Olivine Lattice Research Articles

A first-principles molecular dynamics study of molecular hydrogen diffusion in Fe-free olivine

Molecular hydrogen (H2) may be an important form of water in nominally anhydrous minerals in the Earth’s mantle and plays a critical role in mantle water cycle, but the transport properties of H2 remain unclear. Here, the diffusion of H2 in Fe-free olivine lattice is investigated at pressures of 1–13 GPa and temperatures of 1300–1900 K by first-principles molecular dynamics. The activation energy and activation volume for H2 diffusion in Fe-free olivine are determined to be 55 ± 8 kJ/mol and 3.6 ± 0.2 cm3/mol, respectively. H2 diffusion in Fe-free olivine is faster than H+ by 1–4 orders of magnitude and therefore it is more favorable for hydrogen transportation under upper mantle conditions. H2 can be carried to the mantle transition zone by subducting slabs without releasing to the surrounding mantle. The upper mantle may act as a lid, preventing the releasing of H2 produced in the deep mantle to the surface.

Atomic-scale Element and Isotopic Investigation of 25Mg-rich Stardust from an H-burning Supernova

We have discovered a presolar olivine from ALH 77307 with the highest 25Mg isotopic composition measured in a silicate to date (δ 25Mg = 3025.1‰ ± 38.3‰). Its isotopic compositions challenge current stellar models, with modeling of magnesium, silicon, and oxygen showing a closest match to formation in a supernova (SN) where hydrogen ingestion occurred in the pre-SN phase. Presolar grains within primitive astromaterials retain records of processes and environmental changes throughout stellar evolution. However, accessing these records has proved challenging due to the average grain size (∼150 nm) of presolar silicates, their sensitivity to extraction agents, and instrumental restrictions, limiting the range of isotopic and chemical signatures which can be studied per grain volume. Here, we present the first known detailed geochemical study of a presolar silicate from a hydrogen-burning SN, studied in 3D without contributions to the analysis volume and at unprecedented spatial resolutions (<1 nm), essential for constraining physical and chemical processes occurring within this recently proposed stellar environment. From our results, we infer either (i) condensation within an environment depleted of heavy elements compatible with the olivine lattice under the pressure and temperature conditions during condensation, or (ii) during periods of limited mixing either near the end of the pre-SN phase or from a collapse so rapid localized pockets of different gas compositions formed.

3D Diffusion of Water in Melt Inclusion‐Bearing Olivine Phenocrysts

AbstractOlivine‐hosted melt inclusions are an important archive of pre‐eruptive processes such as magma storage, mixing and subsequent ascent through the crust. However, this record can be modified by post‐entrapment diffusion of H+ through the olivine lattice. Existing studies often use spherical or 1D models to track melt inclusion dehydration that fail to account for complexities in geometry, diffusive anisotropy and sectioning effects. Here we develop a finite element 3D multiphase diffusion model for the dehydration of olivine‐hosted melt inclusions that includes natural crystal geometries and multiple melt inclusions. We use our 3D model to test the reliability of simplified analytical and numerical models (1D and 2D) using magma ascent conditions from the 1977 eruption of Seguam volcano, Alaska. We find that 1D models underestimate melt inclusion water loss, typically by ∼30%, and thus underestimate magma decompression rates, by up to a factor of 5, when compared to the 3D models. An anisotropic analytical solution that we present performs well and recovers decompression rates within a factor of 2, in the situations in which it is valid. 3D models that include multiple melt inclusions show that inclusions can shield each other and reduce the amount of water loss upon ascent. This shielding effect depends on decompression rate, melt inclusion size, and crystallographic direction. Our modeling approach shows that factors such as 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates and recovering water contents in natural magmatic systems.

Partitioning of Ti, Al, P, and Cr between olivine and a tholeiitic basaltic melt: Insights on olivine zoning patterns and cation substitution reactions under variable cooling rate conditions

The mechanism governing the kinetic growth of olivine in dynamic volcanic settings has been the subject of considerable attention in recent years. Under variable cooling rate (CR) and undercooling (−ΔT) regimes, the textual maturation of olivine proceeds from skeletal/dendritic crystals to polyhedral morphologies by infilling of the crystal framework. Owing to the establishment of a diffusion-controlled growth regime, a sharp diffusive boundary layer develops in the melt next to the advancing olivine surface. In this context, we have quantified the apparent partitioning of Ti, Al, P, and Cr between olivine and a Hawaiian tholeiitic basaltic melt at P = 1 atm, fO2 = QFM-2 buffer, and CR = 4, 20, and 60 °C/h over a constant -ΔT = 85 °C. Differences in charge and/or size between the substituent minor cations and the major species in the olivine crystallographic site dominate the energetics of homovalent and heterovalent cation substitutions. While the entry of Ti in the olivine lattice site accounts for the simple exchange [TSi4+] ↔ [TTi4+], more complex charge-balancing coupled-substitution mechanisms have been determined for the incorporation of Al, P, and Cr, i.e., [MMg2+, TSi4+] ↔ [MAl3+, TAl3+], [2 TSi4+] ↔ [TP5+, TAl3+], and [MMg2+, TSi4+] ↔ [MCr3+, TAl3+], respectively. In order to maintain charge balance, the disequilibrium uptake of minor cations in rapidly growing crystals is controlled by the same substitution mechanisms observed under equilibrium crystallization. This finding is consistent with the achievement of a local interface equilibrium at the olivine-melt interface independently of the diffusive boundary in the melt. A statistical approach based on multivariate analysis of olivine/melt compositional parameters confirms that the control of melt structure on the partitioning of Ti, Al, P, and Cr is almost entirely embodied in the olivine structure and chemistry via charge compensation reactions. Therefore, the magnitude of minor element partition coefficients is weakly dependent on diffusion kinetics in the melt but rather strongly governed by olivine zoning patterns resulting from fast crystal growth rates.

Kinetic partitioning of major-minor cations between olivine and Hawaiian tholeiitic basalt under variable undercooling and cooling rate conditions

In order to elucidate the kinetic partitioning of cations between olivine and basalt, we performed undercooling (−ΔT) and cooling rate (CR) experiments at atmospheric pressure and QFM-2 buffer. Starting from the superliquidus temperature of 1250 °C, a Hawaiian tholeiitic basalt was cooled at the rates of 4 (CR4), 20 (CR20), and 60 (CR60) °C/h to the final target temperatures of 1175 °C (−ΔT = 35 °C; −ΔT35) and 1125 °C (−ΔT = 85 °C; −ΔT85). Results show that polyhedral olivine morphologies are obtained at -ΔT35, whereas strong disequilibrium skeletal and/or dendritic textures form at -ΔT85. The amount of forsterite in olivine decreases from to 85% to 78% with increasing both -ΔT and CR. A diffusive boundary layer also develops in the melt next to the olivine surface and its composition becomes progressively enriched in Ca, owing to its incompatible behavior with the lattice site. Residual melts are progressively depleted in silica and enriched in alkali from CR4 to CR60, but silica-rich melts are observed with increasing -ΔT. In terms of Fe2+-Mg exchange, olivines obtained at -ΔT35 are always in equilibrium with the diffusive boundary layer, comprising both the interface melt next to the olivine surface and the far-field melt where all chemical gradients cease. At -ΔT85, however, the Fe2+-Mg exchange indicates two distinct equilibration stages between olivine core and far-field melt, and between olivine rim and interface melt. Partition coefficients (Kd) of Mg, Fe, Mn, Ca, and Cr calculated at the olivine-melt interface preferentially change as a function of -ΔT rather than CR. From -ΔT35 to -ΔT85, KdMg, KdFe, KdMn, and KdCr remarkably increase, whereas the opposite applies to KdCa. Through the application of equilibrium partitioning models, we found that Mg, Fe, Mn, and Ca are incorporated into the olivine lattice site at near-equilibrium proportions. This generally good agreement with modeling data demonstrates that diffusive mass transport of cations in our experiments occurred under the conditions of local equilibrium at the olivine surface. In contrast, marked deviations from the expected equilibrium are found for KCr in response to the major influence of crystal field stabilization energy on cation incorporation.

Quantitative shock measurement of olivine in ureilite meteorites

AbstractUreilites are the second most abundant achondrite meteorite group, yet they are from an unknown source. They are ultramafic in composition and have undergone thermal metamorphism as well as shock deformation since their formation. In this work, olivine grains from six monomict ureilites, Northwest Africa 7059, Elephant Moraine 96042, Shişr 007 (all ˜shock level S3), Northwest Africa 2221 (S3‐S4), Larkman Nunatak 04315 (S5), and Allan Hills A81101 (˜S6) and one polymict ureilite, Elephant Moraine 87720 (S5), were examined by in situ micro‐X‐ray diffraction (XRD). It is observed in this study that with increasing shock, the samples develop more complicated 2‐D XRD patterns from diffracted spots, to streaks, to asterism, and “spotty” Debye rings, and these patterns correspond to shock metamorphic effects observed by optical microscopy from undulatory extinction to mosaicism and annealing or recrystallization textures. These 2‐D XRD patterns, representing olivine strain‐related mosaicity (SRM), as documented by given lattice planes, were each measured as the sum of the full‐width‐half‐maximum along the Debye ring or χ dimension (∑[FWHMχ]) to quantify the shock‐related deformation recorded in the olivine grains. In each meteorite, all olivine lattice planes show similar measured ∑(FWHMχ) values for the diffraction streaks. This work demonstrates an increasing trend of average ∑(FWHMχ) in degrees of arc, 4.0 ± 1.7°, 4.1 ± 1.4°, 3.7 ± 1.8° (all S3), 4.9 ± 2.1° (S3–S4), 7.0 ± 3.5° (S5), 13.4 ± 5.7° (S6), and 7.5 ± 3.3° (polymict, S5), which agrees well with petrographic observations. This quantitative method contributes to a more comprehensive shock classification system specifically for ureilites, complementing recently published shock classification systems for olivine‐bearing rocks.

Electrochemical Analysis of Architecturally Enhanced LiFe0.5Mn0.5PO4 Multiwalled Carbon Nanotube Composite

In this work, the effect of carbon on the electrochemical properties of multiwalled carbon nanotube (MWCNT) functionalized lithium iron manganese phosphate was studied. In an attempt to provide insight into the structural and electronic properties of optimized electrode materials, a systematic study based on a combination of structural and spectroscopic techniques was conducted. The phosphor-olivine LiFe0.5Mn0.5PO4 was synthesized via a simple microwave synthesis using LiFePO4 and LiMnPO4 as precursors. Cyclic voltammetry was used to evaluate the electrochemical parameters (electron transfer and ionic diffusivity) of the LiFe0.5Mn0.5PO4 redox couples. The redox potentials show two separate distinct redox peaks that correspond to Mn2+/Mn3+ (4.1 V vs Li/Li+) and Fe2+/Fe3+ (3.5 V vs Li/Li+) due to interaction arrangement of Fe-O-Mn in the olivine lattice. The electrochemical impedance spectroscopy (EIS) results showed LiFe0.5Mn0.5PO4-MWCNTs have high conductivity with reduced charge resistance. This result demonstrates that MWCNTs stimulate faster electron transfer and stability for the LiFe0.5Mn0.5PO4 framework, which demonstrates to be favorable as a host material for Li+ ions.

Phosphorus Coupling Obfuscates Lithium Geospeedometry in Olivine

Lithium zoning in zircon and olivine provides a powerful record of crystal histories and magmatic processes in volcanic systems. Characterizing Li behavior in olivine is important because it is one of the few elements in basaltic systems that tracks short duration magmatic processes (e.g., < a few days). However, the potential for trace element coupling and the competing effects of crystal growth and subsequent diffusion obfuscate interpretations of Li zoning patterns and their use for geospeedometry. Here, we use diverse analytical techniques (EPMA, LA-ICPMS, nanoSIMS) to untangle records of growth and diffusion in carefully oriented olivine crystals from Kīlauea (Hawai‘i). Li, P, and Al are targeted because they show correlations despite contrasting behaviors during growth and diffusion. Lithium zoning exhibits two styles: 1) non-coupled, wherein Li (1-3 ppm) shows diffuse zoning over 10s-100s of μm between the crystal core and rim with low (10s of ppm) and homogeneous P, and 2) coupled, wherein sub-ppm Li enrichments <40 μm wide are correlated with P-rich zones (100s of ppm) that preserve a blueprint of crystal growth. Non-coupled Li zoning modeled for diffusive re-equilibration yields hours-days timescales, reflecting magma mixing events that primed the system for eruption. Coupled Li peaks consistently occur where analyses traverse the dendritic framework of P zoning that reflects rapid crystal growth. This correlation results from Li+ ions partially satisfying charge balancing requirements of P5+ ions in the olivine lattice. Aluminum balances most of the P budget in olivine but has uncommon correlations with Li peaks. Transects can exhibit both non-coupled and coupled Li behavior, preserving histories of both crystal growth and diffusive re-equilibration. Modeling a Li enrichment peak with the Li diffusion coefficient yields a timescale of <3 minutes, too short to represent the time elapsed following crystallization. If the Li and P peaks are modeled using the diffusion coefficient for P, congruent timescales of 5-11 days suggest they have the potential to record magmatic processes. Several caveats such as poorly constrained initial conditions and analytical challenges limit the practicality of enrichment peak geospeedometry. Only broad zoning of non-coupled Li should be modeled for timescales of magmatic processes.

Ni-doped LiFePO4/C as high-performance cathode composites for Li-ion batteries

LiFe1-xNixPO4/C composites were successfully synthesized by a simple hydrothermal method. X-ray diffraction (XRD) data indicate that doped Ni can enhance the crystallinity of the composite and retain the olivine lattice type of LiFePO4. The scanning electron microscopy (SEM) results show that a suitable amount of doped Ni can promote the formation of a uniform and small morphology. The high-resolution transmission electron microscopy (HRTEM) results verify that the Ni-doped sample is well covered by a carbon layer and has a high crystallinity. Charge/discharge data reveal that an appropriate amount of doped Ni can improve electrochemical performance. The LiFe0.97Ni0.03PO4/C composite has the strongest intensity of the XRD peaks, the smallest particle size, highest capacity and best cycling performance. Its specific capacity is 169.5 mAh·g−1 at 0.2C and can reach 122.9 mAh·g−1 at a 5C rate. Even at 10C, the capacity retention ratio is still 93.9% after 200 cycles. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data indicate that the electrode polarization and electrochemical impedance of LiFe0.97Ni0.03PO4/C are reduced compared with pure LiFePO4. Its specific capacity at 0.2C can be 175.8 mAh·g−1 after sufficient activation, which is higher than the theoretical value of 170 mAh·g−1.

Effect of Transitional Metals (Mn and Ni) Substitution in LiCoPO4 Olivines.

Transition metal substitution is a key strategy to optimize the functional properties of advanced crystalline materials used as positive electrodes in secondary lithium batteries (LIBs). Here we investigate the structural alterations in the olivine lattice of Mn and Ni substituted LiCoPO4 phase and the impact on performance in LIBs. X-ray diffraction (XRD) and extended X-ray absorption experiments have been carried out in order to highlight the structural alterations induced by partial substitution of cobalt by manganese and nickel. XRD analysis suggests that substitution induces an expansion of the lattices and an increase of the antisite disorder between lithium and transition metal ions in the structure. XAS data highlight negligible electronic disorder but a relevant modulation in the local coordination around the different metal ions. Moreover, galvanostatic tests showed poor reversibility of the redox reaction compared to the pure LCP sample, and this failure is discussed in detail in view of the observed remarkable structural changes.

Effect of doped Mn on improving the electrochemical performance of LiFePO4

A series of LiFe1−xMnxPO4/C composites were successfully synthesized through a simple hydrothermal method. The effects of doped Mn on their micro-structures and electrochemical performances were investigated. The XRD results indicate that Mn doping cannot change the olivine lattice type of LiFePO4. Appropriate Mn doping can reduce the particle size of LiFePO4. The electrochemical performance data show that the electrochemical performances of the composite materials are enhanced at first and then decrease as the doping amount of Mn increases. Both the CV and EIS results illustrate that a small amount of doped Mn will reduce the electrode polarization and electrochemical impedance of the samples and simultaneously improve the electrochemical performance. Among all the samples, the LiFe0.98Mn0.02PO4/C sample has a desirable electrochemical performance, with its specific discharge capacity reaching 156.0 mAh g−1 at a rate of 0.1 C. Even at a high rate of 5 C, it can still reach 110.0 mAh g−1. All the results indicate that an appropriate amount of doped Mn has an emphatic effect on the electrochemical performance of LiFePO4/C.

Morphological and Compositional Features of Chromian Spinel from Mantle Ultramafic Rocks of The Nurali Massif (South Urals)

Accessory chromian spinels of lherzolites and dunites from a mantle section of the Nurali ophiolite massif are described in the paper. Lherzolites typically host anhedral chromian spinel grains associated with olivine, pyroxenes and plagioclase. The compositions of silicates and chromian spinels are typical of those from ophiolite mantle sections. Olivine and orthopyroxene are characterized by high Mg content (forsterite and enstatite); clinopyroxene is diopside. The compositions of chromian spinel on the Al–Cr–Fe+3 plot occur close to the Al–Cr side. The #Cr and #Mg values of chromian spinels increase from lherzolites to dunites. Both vermicular spinels trapping olivine and orthopyroxene fragments (type I) and symplectite-like intergrowths of chromian spinel and plagioclase (type II) are most genetically interesting. Type I formed during synkinematic growth in deformed silicate matrix. Type II possibly formed as a result of decompression breakdown of a high-P mineral phase enriched in Cr, Al and Ca (a knorringite-type garnet?). In dunites, numerous tiny chromian spinel rods (type IV) in plastic deformed olivine are observed along with typical euhedral chromian spinel (type III) with inclusions of olivine and pargasite. Latter ones locally occur closely to fne pargasite grains. The formation of chromian spinel rods is explained as a result deformation-induced segregation of trace elements on the structural defects of the olivine lattice. Figures 7. Tables 4. References 48.

Phosphorus and aluminum zoning in olivine: contrasting behavior of two nominally incompatible trace elements

Phosphorus zoning in olivine is receiving considerable attention for its capacity to preserve key information about rates and mechanisms of crystal growth. Its concentration can vary significantly over sub-micron spatial scales and form intricate, snowflake-like patterns that are generally attributed to fast crystal growth. Ostensibly similar aluminum enrichment patterns have also been observed, suggesting comparable incorporation and partitioning behavior for both elements. We perform 1-atm crystallization experiments on a primitive Kīlauea basalt to examine the formation of P and Al zoning as a function of undercooling − ΔT (− ΔT = Tliquidus − Tcrystallization) during olivine growth. After 24 h spent at Tinitial = 1290 °C (10 °C above olivine stability), charges are rapidly cooled to final temperatures Tfinal = 1220–1270 °C, corresponding to undercoolings − ΔT = 10–60 °C (with Tliquidus = 1280 °C). Compositional X-ray maps of experimental olivine reveal that only a small undercooling (≤ 25 °C) is required to produce the fine-scale enrichments in P and Al associated with skeletal growth. Concentration profiles indicate that despite qualitatively similar enrichment patterns in olivine, P and Al have contrasting apparent crystal/melt mass distribution coefficients of $$K_{\text{P}}^{{{\text{ol}}/{\text{melt}}}}$$ = 0.01‒1 and $$K_{\text{P}}^{{{\text{ol}}/{\text{melt}}}}$$ = 0.002‒0.006. Phosphorus can be enriched by a factor > 40-fold in the same crystal, whereas Al enrichment never exceed factors of 2. Glass in the vicinity of synthetic and natural olivine is usually enriched in Al, but, within analytical uncertainty, not in P. Thus, we find no direct evidence for a compositional boundary layer enriched in P that would suffice to produce P enrichments in natural and synthetic olivine. Numerical models combining growth and diffusion resolve the conditions at which Al-rich boundary layers produce the observed enrichment patterns in olivine. In contrast, the same models fail to reproduce the observed P enrichments, consistent with our observation that P-rich boundary layers are insignificant. If instead, P olivine/melt partitioning is made to depend on growth rate, models adequately reproduce our observations of 40-fold enrichment without boundary layer formation. We surmise that near-partitionless behavior ( $$K_{\text{P}}^{{{\text{ol}}/{\text{melt}}}}$$ close to 1) of P is related to the olivine lattice being perhaps less stiff in accommodating P during rapid crystallization, and/or to enhanced formation of vacancy defects during fast growth. Our results confirm that P is a robust marker of initial rapid growth, but reveal that the undercooling necessary to induce these enrichments is not particularly large. The near-ubiquitous process of magma mixing under volcanoes, for instance, is likely sufficient to induce low-to-moderate degrees of undercooling required for skeletal growth.

Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments

X-ray computed tomography of individual olivine crystals in basalts from Ofu and Olosega islands, American Samoa, reveals that a small fraction of the olivines contain the vast majority of the fluid inclusions. Single-grain crushing experiments demonstrate that He and CO2 reside primarily in these inclusions. Low CO2 pressures in most grains, corresponding to depths of less than 1 km, provide evidence of ubiquitous decrepitation and associated pressure reduction in the fluid inclusions. Even so, the olivines with the highest inclusion volumes yielded sufficient He to obtain precise He concentrations and isotopic compositions. Within analytical uncertainty, 3He/4He ratios are homogeneous among the olivines from each basalt, but among basalts, the ratios range from 21 to 35 Ra. The total range in C/3He ratio within the analyzed olivines is from 3.6 × 107 to 1.5 × 1010, and varies by nearly an order of magnitude within the olivines from each basalt. We postulate that this wide range of C/3He ratios is caused by grain-scale decoupling of C and 3He due to extensive He diffusion out of fluid inclusions through the olivine lattice during magma ascent and cooling. If so, primary Ofu-Olosega magmas probably had C/3He ratios less than 4 × 108, which is lower than previous estimates for hotspot magmas.

Rare earth element partitioning and subsolidus exchange behaviour in olivine

The systematics of the rare earth element (REE) abundances in olivine are poorly understood and this undermines their usefulness in deciphering petrogenetic processes. Here we report a novel REE dataset for olivine spanning a wide range in Mg number (Mg# 1–59) from the Skaergaard Layered Intrusion.Olivine La and Lu concentrations vary from 0.68 to 56ppb and 66 to 571ppb, respectively. Apparent olivine/melt partition coefficients for the heavy to middle REE define near-symmetrical parabolas, consistent with lattice strain theory, indicating that these elements are structurally bound within the olivine lattice. The parabolas define peaks with variable optimal radii (ro). This suggests that the Fe-content of olivine exerts a strong influence on REE partitioning, as fayalitic olivines display more open parabolas (higher apparent Young's modulus, E) than forsteritic olivines. The greater site elasticity is manifest as higher REE concentrations in the fayalitic samples. The observation that E and ro are not constant accross a range of Mg# has implications for predictive models for olivine REE.The CI chondrite-normalised olivine REE patterns show smooth trends from the heavy to the middle REE, with greater variation amongst the light REE (LREE). Variation and enrichment of the LREE are ubiquitous within published data for natural olivine, yet are not predicted by experimental results. This study also presents two-dimensional REE maps illustrating the behaviour of the REE in olivine in unprecedented detail. Mapping of magnesian olivine shows core to rim increase in Al, LREE, and Eu and the development of a positive Eu anomaly in olivine, resulting from solid-state, likely diffusive re-equilibration with adjacent plagioclase. We propose that secondary redistribution is the principal cause of elevated LREE patterns in natural olivine.The results from the present investigation show that from a petrological point of view, MREE to HREE systematics of olivine hold promise for study of olivine petrogenesis and that the LREE in olivine may be further developed to understand diffusion time scales and cooling rates in magmatic systems.

A High Voltage Olivine Cathode for Application in Lithium-Ion Batteries.

A new olivine composition (i.e., LiFe0.25 Mn0.5 Co0.25 PO4) is proposed as electrode material with increased energy density for application in lithium-ion batteries. The new formulation increases the working voltage and induces different electrochemical behavior with respect to bare olivine materials based on Fe. The study provides deep insight into the features of the Fe(3+) /Fe(2+), Mn(3+)/Mn(2+), and Co(3+)/Co(2+) redox couples within the olivine lattice in terms of electrochemical activity, Li(+) transport properties, and Li-cell behavior. The electrochemical characterization clearly reveals the voltage signatures corresponding to the various metals; however, the Mn(3+)/Mn(2+) process has higher intrinsic polarization with respect to Fe(3+)/Fe(2+) and Co(3+)/Co(2+). This issue is efficiently mitigated by carbon coating the material, resulting in enhanced electrochemical performances.

Diffusion of phosphorus in olivine and molten basalt

The diffusivity of phosphorus in San Carlos olivine (SCO) was measured at near-atmospheric pressure and 650–850 °C by in-diffusion of P from a surface powder source consisting of pre-reacted SCO and AlPO 4 . The experiments were conducted in evacuated silica-glass ampoules at oxygen fugacities fixed by solid-state buffers, generally Ni-NiO but also including two experiments buffered at wustite-magnetite. Phosphorus uptake profiles were characterized by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). The temperature dependence of P diffusion in SCO conforms to the expected Arrhenius relation D = D 0 exp(− E a /R T ), where the constants are as follows: log( D 0 , m 2 /s) = −10.06 ± 0.80 and E a = 229 ± 16 kJ/mol. These values characterize P as a relatively slow diffuser in olivine—slower by about an order of magnitude than Cr and Ca at basalt near-liquidus temperatures—but substantially faster than Si. With a view toward modeling P uptake during rapid growth of natural olivines, P diffusion was also characterized in dry MORB basalt melt over the temperature range 1250–1500 °C at 1 GPa, using traditional diffusion couples contained in graphite. Phosphorus diffusion profiles in the quenched and depressurized samples were quantified by laser-ablation ICP/MS. Phosphorus diffusion in basaltic melt is similar to that of Si, with log( D 0 , m 2 /s) = −6.30 ± 0.7 and E a = 147 ± 22 kJ/mol. The new data for P diffusion in olivine and basalt melt can be used to explore the acquisition of fine-scale zoning in natural olivine phenocrysts through kinetic models, as well as the survival of P zoning in olivine with time spent at elevated temperature. Models of growth entrapment of a P-enriched near-surface layer in the olivine lattice indicate that crystal growth at plausible sustained rates is indeed likely to result in regions of anomalously high P content in the resulting crystal. Phosphorus concentrations above the equilibrium partitioning value can also result from development of a diffusive boundary layer in the melt against a rapidly growing crystal, but this mechanism is ineffective at typical sustained olivine growth rates, requiring dendrite-forming growth speeds. Preservation of P zoning on the scale of a few micrometers apparently requires cooling within a few months of formation of the zoning.

Lithium-Ion Battery Based on LiMn0.5Fe0.5PO4 Cathode and Lithium Alloying Anode

Polyanion framework olivine materials have attracted large attention as cathodes in LIBs. Recently, the attention of the scientific community has been focused on the substitution of iron by manganese, cobalt or nickel within the olivine lattice [1]. Indeed, Mn3+/Mn2+, Co3+/Co2+ and Ni3+/Ni2+ couples show increasing redox potentials compared to Fe3+/Fe2+, thus leading to an increased energy density. However, the latter two exhibit redox potential of about 4.8 and 5.2 V vs. Li+/Li, respectively, i.e. a value exceeding the conventional carbonate-based electrolytes stability limit [2]. LiMnPO4 has instead an operating voltage of about 4.1 V vs. Li+/Li, still within the working electrochemical window of conventional electrolytes, thus leading to a theoretical energy density of about 700 Wh kg-1 [3]. LiMnPO4 suffers from poor electric conductivity in comparison to LiFePO4 [4]. Several approaches have been studied to improve LiMnPO4 electrochemical activity in lithium cell. Incorporation of conductive additives, such as carbon, by post-synthesis ball milling or formation of a carbon layer on particle surface can increase the electronic conductivity [5, 6]. Significant decrease of the lithium diffusion pathway and consequent conductivity increase may be reached by reduction of particle size by proper synthetic procedures leading to nano-morphologies [7, 8]. Soft-chemistry methods allow more controlled tailoring of morphologies and size than the conventional solid-state approaches [9]. In particular, hydro- and solvo-thermal methods are simple techniques allowing the precipitation of olivines at low temperature and controlled crystal growth [10]. Further improvement of the cathode performance may be reached by mixed compositions, LiMn1-xFexPO4. Iron incorporation in the olivine lattice has proven to enhance the electrochemical activity of the Mn3+/Mn2+ redox reaction by increasing both electronic and ionic conductivity, and by reducing lattice strain due to Jahn-Teller active ion Mn3+ [11, 12].Herein we report the synthesis and characterization of LiMnPO4, LiFePO4 and mixed LiMn0.5Fe0.5PO4 materials through solvothermal approach. Careful analysis of the electrochemical properties by potentiodynamic cycling with galvanostatic acceleration and galvanostatic cycling techniques were carried out in order to investigate the features of the Fe3+/Fe2+ and Mn3+/Mn2+ redox couples in the olivine structure. In addition, an advanced lithium-ion battery formed by combining a LiMn0.5Fe0.5PO4 cathode with a Sn-C anode is reported. The figure shows the galvanostatic cycling results of a Sn-C/LiMn0.5Fe0.5PO4 cell.

Electrochemical and Magnetic Studies of Li-Deficient Li1-xCo1-xFexPO4 Olivine Cathode Compounds

Li-deficient olivine compounds, Li1-xCo1-xFexPO4 (x < 0.15), were prepared by calcination of the sol-gel derived precursors in ambient atmosphere. They were almost single phase at x < 0.10 but some impurity phases were found at x > 0.10. The lattice parameters changed continuously with an increase in the Fe substitution. Their variation in the range of x < 0.10 roughly obeyed Vergard's law. From the 57Fe Mössbauer spectrum at room temperature, it was shown that the introduced Fe took a trivalent high-spin state in the olivine lattice. The temperature-dependent magnetic susceptibility exhibited that the Fe incorporation enhanced the antiferromagnetic ordering: the Neel temperature shifted higher. Furthermore, the electrochemical performances, such as cycling stability and rate capability, were improved significantly with an appropriate substitution (about 10%) with Fe3+. Consequently, the Fe3+ incorporated compounds with Li deficiency as the charge compensation are thought to be good candidates as high-voltage cathode material, which may enhance the energy density of Li ion battery.

The development of magmatism along the Cameroon Volcanic Line: Evidence from seismicity and seismic anisotropy

AbstractThe Cameroon Volcanic Line (CVL) straddles the continent‐ocean boundary in West Africa but exhibits no clear age progression. This renders it difficult to explain by traditional plume/plate motion hypotheses; thus, there remains no consensus on the processes responsible for its development. To understand better the nature of asthenospheric flow beneath the CVL, and the effects of hotspot tectonism on the overlying lithosphere, we analyze mantle seismic anisotropy and seismicity. Cameroon is relatively aseismic compared to hotspots elsewhere, with little evidence for magmatism‐related crustal deformation away from Mount Cameroon, which last erupted in 2000. Low crustal Vp/Vs ratios (∼1.74) and a lack of evidence for seismically anisotropic aligned melt within the lithosphere both point toward a poorly developed magmatic plumbing system beneath the CVL. Null SKS splitting observations dominate the western continental portion of the CVL; elsewhere, anisotropic fast polarization directions parallel the strike of the Precambrian Central African Shear Zone (CASZ). The nulls may imply that the convecting upper mantle beneath the CVL is isotropic, or characterized by a vertically oriented olivine lattice preferred orientation fabric, perhaps due to a mantle plume or the upward limb of a small‐scale convection cell. Precambrian CASZ fossil lithospheric fabrics along the CVL may have been thermomechanically eroded during Gondwana breakup ∼130 Ma, with an isotropic lower lithosphere subsequently reforming due to cooling of the slow‐moving African plate. Small‐scale lithospheric delamination during the 30 Ma recent development of the line may also have contributed to the erosion of the CASZ lithospheric fossil anisotropy, at the same time as generating the low‐volume alkaline basaltic volcanism along the CVL.