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1.
A suite of synthetic titanomagnetites of composition Fe2.4?δAlδTi0.6O4 and Fe2.6?δAlδTi0.4O4 (δ = 0, 0.1 and 0.2 in both cases) have been prepared by a method of partial self-buffering and pulverized in a ball mill to particle size of about 200–500 Å. Magnetic hysteresis parameters-saturation and remanent magnetizations and coercive force were measured between room temperature and the Curie temperatures and other parameters-X-ray cell edge, initial susceptibility and coercive force of remanence were determined at room temperature. The intrinsic magnetic “hardness” increases with increasing content of Al3+ and Ti4+, both probably corresponding to an increase in the concentration of Fe2+ ions on the tetrahedral sites of the spinel structure. The room-temperature hysteresis properties were compared with those resulting from monodomain models for the work done to magnetically saturate an assemblage of grains and the approach to saturation, and the separate contributions from coexisting anisotropies of cubic and uniaxial symmetries (assumed present) inferred. The cubic anisotropy energy constants so derived are larger than those determined from multidomain single crystals. The derived cubic constants are also larger than the derived uniaxial anisotropy constants. The latter, however, dominate the behaviour (e.g., coercive force) because of the lower symmetry. The materials appear to be entirely in the stable monodomain state at room temperature.  相似文献   

2.
Four pyroxenes with compositions En48Fs48Wo4, En47·5Fs47·5Wo5, En45Fs45Wo10 and En40Fs40Wo20, synthesized at 1200°C at atmospheric pressure, were heat-treated at 500, 600, 700, and 800°C for various lengths of time. These pyroxenes are variously ordered with respect to Fe2+ and Mg2+ without unmixing. The Fe2+-Mg2+ distribution over the two nonequivalent sites M1 and M2, determined through Mössbauer spectroscopy, is found to be a function of both temperature and concentration of Ca2+ at the M2 site. The preference of Fe2+ for the M2 site increases with decreasing temperature and increasing Ca2+. These data can be used to determine cation equilibration temperatures of lunar and terrestrial pigeonites. The lunar pigeonites usually indicate equilibration temperatures of 700–860°C, except the pigeonite from rock 14053, which may have been subjected to shock heating due to meteoritic impact.  相似文献   

3.
A comparison of lunar ilmenites (Apollo 11, 10047, 13) with terrestrial ilmenites by means of electron microprobe analysis, X-ray and Mössbauer spectrometry showed that the lunar samples contained no Fe3+ but excess Ti3+. This causes an increase of thec-axis as compared with stoichiometric ilmenite.  相似文献   

4.
Because of the difficulties of laboratory simulation of maghemitization, few studies have been carried out and the results of such studies are not in good agreement. Of particular interest is the evolution of the cation distribution with degree of non-stoichiometry. This indicates how the maghemitization process takes place at the atomic level. In the present study a suite of titanomagnetite specimens containing a range of concentration of Mg2+ ions were maghemitized and the possible cation distribution inferred from saturation magnetization measurements. Although a unique cation distribution cannot be obtained from such measurements, the observed data are consistent with the following model, except for high degrees of oxidation and high Mg concentration: (1) Ti4+ and vacancies in B sites only; (2) a random distribution of Mg2+ which remains frozen in during maghemitization; (3) availability for oxidation of tetrahedral site Fe2+ 20% of that of octahedral site Fe2+; and (4) deficient moment of the oxidized specimens due to small particle size.  相似文献   

5.
The magnetic susceptibility of a terrestrial, synthetic and lunar ilmenite specimen has been measured from 4 to 300 K. All specimens had a single Néel temperature transition which ranged from 56 to 57.7 K. In one case the powdered specimen was partially aligned in the magnetic field prior to the susceptibility measurements and the Néel transition was observed to shift to 60 K indicating magnetic anisotropy. A study of several magnetic parameters calculated from the experimental data showed gross impurities in the terrestrial specimen, single-domain to multi-domain metallic iron in the synthetic specimen, and a small amount of superparamagnetic metallic iron in the lunar sample. No multidomain iron was observed in the lunar ilmenite. The results of electron spin resonance measurements were also in general agreement with these findings. Because of the absence of Fe3+ compared to most terrestrial samples it is suggested that the anisotropic magnetic parameters be determined on lunar ilmenite when a large enough single crystal becomes available.  相似文献   

6.
Exsolution microstructures including ilmenite±garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the Sulu ultrahigh-pressure (UHP) terrane. In order to understand the phase relations and Ti solubility in both garnet and clinopyroxene in a natural TiO2-bearing system, several experiments at 5-15 GPa, 1000-1400°C were carried out using the multianvil high-pressure apparatus. The Hujianlin ilmenite-rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material, because it closely approaches a composition lying in the TiO2-CaO-MgO-FeO-Al2O3-SiO2 system. Except for minor melt in one experiment at 1400°C and 5 GPa, other run products contain majoritic garnet+clinopyroxene±ilmenite (or rutile) and exhibit neoblastic texture. With increasing pressure, Ti and Ca, Mg and Si contents of neoblastic garnet increase with decreasing Al. The principal coupled substitutions are Ca2+Ti4+→2Al3+ and Si4+Mg2+→2Al3+ responding to majorite component increase. Titanium solubility (0.8-4.5 wt% as TiO2) in garnet and GrtTi/CpxTi ratio have a pronounced positive correlation with pressure between 5 and 15 GPa. On the other hand, the coexisting clinopyroxene contains low Ti (0.17-0.53 wt% as TiO2), and shows no significant pressure effect. Rutile exsolution in garnet is coupled to that of pyroxene exsolution; both are exsolved from majoritic garnet on decompression. Therefore, the amount of such exsolved lamellae is a potential indicator of high-pressure metamorphism in exhumed rocks, whereas the TiO2 content of clinopyroxene coexisting with garnet is not sensitive to pressure change.  相似文献   

7.
The elastic moduli of a synthetic single crystal of pyrope (Mg3Al2Si3O12) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)3 (Al, Fe, Cr)2 Si3O12. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe2+/Mg2+ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg2+ and Fe2+ in these different minerals.  相似文献   

8.
X-ray single-crystal studies have been made of armalcolites from the 2–5-mm fraction of Apollo 17 soils 75082 and 78502. Two types of armalcolite, “ortho” and “para” have previously been distinguished on the basis of optical properties and crystal habit in the Apollo 17 samples. “Ortho”-armalcolite has the space group of pseudobrookite, Bbmm, and has cell dimensions: a = 9.743 (5); b = 10.001 (5); and c = 3.728 (2)Å. The crystal structure, refined from 582 symmetry-independent intensities, is ordered with Ti in the site with m symmetry and Mg and Fe in the site with mm symmetry. “Para”-armalcolite has space group Bbmm, cell dimensions: a = 9.712 (20), b = 9.997 (20), and c 3.735 (8)Å; and is structurally identical with “ortho”-armalcolite. The two crystals studied are similar but not identical in composition, but the slight compositional differences and identical structures do not warrant the use of separate mineral names.  相似文献   

9.
Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mössbauer spectroscopy to determine Fe3+/ΣFe. Fe3+ concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3+/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53+O (M=Fe3+, Cr3+, Al3+). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and São Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe-(Mg,Fe)O equilibrium, but more reducing than Re-ReO2 equilibrium. Fe2+/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mössbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3+ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3+ concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle.  相似文献   

10.
Reviewing 92 measurements of lunar sample dielectric constant versus density at frequencies above 100 kHz, gives the relationK′ = (1.93 ± 0.17)p by regression analysis, where K′ is the dielectric constant of a soil or solid at a density ofpg/cm3. This formula is the geometric mean between the dielectric constant of vacuum (1) and the zero porosity dielectric constant of lunar material. Similarly, the loss tangent (D) can be described byD = [(0.00053 ± 0.00056) + (0.00025 ± 0.00009)C]p whereD is the loss tangent at densitypg/cm3 withC percent of total FeO + TiO2 (approximately proportional to ilmenite content). Using the density versus depth relations derived from lunar surface core tubes, and from laboratory studies of lunar soil compression gives a model of the dielectric properties as a function of depth in the lunar regolith. The dielectric constant increases smoothly with depth, as a function of the soil compaction only. The loss tangent, however, is more sensitive to the ilmenite content than it is to density. Neither dielectric constant nor loss tangent varies significantly with the temperature observed in a lunar day.  相似文献   

11.
Aluminum incorporation into the high pressure polymorph of TiO2 with the structure of α-PbO2 has been studied from 10 to 20 GPa and 1300 °C by XRD, high-resolution 27Al MAS-NMR and TEM. Al-doped α-PbO2 type TiO2 can be recovered at atmospheric pressure. Al2O3 solubility in α-PbO2 type TiO2 increases with increasing the synthesis pressure. The α-PbO2 type TiO2 polymorph is able to incorporate up to 35 wt.% Al2O3 at 13.6 GPa and 1300 °C, being the substitution of Ti4+ by Al3+ on normal octahedral sites the mechanism of solubility. The transition to the higher pressure TiO2 polymorph with the ZrO2 baddeleyite structure, Akaogiite, has not been observed in the quenched samples at room pressure. The microstructure of the recovered sample synthesized at 16 GPa and 1300 °C points to the existence of a non-quenchable aluminum titanium oxide phase at these conditions.  相似文献   

12.
The anionic structure of magmatic liquids has been estimated at 1 atm and at pressures corresponding to those of the upper mantle. These estimates are based predominantly on spectroscopic data on binary metal oxide-silica and ternary metal oxide-silica-alumina melts. Structural information on melt compositions in aluminate-silica joins has been used to provide detailed information on the role of Al3+ in natural magma at atmospheric and high pressure.Regardless of pressure, andesitic melts may be described as combinations of chain, sheet, and three-dimensional network units. Nearly all Al3+ in the magmatic liquid resides in the three-dimensional network units. This Al3+ is locally charge-balanced with Na+, K+, Ca2+, and Mg2+. In the latter two cases, Al3+ and Si4+ are ordered, whereas for Na+ and K+, Si4+ and Al3+ are randomly mixed. Solution of water in natural magma results in the formation of new nonbridging oxygens in addition to OH groups attached to Si4+ and metal cations.On the basis of determined solution mechanisms of CO2 and H2O in silicate melts, thermodynamic properties of HO+CO2, fluids and hydrous silicate melts and melting phase relations in peridotite-H2O-CO2, systems, it is found that natural andesitic magma in equilibrium with spinel Iherzolite in the upper mantle (10–20 kbar) must contain at least 5–7 wt.% H2O. Andesitic magma with 5–7 wt.% H2O in solution may be described as a mixture of Al-free three-dimensional units, sheets, and chains with a small proportion (less than 10%) of monomers.  相似文献   

13.
By using the diamond-anvil pressure cell coupled with laser heating, Ca2GeO4 in the K2NiF4-type structure has been found to decompose into the mixture Ca3Ge2O7 plus CaO at pressures greater than 200 kbar and at about 1000°C, and the same type of structure for Ca2MnO4 has been found to decompose into the mixture CaMnO3 (perovskite) plus CaO at pressures greater than 100 kbar and at about 1400°C. The decomposition product of Ca3Ge2O7 is a new compound which is isostructural with Sr3Ti2O7 and has the lattice parameters of a = 3.72 ± 0.01 and c = 19.32 ± 0.05 A? at room temperature and 1 bar pressure. The results of the study of Ca2GeO4 and Ca2MnO4 (both with the K2NiF4-type structure) strongly support the view that compounds possessing the K2NiF4-type structure are unstable relative to corresponding mixtures possessing the perovskite and rocksalt structures. It is concluded that, in the earth's mantle, the K2NiF4-type Ca2SiO4 would ultimately decompose into the mixture CaSiO3 (perovskite) + CaO or would otherwise transform to other as-yet-unknown phase(s), and that the mixture of MgSiO3 (perovskite) + MgO (the post-spinel phase of Mg2SiO4) would not adopt the K2NiF4-type structure.  相似文献   

14.
The fO2 stability relations of ilmenite and ulvöspinel were determined using C-O H-N gas-flow apparatus with fO2 measured by a solid ceramic (calcia-zirconia) oxygen electrolyte cell. For Fe+TiO2 + 1/2 O2 =FeTiO3 (from 850°–1050°C), 1/2 log fO2=(−11,250/T) + 0.98 and for Fe+FeTiO3 + 1/2 O2 =Fe2TiO4 (from 850°–1210°C), 1/2 logfO2 = (−12,170/T) + 1.93. These curves lie at significantly higher values of ?O2 than determined by previous investigators (i.e., 3/4 and1/4 order of magnitude for ilmenite and ulvöspinel, respectively). In addition, for Fe+ 2TiO2 + 1/2 O2 =FeTi2O5 (1210°C), ΔGr0=−45.8 ± 0.6 kcal. The QFI curve crosses the ulvöspinel reduction curve at ∼950°C and is at lower values of fO2 below this temperature. The occurrences of fayalite reduction to SiO2 + Fe in lunar rock 14053, as well as a new finding of this assemblage in 14072, are evidence for extreme sub-solidus reduction, whereas ulvöspinel breakdown alone occurs under less reducing conditions. The ‘complete’ reduction of ulvöspinel to TiO2 + Fe occurs in 2 steps: first, to ilmenite + Fe and then, however more slowly, to rutile + Fe. Thus, the presence of ulvöspinel but lack of ilmenite reduction in lunar rocks cannot be used as evidence that the fO2 was between the associated curves — only upper limits of fO2 can be inferred.  相似文献   

15.
A new type of insoluble potassium ore, newly found in North China, consists of microcline, dolomite, and pyroclastic particles of silty to clayey grade, falling into potassic dolomitic mudstone in petrological no-menclature. The geological surveys and explorations of the deposit by the Tianjin Bureau of Geological Survey evidenced that the main bodies of the potas-sium deposit elongated up to 1600 m with an average thickness about 85 m, and the concentration of potas-sium as K2O was around 1…  相似文献   

16.
Three synthetic Fe3+ bearing λ-Fe2SiO4 were analyzed using electron probe method, and the M?ssbauer spectra of the samples at 298 K, 150 K, and 95 K were measured. Each spectrum at three temperatures is composed of two doublets. These two doublets are assigned to Fe2+ in the octahedral sites and Fe3+ in the tetrahedral sites, respectively. Site occupancies were determined. The results show that Fe3+ and a small amount of Si4+ are in the tetrahedral and octahedral sites, respectively. The average bond lengths of the octahedral and tetrahedral sites were calculated according to the equations primarily given by Hill et al., O’Neill and Navrotsky and modified by the authors. Furthermore, the octahedral and tetrahedral bond lengths were used to calculate cell parameters and oxygen parameters. In addition, Fe3+ line broadening in the M?ssbauer spectra of Fe3+ bearing λ-Fe2SiO4 were interpreted by using the next nearest neighbor effects  相似文献   

17.
Samples from the surface of lava flows discharged by the 2012–2013 Tolbachik Fissure Eruption were found to contain oxysulfates of copper, sodium, and potassium: K2Cu3O(SO4)2 (fedotovite), NaKCu2O(SO4)2, and Na3K5Cu8O4(SO4)8. The last two phases have no naturally occurring or synthetic analogues that we are aware of. They form flattened crystals of prismatic to long-prismatic habits. The crystals of Na3K5Cu8O4(SO4)8 have a chemical composition corresponding to the empirical formula Na2.22K5.47Cu8.02S8.05O36. An X-ray analysis of this compound showed that it has a monoclinic symmetry, P2/c, a = 13.909(4), b = 4.977(1), c = 23.525(6) Å, β = 90.021(5)°, V = 1628.3(7) Å3. The crystal structure was determined by direct techniques and refined to yield R 1 for 3955 reflexes//web// with F 2 > 4σF. The compound NaKCu2O(SO4)2 also belongs to the monoclinic system, P2/c, a = 14.111(4), b = 4.946(1), c = 23.673(6) Å, β = 92.052(6)°, V = 1651.1(8) Å3. The structure was determined by direct techniques to yield a tentative structural model that has been refined up to R 1 = 0.135 for 4088 reflexes with F 2 > 4σF. The crystal structure of Na3K5Cu8O4(SO4)8 is based on chains of [O2Cu4]4+ consisting of rib-coupled oxy-centered tetrahedrons of (OCu4)6+. The chains are surrounded by sulfate radicals, resulting in columns of {[O2Cu4](SO4)4}4? aligned along the b axis. The interchain space contains completely ordered positions of Na+ and K+ cations. The principle underlying the connection of NaKCu2O(SO4)2 columns in the crystal structure of {[O2Cu4](SO4)4}4? is different, in view of the relation Na:K = 1 as contrasted with 3:5 for the compound Na3K5Cu8O4(SO4)8. The presence of oxy-centered tetrahedrons in the structure of these new compounds furnishes an indirect hint at the importance of polynuclear copper-oxygen radicals with centering oxygen atoms as forms of transport of copper by volcanic gases.  相似文献   

18.
The effects of temperature, fO2 and composition on the electrical conductivity of silicate liquids have been experimentally determined from 1200 to 1550°C under a range of fO2 conditions sufficient to change the oxidation state of Fe from predominantly Fe2+ to Fe3+. Oxidation of ferrous to ferric iron in the melt has no measurable effect on the conductivity of melts with relatively low ratios of divalent to univalent cations. Under strongly oxidizing conditions a minor decrease of conductivity is detected inth highΣM2/ΣM+ ratios. It is concluded that for purposes of estimating the conductivity of magmatic liquids, fO2 may be ignored to a first approximation. Both univalent and divalent cation transport is involved in electrical conduction. Melts relying heavily on divalent cations for conduction, i.e. melts with relatively large ΣM2+/ΣM+ ratios, show strong departures from Arrheenius temperature dependence with the apparent activation energies decreasing steadily as the temperature increases. Conductivities dominated by the univalent cations, in melts with relatively small ΣM2+/ΣM+ ratios, show classical Arrhenius temperature dependence. These observations are discussed in terms of the general characteristics of the melt structure.Compositional variations within the magmatic range account for much less than an order of magnitude variation in electrical conductivity at a fixed temperature. This observation, combined with previous measurements of the conductivity of olivine (A. Duba, H.C. Heard and R. Schock, 1974) make it possible to state with reasonable confidence that melts occurring within the mantle will be more conductive by 3–4 orders of magnitude than their refractory residues. Potential applications to geothermometry are discussed.  相似文献   

19.
Gypsum is an authigenic precipitate that forms under periods of accentuated aridity and occurs widely in arid zones. However its use in quantitative paleoclimatology has been limited due to the absence of a method to determine the timing of its formation. We present here the results of a feasibility study that demonstrates that the timing of the formation event of gypsum can be estimated using Electron Spin Resonance (ESR) analysis. We used well documented samples from White Sands in New Mexico, USA, the Thar Desert, India and lakes in the Simpson Desert and Mallee Region, Australia and found that ESR ages could be obtained using radiation sensitive SO4?, SO3? radicals and a photobleachable signal O3?. ESR signals were consistent with control ages based on contextual information. These suggest that the dating signals (SO4?, SO3?) are stable over time scales >100 ka. We propose that this stability of the SO4? signals over geological time scales arises due to hydrogen bonding between the water proton and the SO4? radical and that the suitability of these radiation-induced radicals comes from their being a part of the host matrix. Further, ESR along with Fourier Transform Infrared (FT-IR) Spectroscopy methods additionally inform on the geochemical pathways for gypsum formation and help elucidate complex formation processes even in samples that appeared unambiguous gypsum precipitates. Thus, the presence of Hannebachite (CaSO3.½H2O) and Mn2+ in Thar and Australian samples suggested a reducing environment such that low valence sulfur reacted with CaCO3 to form hannebachite and eventually gypsum. The presence of sulfur, partially as sulfite in Thar gypsum samples suggested that redox cycles were mediated by microbial activity. Absence of these features in White Sands samples suggested oxic conditions during gypsum precipitation.  相似文献   

20.
Magnetic hysteresis effects have been observed in ferromagnetic resonance (FMR) spectra obtained at 9 and 16 GHz for certain simulated lunar glasses which were reduced by H2 in the melt and rapidly quenched. Transmission electron microscopy has revealed that these samples contained spherical particles in the size range ～0.01–0.5 μm. FMR spectra obtained at 35 GHz (applied field ～ 12.5 kOe) exhibited a line shape characteristic of spherical, single-domain (SD) iron particles with no hysteresis. Computer simulations of the latter spectra confirmed that the average deviation from sphericity must be ?3% and that (2K1/Ms) ≈ + 600 Oe for the precipitated magnetic phases. The principal features of the spectra obtained at all three frequencies have been explained on the basis of a simple theoretical treatment for spherical iron particles which have 2 domains in applied fields ?7 kOe, but become saturated at higher fields. Isothermal remanent magnetization (IRM) of these samples has been studied by both FMR and standard static techniques; the mean coercive force measured by the former (～4 kOe) contrasts with the mean value determined by the latter (～550 Oe). Apparently, FMR singles out and even amplifies the contributions of two-domain particles (which are magnetically hard), while the static measurement is more sensitive to the average of all particles present. The intensity of the FMR hysteresis of typical lunar soils is found to be ～1% of the total FMR intensity. In spite of this seemingly small value, two-domain iron particles may be important carriers of natural remanent magnetization (NMR) in certain lunar rocks.  相似文献   

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