Wavelength-dispersive Electron Microprobe Analysis Research Articles

Optimal Raman-scattering signal for estimating the Fe3+ content on the clinozoisite–epidote join

Abstract. To provide a tool for fast estimation of the Fe3+ content in Ca2(Al, Fe3+)3Si3O12(OH) epidote grains, including in thin sections and crude-rock samples, we applied Raman spectroscopy to 33 areas from 15 natural samples with Fe3+ ranging from 0.22 to 1.13 atoms per formula unit (apfu), the chemistry of which was independently determined by wavelength-dispersive electron microprobe analysis (WD-EPMA). The Raman spectra were collected from the very areas subjected to WD-EPMA. We have analysed both the OH-stretching region (3215–3615 cm−1) and the spectral range generated by the framework vibrations (15–1215 cm−1). Similarly to the IR spectra, the Raman peaks in the OH-stretching region shift toward higher wavenumbers with increasing Fe. However, the quantification of Fe3+ based on OH-stretching Raman peaks can be hindered by the multicomponent overlapping and significant intensity variations with the crystal orientation. Among the Raman signals generated by framework vibrations, the position of four peaks (near 250, 570, 600, and 1090 cm−1) exhibit a steady linear regression with the increase in Fe content (in apfu). However, the peak near 250 cm−1 attributed to MO6 vibrations also depends on the crystal orientation and therefore is not always well resolved, which worsens the accuracy in Fe-content determination based on its position. The peaks near 570, 600, and 1090 cm−1 arise from Si2O7 vibrational modes, and although their intensities also vary with the crystal orientation, all three signals are well resolved in a random orientation. However, among the three Si2O7-related signals, the 570 cm−1 peak is the sharpest (peak width <10 cm−1) and is easily recognized as a separate peak. Hence, we propose to use the position of this peak as a highly reliable parameter to estimate the Fe content, via the linear trend given as ω570=577.1(3)-12.7(4)x, where ω is the wavenumber (cm−1) and x is Fe content (apfu), with accuracy ± 0.04 Fe3+ apfu. The peaks near 600 and 1090 cm−1 may be complementarily used for the Fe estimate, based on the following relations: ω600=611.6(2)-13.8(4)x and ω1090=1098.8(3)-13.5(5)x. Analyses of the effect of Sr as a substitution for Ca and Cr at the octahedral sites indicate that contents of Sr <0.12 apfu do not interfere with the quantification of Fe via the ω570 (x) relation, whereas Cr >0.16 apfu leads to overestimation of Fe; Cr presence can be recognized however by the broadening of the peaks near 95 and 250 cm−1.

Glass from the 11th–13th century medieval castle of San Giuliano (Lazio Province, Central Italy)

Compositional analysis of glass from the medieval castle of San Giuliano (Lazio, Italy), occupied from approximately CE 1050–1250, sheds light on the financial wherewithal and integration of the castle’s elite inhabitants into wider economic networks. Portable X-ray fluorescence (pXRF) of 261 shards was used to select 32 for further analysis using wavelength dispersive electron microprobe analysis (EMPA-WDS). This compositional analysis documents the pervasive recycling of earlier glass cullet, some of which pre-dates the 4th century CE. Around 20% of the sample comprised primarily plant ash glass, evidencing the penetration of plant-ash glasses into inland sites on the western central Italian peninsula. Almost 60% of the shards were an intermediate glass combining more recent plant ash glasses with recycled natron-based glass cullet derived from the Roman and Early Medieval periods. Compared to glass assemblages from contemporaneous sites, the levels of both recycled and intermediate glasses are quite high, with a concurrent incidence of trace elements that further precluded the manufacture of perfectly translucent glass vessels. This suggests that while the residents of the castle desired glass as a symbol of prestige, they may not have had the economic resources to obtain glass of the highest quality.

The atomic arrangement and electronic interactions in vonsenite at 295, 100, and 90 K

Abstract Vonsenite, Fe22+Fe3+O2BO3, has been the subject of many studies in the materials science and condensed matter physics communities due to interest in the electronic and magnetic properties and ordering behavior of the phase. One such study, undertaken on synthetic material of end-member composition, reports X-ray diffraction structure refinements that indicate a phase transition from Pbam to Pbnm at or just below ~283 K, determined subsequently to arise from a Peierls-like instability. To compare the stability of the natural phase with that of synthetic material, we performed high-precision X-ray crystal-structure analyses at 295, 100, and 90 K (R1 = 0.0119, 0.0186, and 0.0183, respectively), Mössbauer spectroscopy at 295, 220, 150, 80, and 4.2 K, and wavelength-dispersive electron microprobe analysis on a vonsenite of near-end-member composition from Jayville, New York, U.S.A. The Pbnm structure is observed at 100 and 90 K, suggesting similar phase stability for the natural and synthetic phases. Comparison of Mössbauer data and X-ray site occupancies between the natural and synthetic phases suggests a reinterpretation of Mössbauer site assignments. We conclude that the Peierls-like instability underlying the reported transition from Pbam to Pbnm in synthetic material also occurs in our specimen of natural near-end-member vonsenite at temperatures between 295 and 100 K.

Carletonmooreite, Ni3Si, a new silicide from the Norton County aubrite meteorite

Abstract Carletonmooreite (IMA 2018-68), Ni3Si, is a new nickel silicide mineral that occurs in metal nodules from the Norton County aubrite meteorite. These nodules are dominated by low-Ni iron (kamacite), with accessory schreibersite, nickelphosphide, perryite, and minor daubréelite, tetrataenite, taenite, and graphite. The chemical composition of the holotype carletonmooreite determined by wavelength-dispersive electron-microprobe analysis is (wt%) Ni 82.8 ± 0.4, Fe 4.92 ± 0.09, and Si 13.08 ± 0.08 (n = 6, total = 100.81) giving an empirical formula of (Ni2.87Fe0.18)Σ3.05Si0.95, with an end-member formula of Ni3Si. Further grains discovered in the specimen after the new mineral submission extend the composition, i.e., (wt%) Ni 81.44 ± 0.82, Fe 5.92 ± 0.93, Cu 0.13 ± 0.02, and Si 13.01 ± 0.1 (n = 11, total = 100.51 ± 0.41), giving an empirical formula (Ni2.83Fe0.22Cu0.004)Σ3.05Si0.95. The backscat tered electron-diffraction patterns were indexed by the Pm3m auricupride (AuCu3)-type structure and give a best fit to synthetic Ni3Si, with a = 3.51(1) Å, V = 43.2(4) Å3, Z = 1, and calculated density of 7.89 g/cm3. Carletonmooreite is silver colored with an orange tinge, isotropic, with a metallic luster and occurs as euhedral to subhedral crystals 1 × 5 µm to 5 × 14 µm growing on tetrataenite into kamacite. The dominant silicide in the Norton County aubrite metal nodules is perryite (Ni,Fe)8(Si,P)3, with carletonmooreite restricted to localized growth on rare plessite fields. The isolated nature of small euhedral carletonmooreite single crystals suggests low-temperature growth via solid-state diffusion from the surrounding kamacite and epitaxial growth on the tetrataenite. This new mineral is named in honor of Carleton B. Moore, chemist and geologist, and founding director of the Center for Meteorite Studies at Arizona State University, for his many contributions to cosmochemistry and meteoritics.

Mechanical Properties, Quantum Mechanical Calculations, and Crystallographic/Spectroscopic Characterization of GaNbO4, Ga(Ta,Nb)O4, and GaTaO4.

Single crystals as well as polycrystalline samples of GaNbO4, Ga(Ta,Nb)O4, and GaTaO4 were grown from the melt and by solid-state reactions, respectively, at various temperatures between 1698 and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive electron microprobe analysis, and the crystal structures were determined by single-crystal X-ray diffraction. In addition, a high-P-T synthesis of GaNbO4 was performed at a pressure of 2 GPa and a temperature of 1273 K. Raman spectroscopy of all compounds as well as Rietveld refinement analysis of the powder X-ray diffraction pattern of GaNbO4 were carried out to complement the structural investigations. Density functional theory (DFT) calculations enabled the assignment of the Raman bands to specific vibrational modes within the structure of GaNbO4. To determine the hardness (H) and elastic moduli (E) of the compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load-displacement curves resulted in a high hardness of H = 11.9 ± 0.6 GPa and a reduced elastic modulus of Er = 202 ± 9 GPa for GaTaO4. GaNbO4 showed a lower hardness of H = 9.6 ± 0.5 GPa and a reduced elastic modulus of Er = 168 ± 5 GPa. Spectroscopic ellipsometry of the polished GaTa0.5Nb0.5O4 ceramic sample was employed for the determination of the optical constants n and k. GaTa0.5Nb0.5O4 exhibits a high average refractive index of nD = 2.20, at λ = 589 nm. Furthermore, in situ high-temperature powder X-ray diffraction experiments enabled the study of the thermal expansion tensors of GaTaO4 and GaNbO4, as well as the ability to relate them with structural features.

Assessing dorsal scute microchemistry for reconstruction of shortnose sturgeon life histories

The imperiled status of sturgeons worldwide places priority on the identification and protection of critical habitats. We assessed the micro-structural and micro-chemical scope for a novel calcified structure, dorsal scutes, to be used for reconstruction of past habitat use and group separation in shortnose sturgeon (Acipenser brevirostrum). Dorsal scutes contained a dual-layered structure composed of a thin multi-layered translucent zone lying dorsally above a thicker multi-layered zone. Banding in the thick multi-layered zone correlated strongly with pectoral fin spine annuli supporting the presence of chronological structuring that could contain a chemical record of past environmental exposure. Trace element profiles (Sr:Ca), collected using both wavelength dispersive electron microprobe analysis and laser ablation inductively coupled mass spectrometry, suggest scutes record elemental information useful for tracing transitions between freshwater and marine environments. Moreover, mirror-image like Sr:Ca profiles were observed across the dual-zone structuring of the scute that may indicate duplication of the microchemical profile in a single structure. Additional element:calcium ratios measured in natal regions of dorsal scutes (Ba:Ca, Mg:Ca) suggest the potential for further refinement of techniques for identification of river systems of natal origin. In combination, our results provide proof of concept that dorsal scutes possess the necessary properties to be used as structures for reconstructions of past habitat use in sturgeons. Importantly, scutes may be collected non-lethally and with less injury than current structures, like otoliths and fin spines, affording an opportunity for broader application of microchemical techniques.

Comparative study of three sodium phosphates as corrosion inhibitors for steel reinforcements

A comparative study was performed on the inhibition of steel reinforcement corrosion using three soluble phosphates: sodium monofluorophosphate (Na2PO3F), disodium hydrogen phosphate (Na2HPO4) and trisodium phosphate (Na3PO4). Tests were carried out using ordinary Portland cement (OPC) paste specimens and OPC mortar specimens with embedded steel reinforcements. The corrosion inhibitors were deployed in two different ways: by immersion of OPC specimens in aqueous solutions containing the soluble phosphates (migrating corrosion inhibitor), and by addition of the phosphate powders to a fresh OPC paste (admixture corrosion inhibitor). After curing, the tested specimens were studied using X-ray diffraction, wavelength-dispersive electron microprobe analysis, linear polarisation resistance and electrochemical corrosion potential. A correlation was found between the phosphate content (by migration or admixture) in the OPC matrices and the steel corrosion rate.

M2B5 or M2B4? A Reinvestigation of the Mo/B and W/B System

AbstractThe literature states different compositions (M/B = 1:2 vs. 2:5) and structures for diborides of molybdenum and tungsten. Using X‐ray and neutron powder diffraction as well as energy and wavelength dispersive electron microprobe analysis, the Mo/B and W/B systems were now reinvestigated. Molybdenum diboride crystallizes as a stoichiometric compound Mo2B4 (formerly described as Mo2B5) in space group $R{\bar 3}m$ (No. 166, a, b = 3.01375(2) Å, c = 20.9541(3) Å), and as a non‐stoichiometric compound MoB2−x (formerly described as MoB2) in P6/mmm (No. 191, a, b = 3.043(2) Å, c = 3.067(2) Å), whereas stoichiometric tungsten diboride W2B4 (formerly described as W2B5) is found to crystallize in space group P63/mmc (No. 194, a, b = 2.9864(4) Å, c = 13.896(2) Å). These results seem to be supported by DFT calculations which show the instability of a hypothetic W2B5.

Laser microprobe (U–Th)/He geochronology

A new analytical method had been developed to enable high-spatial-resolution (U–Th)/He dating of accessory minerals. It involves the use of a focused ArF excimer to ablate pits in a polished grain surface, with the evolved gases spiked for isotope-dilution measurement of radiogenic 4He. These data are converted to concentrations by precise measurement of each pit using an optical interferometric microscope. U, Th, and Sm concentration measurements are made using one of several alternative microanalytical techniques (e.g., wavelength-dispersive electron microprobe analysis or laser-ablation, inductively coupled plasma mass spectrometry). By way of illustration, we present both conventional and laser microprobe (U–Th)/He dating results for a Brazilian monazite sample. Laser microprobe data (28 measurements on two crystal fragments) yield a weighted mean (U–Th)/He date of 455.3 ± 3.7 Ma (2SE). This result is statistically indistinguishable from the mean conventional (U–Th)/He date for three separate grain fragments: 449.6 ± 9.8 Ma (2SE). The agreement of conventional and laser ablation dates should encourage a wide variety of applications of the technique, including: (1) detrital mineral dating for provenance and unroofing studies; (2) the dating of broken, included, highly zoned, or irregular grains which are not easily corrected for α-ejection; and (3) measuring 4He loss profiles that can be inverted to determine cooling histories.

Model independent phase equilibrium constraints on the ferrosilite activity in the binary Fe-Mg orthopyroxene solid solution

Compositional reversals of orthopyroxene solid solution (Opx) on the enstatite (En, MgSiO 3 )-ferrosilite (Fs, FeSiO 3 ) join equilibrated with hematite (Hm, Fe 2 O 3 ), magnetite (Mt, Fe 3 O 4 ), and quartz (Qz) were obtained in piston-cylinder apparatus in the pressure range 10–25 kbar at temperatures of 800 and 950 °C. A double-capsule technique with the Hm-Mt external buffer was employed to ensure the presence of both Fe oxides in the run products. Starting materials were crystalline mixtures of synthetic Opx with X Fs [=Fe/(Fe+Mg)] = 0.05 and 0.45, reagent Mt, Hm, and natural Qz. Run products were carefully examined using optical microscopy, back-scattered (BSE) imaging, and wavelength dispersive electron microprobe analysis. Despite the differences in the rates of forward and backward reactions Fs (in Opx) + Hm = Mt + Qz (1), very tight compositional brackets (less than 0.01 X Fs ) were obtained at each P - T . At 800 °C, midpoints of the compositional brackets are: 0.195/10 kbar, 0.24/15, 0.29/20, and 0.34/25; at 950 °C: 0.21/15 kbar, 0.25/20, and 0.295/25 kbar. Activities of Fs, recalculated from these compositions using either Berman and Aranovich (1996) or Holland and Powell (1998) data sets for the pure end-member reaction (1), show negative deviation from ideal mixing. Strongly temperature dependent enthalpy of mixing is required to make these activity values consistent with the configurational entropy of the solid solution deduced from the site occupancy measurements. A simple mixing model with a linearly varying temperature-dependent W term, similar to that of Berman and Aranovich (1996), adequately reproduces the experimental data.

Optimization of Wavelength Dispersive X-Ray Spectrometry Analysis Conditions.

In setting up the conditions for quantitative wavelength-dispersive electron microprobe analysis a number of parameters have to be defined for each element, namely accelerating voltage, beam current, and (for each element) x-ray line, spectrometer crystal, pulse-height analyser settings, background offsets, and counting times for peak and background. The choices made affect both the reliability of the results and the time taken to obtain a complete analysis. It is difficult for even an experienced user to arrive at the optimum set of conditions for any particular application, in view of the large number of interacting factors involved. Furthermore, optimum choices of some parameters are dependent not only on the concentration of the element concerned (for example, counting times) but also the concentrations of other elements which may have peaks that interfere with peak and/or background measurements, requiring alternative selections of x-ray line or spectrometer crystal. The various factors involved in arriving at an optimum routine and practical possibilities for computer-aided optimization are discussed here.

Authigenic Apatite in a Fluvial Sandstone Sequence: Evidence for Rare-Earth Element Mobility During Diagenesis and a Tool for Diagenetic Correlation

ABSTRACT Apatite grains in heavy-mineral separates of samples from the Statfjord Formation (Triassic-Lower Jurassic, North Sea), exhibit variably corroded detrital cores and euhedral diagenetic overgrowths. In the overlying marine deposits of the Nansen Formation, apatite grains show no evidence of either corrosion or overgrowths. Compositions of detrital apatite in both formations are indistinguishable, indicating that the source areas responsible for Statfjord and Nansen deposition included similar apatite-bearing lithologies. Apatite corrosion in the Statfjord Formation is considered to be a response to leaching by acidic groundwaters during sediment storage on the fluvial floodplain. Any variations in the degree of corrosion that may have been generated by differences in floodplain residence time have been homogenized by sediment reworking. Laser-ablation microprobe inductively coupled plasma mass spectrometry and wavelength-dispersive electron microprobe analysis indicate that the authigenic apatite contains high abundances (up to 3.5% total) of the rare earth elements (REE), implying a degree of REE mobilization during diagenesis. REE profiles (NASC normalized) for the authigenic apatites are convex-upwards with enrichment of the MREE. The REE are considered to have been derived from dissolution of other REE-bearing heavy minerals (e.g., garnet) during alluvial storage, with possible contributions from dissolved feldspars (which may account for slight positive Eu anomalies) and REEs adsorbed onto clay surfaces. REE mobilization was probably enhanced by fluoride and/or phosphate complexes, although organic complexing related to pedogenic processes is also a possibility. Through the Statfjord Formation, authigenic apatite displays subtle variations in major-element, minor-element, and trace-element chemistry, which may find application as a tool for diagenetic correlation.

High-speed X-ray analysis

The profile-fitting method was used to analyze data, collected at speeds up to 1° (2θ) per second with a computer-controlled diffractometer, which gave precise values of intensities and angles of reflections above a selected intensity threshold level. Powder diffraction data with 52 reflections in a 40° range were collected and analyzed in a few minutes. This technique has also been applied to X-ray fluorescence spectroscopy, and is applicable to single-crystal diffractometry and wavelength-dispersive electron microprobe analysis.