the positions of the sulfates in the structure will help to better understand exchange processes with oxyanions of trace contaminants, such as arsenate, chromate, or selenate, that strongly influence their biogeochemical cycling in mining ecosystems.

Factors responsible for crystal-chemical variations in the solid solutions from illite to aluminoceladonite and from glauconite to celadonite

Abstract: Several finely dispersed low-temperature dioctahedral micas and micaceous minerals that form solid solutions from (Mg, Fe)-free illite to aluminoceladonite via Mg-rich illite, and from Fe3+-rich glauconite to celadonite have been studied by X-ray diffraction and chemical analysis. The samples have 1 M and 1 Md structures. The transitions from illite to aluminoceladonite and from glauconite to celadonite are accompanied by a consistent decrease in the mica structural-unit thickness (2:1 layer + interlayer) or c sinβ. In the first sample series c sinβ decreases from 10.024 to 9.898 A, and in the second from 10.002 to 9.961 A. To reveal the basic factors responsible for these regularities, structural modeling was carried out to deduce atomic coordinates for 1 M dioctahedral mica based on the unit-cell parameters and cation composition. For each sample series, the relationships among c sinβ, maximum and mean thicknesses of octahedral and tetrahedral sheets and of the 2:1 layer, interlayer distance, and variations of the tetrahedral rotation angle, α, and the degree of basal surface corrugation, Δ Z , have been analyzed in detail. The transitions from illite to aluminoceladonite and from glauconite to celadonite are accompanied by a slight increase in the mean thickness of the 2:1 layers and a steady decrease in the α angles, whereas the interlayer distance becomes smaller. These results are consistent with the generally accepted model where tetrahedral rotation is the main factor for the interlayer contraction in muscovite-phengite structures: the smaller the rotation angle (α) the larger the ditrigonal ring of the tetrahedral sheet and the interlayer pseudo-hexagonal cavity, allowing the interlayer cation to sink and thus shorten the c parameter. A new insight into the interpretation of the contraction of the mica layer thickness in dioctahedral micas has been achieved with the discovery that micas with the same or close mean interlayer distance, on one hand, have the same or nearly the same substitution of Al for Si; and on the other hand, they may have significantly different parameters of the interlayer structure, such as tetrahedral rotation, basal surface corrugation, Δ Z , and minimum and maximum interlayer distance. These results show that in dioctahedral 1 M micas, the mean interlayer distance is determined by the amount of tetrahedral Al because the higher the Al for Si substitution, the stronger the repulsion between the basal O atoms and the larger the interlayer distance and c sinβ parameter.

Fe site occupancy in magnetite-ulvöspinel solid solutions: A new approach using X-ray magnetic circular dichroism

Abstract: Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was > 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrentmore » with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.« less

The behavior of the Hf isotope system in radiation-damaged zircon during experimental hydrothermal alteration

Abstract: The application of the Hf isotope composition of zircon as a geochemical tracer requires the preservation of Lu-Hf systematics within individual grains. We performed hydrothermal experiments on a self-irradiation-damaged zircon to test whether hydrothermal alteration would affect its Hf isotopic composition. Severely radiation-damaged zircon from Sri Lanka was reacted in either Teflon reactors or gold capsules at 200 °C (1080 h, autogeneous pressure), 400 °C (120 h, 1 kbar), and 600 °C (72 h, 1 kbar) in a 1 M HCl–0.2 M HF solution that was spiked with 300 ppm non-natural Hf (98.2% 180 Hf) and 970 ppm Yb. Laser ablation inductively coupled plasma mass spectrometry measurements of the Hf- and U-Pb isotope compositions of the altered domains revealed that the U-Pb system of such domains was severely disturbed, resulting in a discordia pointing toward the origin of the concordia diagram, but that the Hf isotope composition was unaffected. In addition, Yb enrichment was observed in the reacted zircon domains, predominantly near the zircon-solution interface. The Yb has apparently diffused into the altered domains. The new data are fully consistent with a diffusion-controlled aqueous alteration process occurring within radiation-damaged zircon.

Nitrogen and hydrogen isotope compositions and solubility in silicate melts in equilibrium with reduced (N+H)-bearing fluids at high pressure and temperature: Effects of melt structure

Abstract: Solubility and solution mechanism(s) of reduced (N+H)- and H-containing N-O-H volatile components in Na 2 O-SiO 2 composition melts in equilibrium with NH 3 +H 2 +N 2 and H 2 O+H 2 fluid and H- and N-isotope concentrations in these melts were determined experimentally at 1.5 GPa and 1400 °C as a function of hydrogen fugacity, f H 2 , and melt polymerization (composition), NBO/Si (NBO/Si = 0.4–1.18). This NBO/Si-range is similar to that between dacite and olivine tholeiite melt (NBO/Si ~0.4–1). The f H 2 was controlled between that of the iron-wustite + H 2 O [log f H 2 (IW) ~3.42 (bar)] and that of the magnetite-hematite + H 2 O [log f H 2 (MH) ~ −0.91 (bar)] buffer. The N solubility decreases from 0.98 to 0.28 wt% in the melt NBO/Si-range from 0.4 to 1.18 at f H 2 (IW) and decreases by about 50% between f H 2 (IW) and f H 2 (MH). The H solubility at f H 2 (IW) is insensitive to NBO/Si and averages 0.76±0.28 wt% and 0.48±0.07 wt% H in (N+H)-saturated and in N-free and H-saturated melts, respectively. The H solubility in the melts decreases by at least ~70% between f H 2 (IW) and f H 2 (MH). Their N and H isotope ratios are systematic functions of the abundance ratio of structurally bound N and H (as NH 2 − - and OH − -groups bonded to Si 4+ ) relative to molecular H 2 , N 2 , and NH 3 in the melts. Molecular H 2 O plays a subordinate role in these melts, the bulk H 2 O content of which is 2 − /NH 3 and OH − /H 2 abundance ratios vary by ~55 and ~500% between NBO/Si = 1.18 and 0.4 relative to the values at NBO/Si = 0.4. In this same NH 2 − /NH 3 abundance ratio range, the δ 15 N of (N+H)-saturated melts, relative to that of melts with NBO/Si = 0.4, varies by ~2‰, whereas the δ D varies by ~87‰. In N-free melts, the δ D varies by ~12‰. Changing abundance of volatiles dissolved in silicate melts in molecular form and as structural complexes that form bonds with the silicate melt structure is an important factor that can affect stable isotope fractionation during melting and crystallization at high pressure and temperature.

The structure of crystals, glasses, and melts along the CaO-Al2O3 join: Results from Raman, Al L- and K-edge X-ray absorption, and 27Al NMR spectroscopy

Abstract: Calcium aluminate glasses are important materials where AlO 4/2 − is the only network former. Aluminum in crystals or glasses between CaO and Al 2 O 3 can have different environments as a function of the CaO/Al 2 O 3 ratio. Using X-ray absorption at the Al K - and L -edges, Raman and 27 Al NMR spectroscopies, we have determined the structural surroundings of Al in glasses, crystals, and melts in this binary system. Aluminum is in octahedral coordination at high-Al 2 O 3 content (>80 mol%) and essentially in fourfold coordination with 4 bridging O atoms (BOs) at Al 2 O 3 contents between 30 and 75 mol%. At around 25 mol% Al 2 O 3 , Al is in tetrahedral coordination with two BOs. The presence of higher-coordinated species at high-Al 2 O 3 contents and their absence at low Al 2 O 3 imply different viscous flow mechanisms for high- and low-concentration Al 2 O 3 networks.

Methodological re-evaluation of the electrical conductivity of silicate melts

Abstract: Electrical impedance measurements in the laboratory on silicate melts are used to interpret magnetotelluric anomalies. On the basis of 2- and 4-electrode measurements, we show that the influence of the electrodes of the 2-electrode system on the measured resistivity can be of significant importance for low-resistivity melts and increases with temperature. At 1400 °C, the resistivity of very conductive melts measured with two electrodes can reach six times the resistivity value measured with four electrodes. A short-circuit experiment is needed to correct the 2-electrode data. Electrodes contribution is also estimated for samples from other studies, for which the resistance of the electrical cell can be as high as the resistance of the sample. A correction of the resistivity data from the literature is proposed and values of the corresponding Arrhenian parameters are recommended.

Texture analysis of a turbostratically disordered Ca-montmorillonite

Abstract: Turbostratic disorder, consisting in a disorder in which different layers have different rotations with respect to an axis, is commonly found in montmorillonite The effect of this kind of disorder on diffraction profiles is significant and must be taken into account, especially in quantitative phase analysis The effect of the turbostratic disorder in textured materials has never been investigated In the present work, we have developed a strategy to perform quantitative texture analysis on turbostratically disordered Ca-montmorillonite aggregates that were uniaxially compressed Synchrotron diffraction images were analyzed with a Rietveld method and disordered and ordered models are compared The method proved to be reliable and ready for further applications

High-pressure melting of wüstite

Abstract: Iron oxide (FeO) is an important component in the mineralogy of Earth’s lower mantle and possibly its core, so its phase diagram is essential to models of the planet’s interior. The melting curve of wustite, Fe 0.94 O, was determined up to 77 GPa and 3100 K in a laser-heated diamond anvil cell. Melting transition temperatures were identified from discontinuities in the emissivity vs. temperature relationship within the laser-heated spot. The melting curve exhibits no obvious kinks that could be related to a subsolidus transition in wustite, but there is evidence for a two-phase loop at pressures below 30 GPa. Comparison of these results to previous studies on Fe, Fe-O, and Fe-S confirms that the melting point depression in the Fe-O system remains significantly less, by a factor of 2 or more, than that in the Fe-S system up to pressures exceeding 80 GPa.

Radiation damage and uranium concentration in zircon as assessed by Raman spectroscopy and neutron irradiation

Abstract: Radiation damage of natural and synthetic zircon grains is evaluated by Raman spectroscopy to understand annealing and stability of fission tracks. Analyses focus on a suite of 338 Paleozoic detrital zircon grains from metamorphosed strata in the Hellenic forearc that were variably annealed by a Miocene thermal event, as well as a suite of 97 synthetic zircon grains. The Raman wavenumber shift of v 3 [Si0 4] reveals that radiation damage and damage distribution in this suite mainly depends on uranium concentration. In zircon with similar uranium concentration, the Raman wavenumber shift allows for the determination of radiation damage in different crystals, which is a function of effective accumulation time. Nine detrital zircons grains with moderate radiation damage were stepwise annealed at 1000 and 1400 °C, which resulted in progressive removal of radiation damage revealed in an increase of ν 3 [Si0 4] peak positions. For a partly reset sample that was brought to temperatures of ∼350 °C in a geologic setting (Hellenic forearc), we use the Raman measurements and uranium determination to estimate a Zircon Damage Discrimination Factor (Z RDD ), which is our attempt to estimate only radiation damage in single grains by accounting for affects of the uranium atom in the Raman wavenumber. This discrimination allows for a separation of zircon fission track (ZFT) ages of single ages based on grains that have a low track retention (high damage, fully reset grain), thus refining the age determination of cooling in a rock that shows variable resetting.

Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions

Abstract: Tourmaline grains extrcted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora Maira, Western Alps (peak P - T conditions ~3.7 GPa, 750 °C), has a structural formula of X(Na0.90Ca0.05K0.01□0.04)Y(Mg1.78Al0.99Fe2+0.12Ti4+0.03□0.08)Z(Al5.10Mg0.90)(BO3)3TSi6.00O18V(OH)3W[(OH)0.72F0.28]. Dravite from Lago di Cignana, Western Alps, Italy (~2.7–2.9 GPa, 600–630 °C), has a formula of X(Na0.84Ca0.09K0.01□0.06)Y(Mg1.64Al0.79Fe2+0.48Mn2+0.06Ti4+0.02Ni0.02Zn0.01)Z(Al5.00Mg1.00)(BO3)3T(Si5.98Al0.02)O18V(OH)3W[(OH)0.65F0.35]. “Oxy-schorl” from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at >2.9 GPa, 870 °C, has a formula of X(Na0.86Ca0.02K0.02□0.10)Y(Al1.63Fe2+1.23Ti4+0.11Mg0.03Zn0.01) Z(Al5.05Mg0.95)(BO3)3T(Si5.96Al0.04)O18V(OH)3W[O0.81F0.10(OH)0.09]. There is no structural evidence for significant substitution of [4]Si by [4]Al or [4]B in the UHP tourmaline ( distances ~1.620 A), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high- T –low- P tourmaline, which typically has significant amounts of [4]Al. There is an excellent positive correlation ( r 2 = 1.00) between total [6]Al (i.e., YAl + ZAl) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation ( r 2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and YAl content ( r 2 = 1.00) that is best explained by the exchange vector YAlO(R2+F)−1. This is consistent with the W site (occupied either by F, O, or OH), being part of the YO6-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.

Solubility of manganotantalite and manganocolumbite in pegmatitic melts

Abstract: Solubility experiments of MnNb 2 O 6 and MnTa 2 O 6 were conducted in two nominally dry to water-saturated pegmatitic melts with different amounts of Li, F, P, and B at 700 to 1000 °C and 200 MPa to determine the maximum concentrations of Nb and Ta in pegmatitic melts. The Li 2 O, F, B 2 O 3 , and P 2 O 5 contents in the melts were 1.16, 2.99, 1.78, and 1.55 wt% for melt composition PEG1 and 1.68, 5.46, 2.75, and 2.75 wt% for melt composition PEG2 and the resulting Al/(Na+K+Li) ratio for both melts is 0.92. The experimental data show that the solubility product of manganocolumbite increases by a factor of three upon increasing the water concentration from 0 to 4 wt%. Considering that pegmatitic melts at pressures above 50 to 100 MPa are hydrous (>4 wt% H 2 O), the increase in solubility by this magnitude, over the stated range of water concentration, is not significant for pegmatites. The data also point out that the solubility of MnNb 2 O 6 and MnTa 2 O 6 is strongly temperature dependent, increasing by a factor of 50 for manganocolumbite and 15–20 for manganotantalite from 700 to 1000 °C under water-saturated conditions. The solubility also increases with increasing content of fluxing elements like Li, F, B, and P. In the pegmatite melt containing the highest amount of fluxing elements, the maximum concentrations of Ta and Nb are higher by nearly one order of magnitude when compared to a subaluminous rhyolitic melt.

A new method for quantitative petrography based on image processing of chemical element maps: Part I. Mineral mapping applied to compacted bentonites

Abstract: Most natural rocks or engineered materials display a multi-scale heterogeneity ranging from the nanometer to the centimeter. Their spatial textural heterogeneity can be approached from chemical element maps acquired using various techniques (SEM, EPMA, SXAM, synchrotron μ-XRF, TEM), depending on the chosen magnification. Chemical map processing that yields quantitative petrographic information is improved here according to newly developed mineral thresholding methods that accommodate mixtures and solid solutions. The complex case of an MX80 compacted bentonite is used as a test case. The 14 major chemical elements of this sample were mapped using an electron probe microanalyzer, and chemical map processing yielded a quantitative map of the 18 mineral species of bentonite with a spatial resolution of a few micrometers. The textural heterogeneity of the solid part of the sample is thus visualized and quantified on an area ranging between 0.1–1 cm2. The method also provides a complete modal analysis of the sample. The methodology is expected to have broad applications in Earth and materials sciences.

Thermo-chemical and thermo-physical properties of the high-pressure phase anhydrous B (Mg14Si5O24): An ab-initio all-electron investigation

Abstract: aB stra C t Using the hybrid B3LYP density functional method, we computed the ab-initio thermo-chemical and -physical properties of the mineral anhydrous B (Anh-B), which has been recently suggested as a potential phase responsible for the X-discontinuity in the Earth’s mantle at ~300 km depth through the reaction forsterite + periclase = Anh-B, and also to likely split the 410 km discontinuity within the interior of a cold slab through the reaction wadsleyite/ringwoodite = Anh-B + stishovite. We first conducted an investigation of the static properties through a symmetry-preserving relaxation procedure and then computed, on the equilibrium structure, harmonic vibrational modes at the longwavelength limit corresponding to the center of the Brillouin zone (k → 0). While optic modes are the eigenvectors of the Hessian matrix at Γ point, acoustic modes were obtained by solving the non-zero components of the strain matrix. Following the Kieffer model, acoustic branches were assumed to follow sine wave dispersion when traveling within the Brillouin zone. All thermodynamic properties that depend on vibrational frequencies namely, heat capacities, thermal expansion, thermal derivative of the bulk modulus, thermal correction to internal energy, enthalpy, Gibbs free energy, thermal pressure and Debye temperature, were computed on the basis of quasi-harmonic mode-gamma analysis of the volume effects on vibrational frequencies. Moreover, the strain tensor was used to calculate several thermo-physical properties of geophysical interest (transverse and longitudinal wave velocities, shear modulus, Young’s modulus, and Poisson’s ratio). The ab-initio results derived in this study and the available data on molar volumes were used to calculate the univariant equilibrium forsterite + periclase = Anh-B. The results are in satisfactory agreement with the reversed experimental data of Ganguly and Frost (2006).

Determination of manganese valence states in (Mn3+, Mn4+) minerals by electron energy-loss spectroscopy

Abstract: Various manganese valence quantification methods using manganese L 2,3 and oxygen K electron energy loss near-edge spectra (ELNES) were applied to determine the relative portions of individual valence state in a mixed (Mn 3+ , Mn 4+ ) valence system. Multiple linear least-squares (MLLS) fitting of Mn L 2,3 ELNES using reference spectra and Gaussian peak fitting of Mn L 3 edge are newly developed and the feasibility of these methods was tested on a set of cryptomelane minerals with different valence states. The selection of appropriate standards is crucial to the success of the MLLS method. The O K -edge structures for manganese oxides can provide valuable guidance in the selection of appropriate reference spectra for quantitative determination of Mn valence state. Gaussian peak fitting, however, failed to determine the Mn valence for (Mn 3+ , Mn 4+ ) minerals due to the small separation between the primary L 3 peaks from Mn 4+ and Mn 3+ valence. As to the methods based on calibration curves, the energy difference between Mn L 3 and oxygen K , i.e., Δ E ( L 3 -O K ) vs. valence, is the most valence-sensitive method in the range of Mn 3+ and Mn 4+ and yields good agreement with the actual values.

Reduced As components in highly oxidized environments: Evidence from full spectral XANES imaging using the Maia massively parallel detector

Abstract: Synchrotron X-ray fluorescence (SXRF) and X-ray absorption spectroscopy (XAS) have become standard tools to measure element concentration, distribution at micrometer- to nanometer-scale, and speciation (e.g., nature of host phase; oxidation state) in inhomogeneous geomaterials. The new Maia X-ray detector system provides a quantum leap for the method in terms of data acquisition rate. It is now possible to rapidly collect fully quantitative maps of the distribution of major and trace elements at micrometer spatial resolution over areas as large as 1 × 5 cm2. Fast data acquisition rates also open the way to X-ray absorption near-edge structure (XANES) imaging, in which spectroscopic information is available at each pixel in the map. These capabilities are critical for studying inhomogeneous Earth materials. Using a 96-element prototype Maia detector, we imaged thin sections of an oxidized pisolitic regolith (2 × 4.5 mm2 at 2.5 × 2.5 μm2 pixel size) and a metamorphosed, sedimentary exhalative Mn-Fe ore (3.3 × 4 mm2 at 1.25 × 5 μm2). In both cases, As K -edge XANES imaging reveals localized occurrence of reduced As in parts of these oxidized samples, which would have been difficult to recognize using traditional approaches.

Structural relaxation around Cr3+ and the red-green color change in the spinel (sensu stricto)-magnesiochromite (MgAl2O4-MgCr2O4) and gahnite-zincochromite (ZnAl2O4-ZnCr2O4) solid-solution series

Abstract: Optical absorption spectra of flux-grown single crystals in the spinel s.s.-magnesiochromite and gahnite-zincochromite solid solutions were recorded with the aim of exploring variations in local Cr-O bond distance as a function of composition. With increasing Cr contents, the crystals vary in color from pale red to intensely red to dark greenish. These variations are reflected in the optical spectra by the position and intensity of the two spin-allowed electronic d - d transitions in six-coordinated Cr3+ at ~18000 (ν1) and 25000 cm−1 (ν2). From the shift of the ν1 band position, a decrease in crystal field splitting, 10 Dq , for six-coordinated Cr3+ with increasing Cr contents is evident in both solid-solution series. Based on published Cr-O bond distances for the CrO6 polyhedra in magnesiochromite and zincochromite of 1.995 and 1.991 A, respectively, and applying the ligand field relationships, local Cr-O bond distances in gahnite and spinel with Cr contents at trace levels are determined to be 1.974(2) and 1.960(3) A, respectively. These local Cr-O distances result in relaxation parameters (e) equal to 0.69(2) and 0.60(3) for Cr-O bonds in the Mg(Al1−xCrx)2O4 and Zn(Al1−xCrx)2O4 series, respectively. However, the presently obtained Racah B values indicate increasing Cr-O bond covalency with increasing Cr3+ contents. This suggests that color changes and accompanying 10 Dq variations may be due to variations in Cr-O bond covalency along the two solid-solution series, without or with very minor local Cr-O bond distance variation. Consequently, the e values obtained from the present optical absorption spectra should be regarded as minimum values.

REE diffusion in olivine

Abstract: Diffusion of rare earth elements has been characterized in synthetic forsterite and natural olivine (Fo90) under dry conditions. Experiments were prepared by enclosing source material [mixtures of rare-earth element (La, Dy, or Yb) aluminate and synthetic forsterite powders in 3:1 proportions] and polished forsterite in platinum capsules. For buffered experiments on natural olivine, samples were placed with the source in AgPd capsules, sealed under vacuum in silica glass ampoules with a solid buffer (NNO, IW, or graphite). In some experiments, ground natural olivine was also incorporated into the source material. Prepared capsules were annealed in 1 atm furnaces for one hour to several weeks at 850 to 1300 °C. REE distributions in olivine were profiled by Rutherford backscattering spectrometry (RBS). The following Arrhenius relation is obtained for Dy diffusion in forsterite: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{D}_{Dy}\ =\ 8.0\ {\times}\ 10^{{-}10}\ exp({-}289\ {\pm}\ 21\ kJ\ mol^{{-}1}/RT)\ m^{2}/s.\] \end{document} Diffusivities of Yb and La in forsterite and for Dy in natural olivine are similar. Experiments run on natural olivine show little dependence of diffusion on crystallographic orientation. The Arrhenius relation yields diffusivities about three orders of magnitude smaller than those determined for REE diffusion in olivine by Spandler et al. (2007) and suggests that timescales for preservation of REE signatures in olivine-hosted melt inclusions may be relatively long. REE diffusivities in olivine are an order of magnitude faster than those of REE+3 in enstatite (Cherniak and Liang 2007) and in diopside under most conditions (Van Orman et al. 2001). Hence, REE signatures in olivine are more likely to be altered by diffusion than those of clinopyroxene or orthopyroxene.

Multilevel modular mesocrystalline organization in red coral

Abstract: Biominerals can achieve complex shapes as aggregates of crystalline building blocks. In the red coral skeleton, we observe that these building blocks are arranged into eight hierarchical levels of similarly (but not identically) oriented modules. The modules in each hierarchical level assemble into larger units that comprise the next higher level of the hierarchy, and consist themselves of smaller, oriented modules. EBSD and TEM studies show that the degree of crystallographic misorientation between the building blocks decreases with decreasing module size. We observe this organization down to a few nanometers. Thus, the transition from imperfect crystallographic order at millimeter scale to nearly perfect single crystalline domains at nanometer scale is progressive. The concept of "mesocrystal" involves the three-dimensional crystallographic organization of nanoparticles into a highly ordered mesostructure. We add to this concept the notion of "multilevel modularity." This modularity has potential implications for the origin of complex biomineral shapes in nature. A multilevel modular organization with small intermodular misorientations combines a simple construction scheme, ruled by crystallographic laws, with the possibility of complex shapes. If the observations we have made on red coral extend to other biominerals, long-range crystallographic order and interfaces at all scales may be key to how some biominerals achieve complex shapes adapted to the environment in which they grow.

Characteristics of emission centers in alkali feldspar: A new approach by using cathodoluminescence spectral deconvolution

Abstract: Alkali feldspars in syenite from the Cerro Balmaceda pluton in the Patagonian Andes, Chile, show various petrographic microtextures formed during the magmatic to high- and low-temperature hydrothermal stages in which cathodoluminescence (CL) shows a wide range of blue, violet, and pink to red colors with variable brightness. Their CL spectra exhibit two emission bands: one at 405–420 nm in the blue region and the other at 700–760 nm in the red-infrared (IR) region. Asymmetrically shaped spectral peaks in energy units suggest overlapping of each individual emission, which corresponds to various luminescence centers. Blue emission bands were separated into two spectral peaks fitted by Gaussian curves centered at 3.055–3.076 and 2.815–2.845 eV. A positive correlation is found between emission intensities at 3.055–3.076 eV and TiO 2 contents, suggesting the activation of a Ti 4+ impurity as an emission center. The intensities at 2.815–2.845 eV, where clear and featureless feldspar (CF; not affected by hydrothermal metasomatism) is shown under optical microscopy, which have intensities appreciably higher than those showing patched microperthite (PMP), formed during low-temperature hydrothermal reactions, correlate reciprocally with the intensities of red-IR emission caused by a Fe 3+ impurity center. The peak at 2.815–2.845 eV can be attributed to oxygen defects associated with Al-O-Al and Al-O-Ti bridges. Most of the areas show CL emissions at 700–760 nm in the red-IR region, in which intensities increase with an increase in Fe 2 O 3 contents as impurities. The Fe 3+ ion acts as an activator for the red-IR emission. The Ab-rich and Or-rich phases of PMP have emission components at 1.644 eV (754 nm) and 1.727 eV (717 nm), respectively. The red-IR emission from CF consists of emission components at 1.677 eV (739 nm) and 1.557 eV (796 nm), according to an Fe 3+ impurity center in the Or-rich phase and in the Ab-rich phase as cryptoperthite, respectively. Both components are centered at a wavelength longer than the emission band of Ab-rich and Or-rich phases of PMP, suggesting a change in configurational state around the Fe 3+ ion from the T2 to the T1 site by low-temperature hydrothermal metasomatic reactions. Accordingly, the peak positions of the red-IR emission are controlled by the ordering state of Fe 3+ ion into the T1 site, the existence of multiphase perthite and chemical composition.

The determination of sulfate and sulfide species in hydrous silicate glasses using Raman spectroscopy

Abstract: Raman spectroscopy is used to identify the sulfur speciation (sulfur valence state) in “technical” iron-free soda-lime and potassium-silicate glasses and in “natural” glass compositions such as basalt, andesite, and rhyodacite. The presence of sulfate (S 6+ ) is marked in Raman spectra of oxidized synthetic and natural glasses by bands at ~990 and ~1000 cm −1 , respectively. The presence of sulfide (S 2− ) in the reduced technical glasses is marked in Raman spectra by a band at 2574 cm −1 indicating that S 2− is present as HS − in these Fe-free glasses. Such a band is absent in the Raman spectra of reduced basaltic, andesitic, and rhyodacitic glasses. However, an additional band at ~400 cm −1 appears in the Raman spectra of the reduced S-bearing “natural” glasses when compared to that of S-free reduced “natural” reference glasses indicating that S 2− is most likely complexed with Fe in these glasses. Thus, the dissolution mechanism of S 2− appears to be different in Fe-free and Fe-bearing glasses and S 2− is dissolved as either HS − -species or Fe-S complexes, respectively. The data shown here demonstrate the potential of Raman spectroscopy in identifying the sulfur valence state in silicate glasses. In addition, S 2− is dissolved as completely different complexes when comparing “technical” iron-free and “natural” iron-bearing, hydrous silicate glass compositions.