Molybdenite Crystals Research Articles

Using pectin to separate talc and molybdenite through the surface modification with Cu2+: Applications and mechanism

Organic depressants have low selectivity in separating molybdenite and talc because their metal sites lack activity for organics chemisorption. In this study, surface modification by copper sulfate was used to induce the differential adsorption of pectin onto molybdenite and talc surfaces for enhanced flotation separation. Contact-angle experiments, scanning electron microscopy, adsorption measurements, time-of-flight secondary-ion mass spectrometry, and X-ray photoelectron spectroscopy analyses were conducted to reveal the interaction mechanism. Results illustrated that molybdenite and talc could not be separated using pectin alone, while molybdenite was selectively depressed after surface modification by copper sulfate and this effect was strengthened under alkaline conditions. Metal sites (Mg, Si and Mo) of talc and molybdenite themselves were unable to react with pectin, whereas Cu+ would deposit and further function as active site for pectin chemisorption after surface modification. However, the quantity of deposited Cu sites dropped on talc surface and increased on molybdenite surface with increased pH, and the Mo atoms of molybdenite crystal were activated to take part in pectin chemisorption. Therefore, more pectin was adhered on molybdenite surface, which imparted molybdenite stronger wettability. Herein, surface-modification through metal ions can enable the differential adsorption of organic depressants and enhance the flotation separation of minerals.

Scroll-like and platy molybdenite-3R from the Ufaley metamorphic block, South Urals: EBSD, XRD, SEM, EPMA and ICP-MS study

AbstractScroll-like crystals of molybdenite, 2–5 mm in size, were found in phengite rock from the outer contact of the granular quartz vein of the Kyshtym quartz deposit. Platy and partly scrolled molybdenite occur in the same phengite rock from the outer contact of the quartz–feldspar pegmatite of the Slyudyanogorsk mica deposit. Both occurrences are located in the Ufaley metamorphic block in the South Urals. Scroll-like molybdenite crystals can associate with platy and partly twisted crystals in the same samples. The chemical composition of molybdenite was studied by inductively coupled plasma mass spectrometry (ICP-MS) and electron probe microanalysis (EPMA). Polytypes of molybdenite were identified with electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD). Both scroll-like and platy molybdenite crystals are only represented by the 3R polytype, are enriched in Re up to 1 wt.% and contain no other significant impurities. Scroll-like molybdenite is twisted mainly around the crystallographic axis X. Twinning with a rotation of 60 degrees around the Z crystallographic axis is fixed in the plane (ab). The most probable origin of scroll molybdenites is the consequent growth of molybdenite around nucleation centres, which are commonly represented by mica crystals. The formation of the 3R polytype is caused by the difference in dimension of the layers enriched and depleted in rhenium.

Periodic and non-periodic stacking in molybdenite (MoS2) revealed by STEM

Abstract Polytypism is a typical feature of layered minerals with differences only in stacking sequences. There is no obvious “phase” boundary among different polytypes, although the frequency of polytypes occurrence is related to its crystallization environment. In the past decades, X-ray studies of molybdenite specimens from a variety of geological environments have revealed that most molybdenite crystals contain both 2H1 (hexagonal) and 3R (rhombohedral) polytypes. However, the stacking sequences of these molybdenite polytypic intergrowths and their formation mechanism are not well understood. Here, we report stacking faults and domains of long-period polytypes identified by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in a molybdenite sample from a carbonatite vein in the Huanglongpu Mo-Pb ore deposit in Qinling orogenic belt, Northern China. Several layers of disordered domains intergrown with ordered 2H1 domain were recognized based on the contrast in HAADF image with one-dimensional lattice fringes. In addition, a 30-layer long-period polytype was unambiguously identified by a STEM image. The stacking sequences of 4-, 6-, and 8-layer disordered domains and the 30-layer long-period polytype were further examined using HRSTEM images at the atomic resolution. A 2H3 polytype with three repetitions was also discovered in the sample. We propose that non-equilibrium conditions related to the fluctuation of fluid composition during crystallization resulted in the oscillation of 2H1 and 3R polytypes and intergrowth of various disordered domains. More broadly, our study demonstrates that HAADF-STEM imaging method may be applicable for studying other disordered layered crystals and twinned minerals.

Variations of paramagnetic defects and dopants in geo-MoS2 from diverse localities probed by ESR.

Exfoliated flakes from molybdenite crystals often still serve as benchmark substrates for two-dimensional MoS2 fundamental and device-oriented research. In this article, results are reported of a multi-frequency electron paramagnetic resonance (EPR) study on a series of natural 2H MoS2 crystals taken from various (seven) geological sites with the intent to explore the variations in quality and properties in terms of occurring paramagnetic point defects, with particular focus on the assessment of the predominant type of impurity dopant. The sample set covers three types of overall doping regimes, i.e., p-type, n-type, and mixed (n-type and p-type parts in one sample). The doping type appears primarily governed by substitutional impurities as evidenced by the observed As and N acceptor (both substituting for S) and Re donor (substituting for Mo) signals. For all p-type specimens, doping is found to be ruled by As where, however, a strong variation is revealed in doping uniformity, which appears not directly correlated with the As dopant density. Without specific precautions taken, surface contamination related EPR signals are observed in virtually all As-excavated geo-MoS2 specimens. While several of these signals are of unassigned origin, two prominent ones are identified, one as concerning oxo-Mo5+ compounds and the other Mn2+ centers. The geo-MoS2 sample with the foremost n-type doping shows, besides the prime Re donor EPR signal, an intense powder-pattern signal, tentatively typified by g∥ = 2.076, g⊥ = 2.253, which is suggested to originate from intercalation-related defects. The results bear out the necessity of rigorous surface cleaning, even including invasive removal of surface layers, to obtain pristine MoS2 parent crystals suitable for enabling exfoliation of high quality flakes.

Understanding the microscale spatial distribution and mineralogical residency of Re in pyrite: Examples from carbonate-hosted Zn-Pb ores and implications for pyrite Re-Os geochronology

Accurate and precise geochronology using the Re-Os isotopic system in pyrite is an invaluable tool for developing and confirming genetic models of ore systems. However, as a bulk method, the results produced by pyrite Re-Os geochronology are commonly complex, and many imprecise isochrons exist in the literature. Using LA-ICPMS methods it is now possible to map and quantify Re distribution at the ppb level, allowing an unprecedented look into the Re-Os systematics of pyrite-bearing ore.Two samples from the Lisheen Zn-Pb ore deposit in Ireland showing disparate Re-Os isotopic behavior were investigated. In-situ sulfur isotope measurements using SIMS, an analytical technique not previously attempted on the Irish deposits, was used to supplement the Re-Os dataset. A massive pyrite sample from the Main Zone produced a precise, low-scatter isochron (346.6 ± 3.0 Ma, MSWD = 1.6). The Re distribution in this sample is relatively homogeneous, with the Re budget dominated by pyrite containing 1–5 ppb Re, but the δ34S varies significantly from −45.2‰ to 8.2‰. A second, more paragenetically complex, sample from the Derryville Zone produced a younger age with high scatter (322 ± 11 Ma, MSWD = 206) and this also displays a large variation in δ34S (−53‰ to +4‰). The cores of grains of main-stage iron sulfide are depleted in trace elements and show low Re abundances (<10 ppb) but have been altered in an irregular fashion leading to Re-enriched domains that exceed 100 ppb. Additionally, micron-scale molybdenite crystals, found in close association with altered sulfides, contain Re at levels that locally exceed 10 ppm. The highly scattered (MSWD = 206) and younger age (322 Ma), produced by the Derryville Zone sample are interpreted to result from mixing of different generations of sulfide, potentially involving fluids associated with Variscan deformation (<310 Ma). Therefore, the Re-Os data produced from the Derryville Zone sample does not reflect the timing of iron sulfide mineralization, even though a relatively precise age was obtained.A second Re-Os dataset from Zn-Pb mineralization at Hawker Creek, Nunavut, Canada was produced from massive pyrite that displays low Re concentrations (<1 ppb). However, on grain boundaries and in fractures, silicate-rich material contains Re at levels that can locally exceed 500 ppb. Analyses of fracture-free pyrite produced by bulk separation using magnetic separation yielded the oldest model age (1083 Ma), whereas mineral separates containing the highest fracture density produced the youngest age (413 Ma).In general, therefore, the complexities of pyrite Re-Os geochronology can result from impurities in mineral separates. Attempts to eliminate impurities through different mineral separation techniques (e.g. crushing, heavy liquid separation, magnetic separation, acid leaching) are frequently only partially successful and therefore full characterization of any resulting mineral separates is extremely important. We conclude that LA-ICPMS mapping of Re and Mo distributions is essential for the identification of such impurities. Although other trace element LA-ICPMS maps, in-situ sulfur isotope measurements, and petrographic evidence were of limited use in assessing the Re budget of a sample, they are invaluable in linking the documented Re distribution obtained through LA-ICPMS to Re-Os geochronological results.

Thermoelectric Properties of n-Type Molybdenum Disulfide (MoS2) Thin Film by Using a Simple Measurement Method.

In this paper, a micrometre thin film of molybdenum disulfide (MoS2) is characterized for thermoelectric properties. The sample was prepared through mechanical exfoliation of a molybdenite crystal. The Seebeck coefficient measurement was performed by generating a temperature gradient across the sample and recording the induced electrical voltage, and for this purpose a simple measurement setup was developed. In the measurement, platinum was utilized as reference material in the electrodes. The Seebeck value of MoS2 was estimated to be approximately −600 µV/K at a temperature difference of 40 °C. The negative sign indicates that the polarity of the material is n-type. For measurement of the thermal conductivity, the sample was sandwiched between the heat source and the heat sink, and a steady-state power of 1.42 W was provided while monitoring the temperature difference across the sample. Based on Fourier’s law of conduction, the thermal conductivity of the sample was estimated to be approximately 0.26 Wm−1 K−. The electrical resistivity was estimated to be 29 Ω cm. The figure of merit of MoS2 was estimated to be 1.99 × 10−4.

First in situ Re-Os dating of molybdenite by LA-ICP-MS/MS

Conventional dating of molybdenite (187Re-187Os) provides one of few options for direct dating of sulfide mineralization. Unfortunately, in situ dating of molybdenite is considered unreliable due to intra-granular decoupling of 187Re-187Os. In this study, we developed a new analytical protocol for studies of micron- to grain scale 187Re-187Os systematics in molybdenite. Online chemical separation using ICP-MS/MS technology enables in situ dating by β-decay systems (e.g., Rb-Sr and K-Ca in micas) using laser ablation. Here, the methodology is extended to the 187Re-187Os system, another β-decay system that cannot be resolved by mass spectrometry. Several reaction gases were evaluated, and production of OsCH2 by reaction with CH4 was found to produce strong separation of Os from Re. However, in contrast to the e.g., 87Rb-87Sr system, 1–2% of the parent isotope Re also reacted to ReCH2, leaving a significant interference. A mathematical correction of this remaining interference is possible, and 187Re-187Os (mass-shifted) can be measured accurately even for fairly extreme ratios. For laser ablation, standards were developed by pressing particulate pellets of conventionally dated molybdenite (Moly Hill and Merlin), because there are no appropriate reference materials available. Six natural molybdenite samples from a range of geological settings, containing > 10 ppm Re, were analyzed by 70 μm laser ablation spots, and ages were calibrated by analysis of molybdenite pellets. Contrary to our expectation, weighted average ages obtained were in agreement (within 1%) with conventional age determinations, with fairly good precision (from ~ 1 to 5% 2σ depending on Re concentration), suggesting limited or essentially nonexistent decoupling within crystals. Two important implications of this result are that decoupling Re-Os is not universal, and that our new analytical protocol is useful both for dating and for studies of decoupling. The benefit of in situ dating compared to conventional dating is, apart from lower cost and time consumption, the possibility of targeting smaller molybdenite crystals (≥ 100 μm) in thin sections and epoxy mounts. The youngest sample in the study is 920 Ma, but we see potential of dating significantly younger Re-rich molybdenite.

Strain transfer across grain boundaries in MoS2 monolayers grown by chemical vapor deposition

Monolayers of transition metal dichalcogenides (TMDC) mechanically exfoliated from bulk crystals have exceptional mechanical and optical properties. They are extremely flexible, sustaining mechanical strain of about 10% without breaking. Their optical properties dramatically change with applied strain. However, the fabrication of a large number of mechanical devices is tedious due to the micromechanical exfoliation process. Alternatively, monolayers can be grown by chemical vapor deposition (CVD) on the wafer scale, with the drawback of cracks and grain boundaries in the material. Therefore, it is important to investigate the mechanical properties of CVD-grown material and its potential as a material for mass production of nanomechanical devices. Here, we measure the optical absorption of CVD-grown MoS2 monolayers with applied uniaxial tensile strain. We derive a strain-dependent shift for the A exciton of −42 meV/%. This value is identical to MoS2 monolayers, which are mechanically exfoliated from natural molybdenite crystals. Using angle-resolved second-harmonic generation spectroscopy, we find that the applied uniaxial tensile strain is fully transferred across grain boundaries of the CVD-grown monolayer. Our work demonstrates that large-area artificially grown MoS2 monolayers are promising for mass-produced nanomechanical devices.

A double Fe-Ti oxide and Fe-sulphide liquid immiscibility in the Itsindro Gabbro Complex, Madagascar

The petrology and mineralogy of the Itsindro complex in south-central Madagascar has been investigated through samples obtained from the 320.7 m-deep Lanjanina borehole. The section consists of a 254 m-thick pyroxenite unit with interbedded gabbro layers that overlies a gabbro unit and is itself overlain by a 19 m-thick granite unit. Most of the structures are sub-horizontal. A weak magmatic layering is locally observed but at the scale of the core, the intrusion does not appear to be a layered complex.Pyroxenite and gabbro show a systematic disseminated mineralization consisting of Fe-Ti-P oxides and Fe-(Cu-Ni) sulphides that takes the form of ilmenite-titanomagnetite ± apatite and pyrrhotite ± chalcopyrite ± pentlandite. In the upper zone, from 90 to 72 m, sub-massive centimetre-to decimetre-sized layers of oxides and sulphides comprise a total of 16 m of sub-massive sulphide (the main mineralized zone). In this mineralized zone the oxide/sulphide ratio is close to 1/1. The sulphide is strongly dominated by pyrrhotite, which may locally contain inclusions of molybdenite crystals with the Re sulphide rheniite (ReS2). Oxides are generally euhedral, included in or attached to the Fe-sulphide, and also locally form sub-massive centimetre-sized bands.Apatite as a cumulus phase is ubiquitous. Locally it may account for 30% of the ore-rich samples and some samples consist of apatite-Fe-Ti oxides-Fe-Cu-Ni sulphides with virtually no silicate. Apatite is the main REE carrier but the total REE content remains low (<90 ppm).Mineral compositions and whole rock geochemistry indicate that the rocks are highly differentiated, and in spite of a relatively limited thickness, the differentiation process is observed. Two zones can be distinguished: from the bottom to 162.8 m we see a decrease in the Mg number of olivine and pyroxene, and a drop in TiO2 and Al2O3 for the latter. A reverse trend is then observed within the pyroxenite unit from the 162.8 m level upwards. The main mineralized zone is located in the upper part of this unit, about 70 m above this discontinuity.The highly differentiated Fe-Ti-P facies of the Lanjanina series in the Itsindro Gabbro Complex have been interpreted as corresponding to the Fe-Ti-P rich, Si-poor member remaining after an immiscible segregation of an evolved mafic magma. The granite dykes and the overlying granite unit represent the second, Si-rich member of the immiscibility process.The presence of large amounts of sulphide is attributed to sulphur contamination of the Fe-Ti-rich liquid. Fe-Ti oxides will tend to crystallize on the sulphide droplets and the accumulation of dense Fe-sulphides (liquid) and associated Fe-Ti oxides (solid) will result in this complex and unusual association taking the form of a net texture.

Electrochemical maps and movies of the hydrogen evolution reaction on natural crystals of molybdenite (MoS2): basal vs. edge plane activity.

Two dimensional (2D) semiconductor materials, such as molybdenum disulfide (MoS2) have attracted considerable interest in a range of chemical and electrochemical applications, for example, as an abundant and low-cost alternative electrocatalyst to platinum for the hydrogen evolution reaction (HER). While it has been proposed that the edge plane of MoS2 possesses high catalytic activity for the HER relative to the "catalytically inert" basal plane, this conclusion has been drawn mainly from macroscale electrochemical (voltammetric) measurements, which reflect the "average" electrocatalytic behavior of complex electrode ensembles. In this work, we report the first spatially-resolved measurements of HER activity on natural crystals of molybdenite, achieved using voltammetric scanning electrochemical cell microscopy (SECCM), whereby pixel-resolved linear-sweep voltammogram (LSV) measurements have allowed the HER to be visualized at multiple different potentials to construct electrochemical flux movies with nanoscale resolution. Key features of the SECCM technique are that characteristic surface sites can be targeted and analyzed in detail and, further, that the electrocatalyst area is known with good precision (in contrast to many macroscale measurements on supported catalysts). Through correlation of the local voltammetric response with information from scanning electron microscopy (SEM) and atomic force microscopy (AFM) in a multi-microscopy approach, it is demonstrated unequivocally that while the basal plane of bulk MoS2 (2H crystal phase) possesses significant activity, the HER is greatly facilitated at the edge plane (e.g., surface defects such as steps, edges or crevices). Semi-quantitative treatment of the voltammetric data reveals that the HER at the basal plane of MoS2 has a Tafel slope and exchange current density (J0) of ∼120 mV per decade and 2.5 × 10-6 A cm-2 (comparable to polycrystalline Co, Ni, Cu and Au), respectively, while the edge plane has a comparable Tafel slope and a J0 that is estimated to be more than an order-of-magnitude larger (∼1 × 10-4 A cm-2). Finally, by tracking the temporal evolution of water contact angle (WCA) after cleavage, it is shown that cathodic polarization has a 'self-cleaning' effect on the surface of MoS2, consistent with the time-independent (i.e., time after cleavage) HER voltammetric response.

Raman Signatures of Polytypism in Molybdenum Disulfide.

Since the stacking order sensitively affects various physical properties of layered materials, accurate determination of the stacking order is important for studying the basic properties of these materials as well as for device applications. Because 2H-molybdenum disulfide (MoS2) is most common in nature, most studies so far have focused on 2H-MoS2. However, we found that the 2H, 3R, and mixed stacking sequences exist in few-layer MoS2 exfoliated from natural molybdenite crystals. The crystal structures are confirmed by HR-TEM measurements. The Raman signatures of different polytypes are investigated by using three different excitation energies that are nonresonant and resonant with A and C excitons, respectively. The low-frequency breathing and shear modes show distinct differences for each polytype, whereas the high-frequency intralayer modes show little difference. For resonant excitations at 1.96 and 2.81 eV, distinct features are observed that enable determination of the stacking order.

Geology, geochemistry and tectonic setting of the Indosinian Mo deposits in southern Great Hinggan Range, NE China

The southern Great Hinggan Range in northeast China, mainly bounded by the Kangbao–Chifeng Fault to the south and the Hegenshan–Heihe Suture to the north, is a Palaeozoic subduction‐related accretionary belt added to the northern margin of the North China Craton. The area has been considered as a Yanshanian metallogenic province, with more than 20 Mo deposits being discovered. Recently, six of the Mo deposits were proven to have formed during the Indosinian (Triassic) Orogeny, indicating the significance of the Indosinian mineralization, orogeny and magmatism. In this contribution, we summarize available data of the Indosinian Mo deposits, and show that Triassic Mo mineralization occurred in the time range of 248–232 Ma, coincident with a regional granitic event, and was presented as porphyry and intrusion‐related quartz vein systems. The Indosinian ore‐related granitoids show geochemical features of high‐K calc‐alkaline metaluminous–peraluminous and adakitic rocks, and have been formed by partial melting of thickened lower crust in a post‐subduction tectonic setting. Rhenium contents of molybdenite crystals and sulphur–lead isotopic compositions indicate that the ore‐forming materials originated from continental crust with some mantle components involved. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Magmatic–hydrothermal molybdenum isotope fractionation and its relevance to the igneous crustal signature

We analysed the Mo isotope composition of a comprehensive series of molybdenite samples from the porphyry-type Questa deposit (NM, USA), as well as one rhyolite and one granite sample, directly associated with the Mo mineralization. The δ98Mo of the molybdenites ranges between −0.48‰ and +0.40‰, with a median at −0.05‰. The median Mo isotope composition increases from early magmatic (−0.29‰) to hydrothermal (−0.05‰) breccia mineralization (median bulk breccia=−0.17‰) to late stockwork veining (+0.22‰). Moreover, variations of up to 0.34‰ are found between different molybdenite crystals within an individual hand specimen. The rhyolite sample with 0.12μgg−1 Mo has δ98Mo=−0.57‰ and is lighter than all molybdenites from the Questa deposit, interpreted to represent the igneous leftover after aqueous ore fluid exsolution. We recognize three Mo isotope fractionation processes that occur between about 700 and 350°C, affecting the Mo isotope composition of magmatic–hydrothermal molybdenites. ∆1Mo: Minerals preferentially incorporate light Mo isotopes during progressive fractional crystallization in subvolcanic magma reservoirs, leaving behind a melt enriched in heavy Mo isotopes. ∆2Mo: Magmatic–hydrothermal fluids preferentially incorporate heavy Mo isotopes upon fluid exsolution. ∆3Mo: Light Mo isotopes get preferentially incorporated in molybdenite during crystallization from an aqueous fluid, leaving behind a hydrothermal fluid that gets heavier with progressive molybdenite crystallization. The sum of all three fractionation processes produces molybdenites that record heavier δ98Mo compositions than their source magmas. This implies that the mean δ98Mo of molybdenites published so far (~0.4‰) likely represents a maximum value for the Mo isotope composition of Phanerozoic igneous upper crust.

RHENIUM-RICH MOLYBDENITE AND RHENIITE IN THE PAGONI RACHI Mo-Cu-Te-Ag-Au PROSPECT, NORTHERN GREECE: IMPLICATIONS FOR THE Re GEOCHEMISTRY OF PORPHYRY-STYLE Cu-Mo AND Mo MINERALIZATION

The porphyry-type Pagoni Rachi Mo–Cu–Te–Ag–Au prospect, in northern Greece, is a porphyry-epithermal system hosted by an Oligocene dacite porphyry and quartz–feldspar porphyry dikes. The rare mineral rheniite (ReS2) and molybdenite, with very high contents of Re (up to 4.7 wt% Re), occur in quartz veins along with Fe–Cu sulfides, Pb-, Sn-, and Cl-bearing oxides, hematite, ilmenite and tellurides of Bi; these veins are spatially related to sericitic and transitional sericitic–sodic–potassic alteration. Earlier-formed gold-bearing quartz and magnetite veins with sodic–potassic–calcic alteration and late precious-metal telluride-rich carbonate–quartz veins with argillic alteration contain only minor amounts of molybdenite. The composition of rheniite ranges from almost pure, stoichiometric rheniite with low Mo content to rheniite with up to 5.99 wt% Mo. Petrographic, scanning electron microscope, and structural studies suggest that the high Re content of molybdenite from Pagoni Rachi is the result of the isovalent Re-for-Mo substitution in molybdenite. This fact is corroborated by the progressive shortening of the Mo–S mean bond-distance (from 2.414 A in pure molybdenite to 2.355 A), as well as by the isotropic decrease of the unit-cell values with the increase of the Re:Mo ratio. The structural analysis of four molybdenite crystals, with the highest Re content ever reported in nature, demonstrates that they crystallize as the 2 H polytype and not the 3 R polytype, as previously hypothesized, thus suggesting that Re concentration does not correlate with a specific polytype. The fluid inclusions in quartz in the Re-bearing molybdenite–rheniite veins at Pagoni Rachi show that they homogenize to either the liquid or vapor phases (354 to 428°C) or by halite dissolution at 317 to 585°C, which equates to salinities of 40 to 59 wt% NaCl equiv. Rheniite and molybdenite likely precipitated as temperatures fell below 400°C during phase separation under relatively oxidizing conditions, at elevated chlorine activity, and from relatively acid hydrothermal solutions. However, the presence of Pb oxides, Sn-bearing minerals and tellurides is compelling evidence that rheniite and Re-rich molybdenite may have formed directly from the vapor as sublimates, in a manner similar to the way they are deposited at the Kudriavy volcano, Kurile Islands.

Reply to “Accurate and precise Re-Os molybdenite dates from the Galway Granite, Ireland” by D. Selby et al.: Critical comment on “Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation”

of our previous papers (Suzuki et al., 2000, 2001) was focussed on improving our understanding of the behaviour of Re and Os. Therefore, we sampled a part of molybdenite crystal(s) and analyzed each part. This allowed us to examine the micro-scale movement of Re and/ or 187Os and was similar to the later LA-ICP-MS spot dating studies (Kosler et al., 2000; Stein et al., 2003) in concept. Irreproducible Re-Os dates would be expected using such micro-sampling techniques in Cases (b) and (c). Consequently, the sampling methodology promoted by Stein and co-workers and Selby and co-workers would have been inappropriate as bulk sampling averages out any variations present on a scale smaller than the bulk sample and it was precisely these variations we wished to study. Thus, I dispute that our sample preparation technique was “inadequate”, for our purpose. I would argue that Re-Os system disturbance in molybdenite has been shown to have occurred in certain environments. McCandless et al. (1993) found that certain molybdenites experienced Re loss during hydrothermal alteration in hypogene and supergene environments and that Re was enriched in K-Al-silicate intergrowths. They also proposed that Re may be enriched in powellite, an alteration product of molybdenite, in the weathering environment. On the other hand, we (Suzuki et al., 2000) experimentally showed that the outer parts of molybdenite crystals exposed to 0.1 mol/L NaCl and NaHCO3 solutions produced younger Re-Os dates than that of the original unaltered molybdenite, while the inner parts of the molybdenites yielded dates in agreement with the original within analytical uncertainties. These results indicated that the molybdenite lost Re and/or 187Os during experimental alteration. Also, Suzuki et al. (1994) had previously reported that approximately 5% of Re was found to dissolve into the altering NaCl solution, which also indiI welcome the opportunity to reply to the critical comments of Selby et al., which may invite much concern about the Re-Os isotope system. I hope that we have a fruitful discussion in Geochemical Journal. Prior to opening my reply, I exhibit 4 possible cases of Re and Os behaviour in molybdenite and the resulting apparent Re-Os ages (Fig. 1). Case (a): No movement of either Re or radiogenic 187Os after formation of molybdenite. Case (b): Movement of Re and/or 187Os “within” a molybdenite crystal (decoupling). Case (c): Movement of Re and/or 187Os through the crystal surface, but not through the whole rock system. Case (d): Movement of Re and/or 187Os through the whole rock system. Obviously, the Re-Os system is undisturbed in Case (a) and thus a robust Re-Os age can be obtained. In Case (b), we can obtain a reliable age, although the Re-Os system is disturbed within individual crystals. Each crystal should yield a concordant Re-Os age, because the Re-Os system has remained closed within the crystal. Case (c) presents the more complicated situation where Re and/or 187Os have moved beyond the crystal surface. Since the whole rock system has remained closed, a reliable Re-Os age may still be obtained if complete collection of the molybdenite sample is possible. In Case (d), Re and/or Os have moved in an open system and a robust Re-Os age is not possible.

Subgrain-scale decoupling of Re and 187Os and assessment of laser ablation ICP-MS spot dating in molybdenite

Reproducibility of Re-Os molybdenite ages depends on sample size and homogeneity, suggesting that Re and Os are decoupled within individual molybdenite crystals and do not remain spatially linked over time. In order to investigate the Re-Os systematics of molybdenite at the subgrain (micron) scale, we report LA-ICP-MS Re-Os ages for an Archean molybdenite crystal from Aittojärvi, Finland, analyzed in situ in a white aplite matrix. A related Aittojärvi molybdenite (A996D), in the form of a very fine-grained mineral separate, is used as one of our in-house NTIMS standards, and thus its age of 2760 ± 9 Ma is well established. Measurements of ( 187Re + 187Os)/ 185Re on micron scale spots along 200 μm traverses across the crystal yield a wide range of ages demonstrating that, in this case, microsampling of molybdenite does not produce geologically meaningful ages. Experimentation with mineral separations and sample size over a 7-yr period predicted that this would be the outcome. We suggest that 187Os is more likely to be the mobile species, based on its charge and ionic radius, and that 187Os becomes decoupled from parent 187Re with time on the micron and larger scale. Incompatible charge and ionic radius for Os ions formed during reduction of molybdenite-forming fluids may explain the widely observed absence of common (initial) Os in molybdenite. Geologically accurate ages for molybdenite can only be obtained for fully homogenized crystals (or crystal aggregates) so that any post-crystallization 187Re- 187Os decoupling is overcome. A growing number of geologically accurate ID-NTIMS 187Re- 187Os ages for homogenized molybdenite suggest that postcrystallization mobility of radiogenic 187Os must be limited to within the molybdenite mineral phase. We suggest that radiogenic 187Os may be stored in micron scale dislocations, kink bands, and delamination cracks produced by deformation, and that the unusual structure and deformation response of molybdenite results in an increased chemical stability in this mineral. Migration of 187Os into adjacent silicate phases is highly unlikely, but other contacting sulfides may take in Os. In an example from a Proterozoic skarn deposit at Pitkäranta (western Russia), we demonstrate minor loss of radiogenic 187Os from molybdenite and a corresponding gain in adjacent chalcopyrite such that the molybdenite age is not perceptibly disturbed, whereas the resulting chalcopyrite ages are impossibly old. Therefore, it is unadvisable to perform Re-Os analytical work on any sulfide in contact or intimate association with molybdenite. In addition to large errors in the age, if the isochron method is employed, initial 187Os/ 188Os ratios could be erroneously high, leading to seriously errant genetic interpretations.

The remarkable Re–Os chronometer in molybdenite: how and why it works

The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS2) is now demonstrated to be remarkably robust, surviving intense deformation and high‐grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of 187Re parent and 187Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID‐NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP‐MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and 40Ar/39Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid‐state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point‐in‐time markers for age comparisons.

A Low F Pegmatite-Related Mo Skarn from the SouthwesternGrenville Province, Ontario, Canada: Phase Equilibria and PetrogeneticImplications

The Hunt Mo skarn deposit is one of the best examples of a late tectonic, granitic pegmatite-related, skarn system in the Grenville province. An Re-Os age of 1069 ± 11 Ma was obtained from a molybdenite crystal (6.4 ppm Re) hosted in proximal skarn (sample 50O) consistent with a contact metasomatic origin related to the granitic body. Although less evolved than other Grenvillian pegmatites, the Condon Lake granitic pegmatite is a low-temperature, A-type intrusion, with moderate redox characteristics. In contrast to many Mo-bearing skarns in the region, this deposit has low U, Th, REE, F, and P contents. The reduced marginal magnesian skarn is well zoned geochemically, mineralogically, and texturally from the contact outward into the dominant graphite-bearing calc-silicate-calcite-dolomite marble, which is locally intercalated with clinopyroxenite, orthoamphibolite, and pyroxene-biotite quartzofeldspathic gneiss. A narrow zone of endoskarn (<1 m; scapolite-K feldspar-Ca clinopyroxene) and wider, zoned exoskarn (<10 m; scapolite-Ca clinopyroxene (proximal), Ca clinopyroxene-phlogopite, Ca clinopyroxene-tremolite-phlogopite, tremolite-phlogopite (distal), marble), which hosts the bulk of the primary molybdenite (± pyrrhotite) and minor secondary pyrite-pyrrhotite-molybdenite veins. In addition to the obvious Si, Fe, Ti, Mn, S, and Mo addition, Cu, Zn, Y, Nb, and Zr are also slightly enriched. The high f (H 2 O) / f (HF) ratios (138,000–204,000) and moderate f (H 2 O) / f (HCl) ratios (2,200–1,000) at the Hunt deposit contrast with the lower fugacity ratios from other Mo-bearing U-Th-REE skarns in the region, indicating that Mo transport and deposition is principally related to a hydroxide complex. The calc-silicate phase equilibria used to describe the zonation sequence have been reinterpreted using quartz-undersaturated (silica) activities; this shifts the calc-silicate-forming reaction boundaries to very low X (CO2 ) (<0.005), consistent with decarbonation reactions induced via dissolved silica infiltration. The modeled fluid/rock in the skarn based on dissolved molybdenum is greater than 20. The carbon and oxygen isotopes of calcite are consistent with infiltrative metasomatic skarns from the pegmatite and a dominantly igneous source for the sulfur ( δ 34 S = 2–9‰). Overall, the deposition of Mo is probably facilitated by a reaction with carbonate (increase in Ca(OH) 2 , CaCl 2 resulting in complex destabilization), a decrease in f O2 triggering an increase H 2 S/SO 2 , S in the host rocks ( f S2 ), and a slight decrease in temperature (650°–500°C).

モリブデナイト結晶中の水素の拡散，脱離の観察

Diffusion and desorption of hydrogen in a bulk of molybdenite crystal are studied by measurement of gas emitted from the bulk crystal during cleavage experiment for observation of gases in a bulk, electron stimulated desorption (ESD) for measurement of hydrogen concentration on the surface and temperature programmed desorption (TPD) for estimating ion of an activation energy of diffusion and desorption process. The cleavage and ESD experiments are performed to observe the change of hydrogen concentration in the bulk and on the surface by the sample heating. These experiments show that a mount of hydrogen in the bulk reduces at about 500°C and that the hydrogen concentration on the surface and thermal desorption of hydrogen from the bulk also begin to increase around 500°C. The increase of hydrogen concentration on the surface suggests that the activation energy for thermal desorption is lager than the diffusion energy in the bulk to (0001) surface. However, in this case this may not be applicable because of similar temperature behavior of surface hydrogen and desorbed hydrogen. The mechanism of the diffusion and the desorption process is discussed. The activation energy for the desorption induced by diffusion of hydrogen in the bulk to (0001) surface is about 76 kJ/mol.

Principle and application of phase-shifting electron holography

Electron holography using an improved phase-shifting (fringe-scanning) interferometric technique is described. The improvements it makes in phase measurement, in spatial resolution of reconstructed images and in hologram formation are clarified and demonstrated with three experiments: (1) quantitative detection of atomic surface steps on molybdenite crystal film, (2) topographic measurement of gold platelets epitaxially grown on a molybdenite substrate, and (3) sensitive observation of a bacterial flagellum filament.