Fe-poor Composition Research Articles

Crustal differentiation in the early solar system: Clues from the unique achondrite Northwest Africa 7325 (NWA 7325)

The unique achondrite NWA 7325 is an unusual olivine gabbro composed chiefly of calcic plagioclase (An85–93), diopsidic pyroxene (En50.1–54.0 Wo44.8–49.3 Fs0.6–1.3), and forsteritic olivine (Fo97). It is Al and Mg-rich and Fe and Na-poor and displays very low concentrations of incompatible trace elements, much below 0.3×CI abundances for many of them. It is also characterized by huge Eu and Sr anomalies (Eu/Eu∗=65, Srn/Cen=240). Although the O isotopic composition of NWA 7325 and some ureilites (those with olivine cores in the range Fo75–Fo88) are similar, a genetic relationship between them is unlikely due to the Fe-poor composition of NWA 7325. It is almost certainly derived from a distinct planetesimal, not previously sampled by other achondrites. The low Na/Al, Ga/Al, Zn/Al ratios as well as the low K, Rb and Cs shown by NWA 7325, suggest a volatile-depleted parent body. This unique gabbro is demonstrably a cumulate, but the composition of its parental melt cannot be precisely assessed. However, the liquid from which NWA 7325 crystallized would have been very poor in incompatible trace elements (Yb in the range of 0.25–1.5 ×CI abundance) with a very large positive Eu anomaly. Such a melt cannot be the product of the early magmatic activity on a small parent body. Instead, we propose that the parental melt to NWA 7325 formed as a consequence of the total melting of an ancient gabbroic lithology, possibly upon impact, in agreement with the systematics of highly siderophile elements and 26Al–26Mg. Based on recent dating, the crustal material that was parental to NWA 7325 must have been older than 4562.8Ma, and formed possibly ≈4566Ma ago. If this scenario is correct, NWA 7325 provides evidence of one of the earliest crusts on a differentiated body so far studied.

The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities ( fO 2) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO 2 < −9) or oxidised (log fO 2 > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S 2−. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV. The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO 4 2−. Sulfite (SO 3 2−) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S 6+). S 4+ does, however, occur unambiguously with S 6+ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO 4 at 4–16 kbar, and when buffered with Re/ReO 2 at 10 kbar. Solubility of S 4+ thus requires partial pressures of SO 2 considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO 2s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S 2− and S 6+ were found to coexist in some samples that were not in equilibrium with the imposed fO 2. The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S 2−, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S 2− + 8Fe 3+ = S 6+ + 8Fe 2+, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S 2− and S 6+ are frequently found together despite the very limited fO 2 range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.

Study on electromagnetic properties of MnZn ferrites with Fe-poor composition

Curie temperature, initial permeability and resistivity of MnZn ferrites with Fe-poor composition (less than 50 mol% of Fe 2O 3) are studied. The experience formula of Curie temperature for MnZn ferrites with Fe-rich composition is modified to calculate the Curie temperature of Fe-poor MnZn ferrites. Tri-segment frequency dispersion model, which consists of domain wall motion, magnetization rotation and gyro-magnetic spin rotation, is used to simulate the permeability spectra of ferrites. The permeability of Fe-poor MnZn ferrites compares with that of Fe-rich MnZn ferrites in low frequency region, and it is still high enough to compare with that of NiZn ferrites in high frequency region. The resistivity of Fe-poor MnZn ferrites is about 10 3 Ω m, much higher than that of Fe-rich MnZn ferrites.

The Mn, Mg-intracrystalline exchange reaction in donpeacorite (Mn0.54Ca0.03Mg1.43Si2O6) and its relation to the fractionation behavior of Mn in Fe, Mg-orthopyroxene

The equilibrium intracrystalline distribution of Mn and Mg between the M1 and M2 sites of a Mn-rich/Fe-free orthopyroxene (donpeacorite) was investigated by means of annealing experiments at temperatures between 980 and 800 °C and single-crystal X-ray diffraction. The data show that Mn, as does Fe 2+ in Fe-Mg orthopyroxene, preferentially orders at the M2 site. However, comparison of the distribution coefficient k D(Mn-Mg) determined in this study with k D+ measured for Fe-Mg orthopyroxene shows that Mn has a much stronger preference for the M2 site relative to Fe 2+ . This result implies that the practice to partition Fe 2+ + Mn = Fe* as one species, typically implemented to determine the quenched-site occupancies in Fe-rich/Mn-poor orthopyroxene, should be abandoned and that Mn should be considered totally ordered at M2. The partitioning method, i.e., Fe vs. Fe*, has implications for the determination of cooling rates from the observed ordering state of orthopyroxene, particularly for Fe-poor compositions (Fs &lt; 0.16).

Fe-poor 組成 Mn-Zn-Ti フェライトの電気伝導性

The real parts of the complex permittivity and complex impedance spectra of Mn-Zn-Ti ferrite with Fe-poor composition (FPF:Fe2O3 less than 50 mol%) were measured. The relaxation time was estimated by numerical fitting of the permittivity spectra data. The relaxation time of FPF was about 10-5-10-6 s, because dielectric relaxation occurred below 1 MHz. The resistance of the grains is separated from that of the grain boundary in the complex impedance spectra. As a result, it was found that the relaxation time is proportional to the resistance of the grains. It is reasonable to suppose that the relaxation time is related to the electrical mobility.The temperature dependences of the real parts of the complex permittivity and the complex impedance spectra of the same sample were measured. The activation energy of the resistances of the grains was a little larger than that of the relaxation times.

Ｆｅ‐ｐｏｏｒ組成Ｍｎ‐Ｚｎ‐Ｔｉフェライト磁心を用いたコモンモードチョークコイル

To enhance the electrical resistivity of a Mn-Zn ferrite markedly, Fe2O3 composition was reduced to less than 50mol%. Then to obtain good soft magnetic properties with this composition, TiO2 was added to the component composition. We propose a Mn-Zn-Ti ferrite with Fe-poor composition, which has about 1,000 times higher resistivity and 100 times smaller permittivity than conventional Mn-Zn ferrite with Fe-rich composition, although it shows high initial permeability (4,500 at 100kHz). The common-mode impedance of the choke coil and the attenuation of the LC filter were investigated in order to compare this material with conventional Mn-Zn ferrite and Ni-Zn ferrite. It was found that this material was able to retain good magnetic properties in the high-frequency range the use of where Ni-Zn ferrite has been predominant. As a result, this material has potential for EMI applications over a wide frequency range which both conventional Mn-Zn ferrite and Ni-Zn ferrite have been used.

Ｆｅ‐ｐｏｏｒ組成Ｍｎ‐Ｚｎ‐Ｔｉフェライトの初透磁率の周波数依存性

Initial permeability, permittivity and resistivity spectra of Mn-Zn-Ti ferrite with Fe-poor composition (less than 50 mol% of Fe2O3) (FPF) are compared with Fe-rich Mn-Zn ferrite and Ni-Zn ferrite.Analysis of the electromagnetic characteristics shows that the skin effect and dimensional resonance can be disregarded for FPF of the measured sample size below 1 MHz.The contribution of the domain wall and spin rotation resonance to the permeability spectra was estimated by numerical fitting of the permeability data. It was found that natural resonance occurred in FPF, as in Ni-Zn ferrite. In addition, FPF has a damping factor β smaller than the conventional Fe-rich Mn-Zn ferrite, because it does not contain Fe2+ ions that have non- zero orbital moment, and large magnetic anisotropy factor.

窒素中焼成したＦｅ‐Ｐｏｏｒ組成Ｍｎ‐Ｚｎ‐Ｔｉフェライトの磁気特性

Our previous report shows that Mn-Zn-Ti ferrite with Fe-poor composition (less than 50 mol% of Fe2O3) sintered in air exhibits a high resistivity and initial permeability at frequencies above 1 MHz. We examined Fe-poor ferrite sintered in N2 like conventional Mn-Zn ferrite with Fe-rich composition in order to further improve the initial permeability. Sintering the Fe-poor ferrite in N2 leads to a decrease in the phase of α-Fe2O3 in the surface layer of the ferrite, resulting in an improvement of the initial permeability at frequencies below 1 MHz. As a result, the novel ferrite exhibits a high initial permeability in a wide range of frequencies.

ＴｉＯ２を添加したＦｅ‐ｐｏｏｒ組成Ｍｎ‐Ｚｎフェライトの周波数特性

It has been difficult to use Mn-Zn ferrite at a frequency range beyond 1 MHz due to its low electrical resistivity. We have been trying to improve the resistivity and magnetic properties of Mn-Zn ferrite by alterations of the composition and additives. We found that the combination of Fe-poor composition (less than 50 mol % of Fe2O3) and the addition of TiO2 to Mn-Zn ferrite produced a remarkable increase in resistivity and an improvement in initial permeability at high frequencies. In addition, this ferrite unexpectedly could be sintered in air, unlike the conventional Fe-rich ferrite. The representative Fe-poor ferrite (48.5F2O3-30.3MnO-20.2ZnO-1.0TiO2) showed a very high permeability of μi = 200 at 10 MHz. The manufacturing process and the attractive magnetic properties at high frequency of the Fe-poor Mn-Zn ferrite are discussed.

The Triple Group and the Platinova gold and palladium reefs in the Skaergaard Intrusion; stratigraphic and petrographic relations

This paper describes the setting, structure, and textural relations of the Platinova reefs in the Skaergaard intrusion. The reefs comprise a strata-bound occurrence of Au + PGE estimated at 91 million tons of rock with 1.8 ppm Au and similar amounts of Pd. The precious metals occur in two major zones, a lower major Pd reef which is concordant with the modal layering and followed by at least ten minor Pd reefs (also concordant), and an Au concentration which fringes the Pd reefs outward and upward through the Triple Group units. The major Pd concentration and the Au concentration are separated in a systematic way across the intrusion. Near the center of the intrusion, the Au concentration occurs some 60 m higher in the stratigraphy than the main Pd reef, but the separation decreases gradually outward, being only a few meters near the margins.The dominant precious metal minerals are small grains of (Cu,Fe)(Au,Pd,Pt) alloys which typically are included in interstitial sulfides (bornite, digenite, and smaller amounts of chalcopyrite). The textural relations suggest a close genetic association between the sulfides and Au + PGE minerals, an association which is confirmed by whole-rock compositional data.Our observations demonstrate that the Platinova reefs formed after significant concentration of the Au + PGE and S in the residual magma during fractional crystallization, before it became saturated with respect to a sulfide (Fe-Cu-S-O) liquid. This sulfide liquid acted as a collector of Au + PGE and settled into the Layered series cumulates. We propose that the structure of the deposit evolved through fractional crystallization of the Au + PGE-bearing sulfide liquid as it was filter-pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates. The fractional crystallization resulted in an evolution of the Au + PGE-bearing phases with progressive fractionation from Pd-rich alloys at the base of the deposit to Au-rich alloys at the top.We argue that the sulfides evolved to their present extreme Cu-rich, Fe-poor composition during oxidation-desulfurization reactions in the same event which oxidized the iron-titanium oxides of the intrusion. This oxidation-desulfurization probably had no overall effect on the distribution of Au and PGE.

Formation of regularly interstratified serpentine-chlorite minerals by tetrahedral inversion in long-period serpentine polytypes

Serpentinite from Lancaster County, Pennsylvania, consists of a variety of fme-grained serpentine minerals, chlorite, randomly interstratified serpentine-chlorite, and a series of phases based on regular interstratification of serpentine and chlorite (SxCy, where x and y are integers). Within the resolution of the AEM technique, all layer silicates have the same Mg-rich, AI-rich, Cr-rich, and Fe-poor compositions. Regularly interstratified serpentinechlorite minerals are frequently intimately intergrown with serpentines that have repeat distances identical to those of the regular interstratifications. Thus, dozyite (SICI, (3= 90°) is intimately associated with serpentine with three-layer octahedral order (1,1,11).Longer period polysomes (S2C SIC2, S2C2, S,C3, S3C2, and S,C4, all with (3 = 90°) are each accompanied by serpentines with equivalent c-axis periodicities. SxCy phases apparently form by selective growth ofIbb chlorite units from 1,11octahedral sequences in long-period serpentines. All microscopic structural evidence is consistent with the formation of regular interstratifications by tetrahedral inversion within existing serpentine. Atomic resolution images reveal that the tetrahedral sheet is displaced by a13.where it inverts to form the 2:1 layer. A :ta13 shift is required for hydrogen bonding between OH of the newly formed brucite-like interlayer and 0 atoms of the 2: 1 layer. The sense of the shift is determined by the strong interactions between the octahedral cations in the brucite-like interlayer and the Si in the 2: 1 layer (direct superimposition, previously described as a type-a interaction, is strongly unfavorable). Distortion at the inversion point probably lengthens Si-O bonds in the next tetrahedra, facilitating relocation of Si on the other side of the basal 0 plane. Reversal of the octahedral slant in the 2:1 layer occurs because the +a13 tetrahedral shift necessitates repositioning of 0 and OH coordinating octahedral cations, requiring movement of octahedral cations from type-II to type-I positions. Except in the 2: 1 layers, the stacking and octahedral slants are inherited. The result is a series of regular interstratifications characterized by a single octahedral slant (specifically, Ibbb,l) and bl3 stacking disorder. This analysis reveals the importance of cation-cation interactions in determining the relative stability of pairs of 1:1 layers and in controlling the detailed structures oflayer silicates formed in solid-state serpentine-to-chlorite reactions. Because similar constraints apply to formation of serpentine from chlorite by direct structural modification, the common 1T lizardite polytype may be produced from both IIbb and Ibb chlorites.