Elasticity of ilmenites

Ultrasonic data for the velocities of the ilmenites MgTiO₃ and CoTiO₃ have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed in a piston-cylinder apparatus at pressures up to 30 kbar. Titanate and germanate ilmenites define divergent isostructural trends on a Birch diagram of bulk sound velocity (υ_φ) vs. density (ρ). On a υ_φ vs. mean atomic weight ($\text{M}$) diagram, however, all of the ilmenite consistent with a single $\text{υ}{}_{\mathrm{\phi }}\text{M}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{=}\text{constant trend}$. Elasticity systematics for isostructural sequences are used to e the bulk modulus (2.09 Mbar) and bulk sound velocity (7.4 km/sec) of MgSiO₃-elmenite.     

The Néel transition and magnetic properties of terrestrial,synthetic, and lunar ilmenites

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.     

Magnetic properties of natural Mn-oxides

Summary The paper is devoted to the study of the magnetic properties, of the volume magnetic susceptibility in a weak magnetic field, of the specific magnetic susceptibility in a strong magnetic field, of natural remanent magnetization and its stability in A.C. fields, and of the temperature dependence of magnetic susceptibility of natural Mn-oxides. Samples of pyrolusite, polianite, manganite, hausmannite, psilomelan and wad from different localities all over the world were studied; the measured values are statistically treated so that the given values are valid in general. Hausmannite displayed typical values of the magnetic susceptibility, different from the values of the other Mn-oxides. In general, many natural Mn-oxides contain heterogeneous admixtures of Fe-oxides, which are responsible for their ferromagnetic properties. A high degree of magnetic hardness, predetermining the minerals for palaeomagnetic research, was proved with many of the natural Mn-oxides.     

Magnetic properties of synthetic titanomagnetites

Magnetic properties and crystal structure parameters of synthetic solid solutions Fe₃O₄Fe₃TiO₄, Fe₂O₄MgFe₂O₄ and Fe₃O₄Mg₂TiO₄ have been studied. Basic regularities in the behaviour of saturation magnetisation (I_s), Curie temperature (T_C) and cubic lattice parameter a during the substitution of Ti and Mg ions for Fe ions have been found. As the concentration of Ti ions increases, I_s reduces from 70 Gs·cm³ g^?1 to 0, T_C changes from 580 to 130°C and a from 8.391 to 8.520 Å. Growth of the Mg concentration leads to changes in I_s to 19.8 Gs·cm³, g^?1, T_C, to 440°C and a, to 8.360 Å. The full Fe ions substitution gives $\text{“a”=8.440}\text{A}\text{?}$.Chemical compositions of the samples, in which the valency variation of Fe ions at oxidation leads to an increase in susceptibility and T_C, have been determined.     

Magnetic properties of monodomain aluminium-substituted titanomagnetite

A suite of synthetic titanomagnetites were prepared with compositions Fe_2.6?δTi_0.4Al_δO₄ and Fe_2.4?δTi_0.6Al_δO₄ (δ = 0, 0.1, 0.2 in both cases). Ball-milling of the synthesized samples produced material in the magnetic monodomain state as indicated by hysteresis loops and the Lowrie-Fuller test. The coercive force of the specimens depends on the Al concentration and lies in the range 1–2 kOe. The TRM induced in the samples is correspondingly “hard”. The low-field (0–1 Oe) TRM acquisition curve is linear. The higher field TRM-H curve is not in agreement with either monodomain or two-domain theoretical models.     

Magnetic properties of ulvöspinel

Magnetisation measurements on ulvöspinel have shown that there is a transition from the weakly ferromagnetic state to an essentially antiferromagnetic one at T ～ 60–100 K when moderate measuring fields (24 kOe) are used. Cooling from above 100 K in the presence of a magnetic field of several kilooersteds produces a reversed remanence for $\text{T ? 40}\text{K}$ and the resulting thermomagnetic curve is Néel N-type. Magnetisation in 80 kOe produces a spontaneous moment extrapolated to 0 K of 0.015 μ_B, although this may not be completely saturated. An explanation for the magnetic transition is suggested in terms of an increased anisotropy possibly associated with a crystal transition.     

Magnetic and microstructural properties of some lodestones

An iron ore which behaves as a permanent magnet is lodestone. The intrinsic magnetic properties and the microstructual characteristics which are responsible for the permanent-magnet properties of the lodestone have never been described or explained. Iron ores capable of being charged sufficiently strongly to behave as permanent magnets are defined here as proto-lodestones and fall into two categories — Class I which contains Fe as the only significant cation, and Class II which contains considerable Ti, Mg and Al as well. Proto-Lodestones are magnetically hardened by oxidation and precipitation processes which produce the microstructure responsible for the permanent-magnet properties. Lodestones are charged proto-lodestone iron ores. Lodestones have R_H values (ratio of remanent coercive force, H_R, to coercive force, H_C) between 2.0 and 2.5; R_I values (ratio of saturation remanence, I_SR, to saturation magnetization, I_S) >0.1–0.25 and the ratio of NRM/SIRM (natural remanence to saturation remanence) is 0.15–0.7. The intrinsic magnetic properties and microstructural characteristics of proto-lodestones and other iron ores are described and explained. The mechanism of charging the proto-lodestone appears to be either transient magnetic fields associated with lightning-discharge currents or presently obscure aspects of magnetization intensity enhancement associated with maghemitization of massive iron ores.     

Magnetic and crystallographic properties of synthetic titanomaghemite

Systematic magnetic and crystallographic studies were made on 57 titanomaghemite samples produced from sintered titanomagnetites with x values of 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0. Curie temperature and lattice parameter values are often significantly different from results of previous studies (Ozima and Sakamoto, 1971; Readman and O'Reilly, 1972; Nishitani, 1979), particularly for x values of 0.4 and 0.6. Other important results are: (1) Oxidation of titanomagnetite is mainly a function of temperature. Atmosphere and Ti content have little influence. (2) Progressive oxidation of homogeneous titanomaghemites can take place in the presence of a rhombohedral phase. The amount of rhombohedral phase produced during oxidation increases with increasing temperature and (less conclusively) with decreasing Ti content. (3) Saturation magnetization measurements at low temperatures show both P- and Q-type ferrimagnetic behavior in titanomaghemites, and also show the Verwey (?) transition in titanomaghemites with composition x=0.0. (4) The temperature of spinel inversion increases with degree of oxidation. Slightly oxidized samples invert near 300°C; for samples with z > 0.8 the inversion temperature is above 450°C. This last result, although neither expected nor understood, is supported by results of thermomagnetic studies on some oceanic basalts.     

Magnetic properties and paleoclimatic implications of loess-paleosol

Detailed rock magnetic investigations and X-ray diffraction (XRD)were carried out on loess-paleosol sequences of the last interglacial-glacial at Znojmo section in Czech Republic. The results indicate that pedogenesis causes susceptibility enhancement in the paleosols, which is similar to that observed in the Chinese Loess Plateau. k-T curves, IRM, and XRD show that magnetite is the dominant magnetic mineral in the loess-paleosol sequences at the Znojmo section, while maghemite, hematite, and pyrite/pyrrhotite are minor minerals. Measurements of anisotropy of magnetic susceptibility (AMS) indicate that the magnetic lineation is smaller than the foliation. The susceptibility ellipsoids are oblate and the directions of the maximum principal axes (Kmax) are distributed randomly, and cannot be used to determine the paleo-wind direction.     

Magnetic properties of synthetic six-line ferrihydrite nanoparticles

Three synthetic samples of six-line ferrihydrite (Fh5.4, Fh4.1, and Fh3.0), with average particle sizes of 5.4, 4.1, and 3.0 nm (respectively), have been studied by low-temperature magnetic techniques, Mössbauer spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy. Magnetic data indicate that these nanoparticles are not only antiferromagnetic, but that they also possess a ferromagnetic-like moment due to the presence of uncompensated spins. Both magnetic and Mössbauer data indicate that nanoparticles of six-line ferrihydrite are superparamagnetic at room temperature, with a low-temperature transition between blocked and unblocked magnetic states dependent on average particle size. In particular, low-field magnetic susceptibility data display a peak in amplitude at 45 K (Fh3.0), 55 K (Fh4.1), and 80 K (Fh5.4). Low-temperature induced magnetization data, acquired in magnetic fields up to 5 T, also display clearly a superparamagnetic behavior. These data were tentatively modeled as the sum of two contributions: a linear term due to the antiferromagnetic susceptibility and a nonlinear term due to the uncompensated spins. Model estimates of the magnetization carried by the uncompensated spins (M_nc) show a decrease in M_nc with increasing temperature. Extrapolation of M_nc values down to zero provided an estimate of the Néel temperature for six-line ferrihydrite on the order of 500 K.     

X-ray study and mössbauer spectroscopy on lunar ilmenites (Apollo 11)

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.     

Magnetic properties related to thermal treatment of pyrite

Detailed rock magnetic experiments were conducted on high-purity natural crystalline pyrite and its products of thermal treatments in both argon and air atmospheres. In argon atmosphere (reducing environment), the pyrite is altered by heating to magnetite and pyrrhotite; the latter is stable in argon atmosphere, and has coercive force and coercivity of remanence of ～20 and ～30 mT, respectively. Whereas in air, the pyrite is ultimately oxidized to hematite. First order reversal curve (FORC) diagram of the end product shows that the remanence coercivity of hematite is up to ～1400 mT. The corresponding thermal transformation process of pyrite in air can be simply summarized as pyrite→ pyrrhotite→magnetite→hematite. These results are helpful for understanding of sedimentary magnetism, secondary chemical remanence and meteorolite magnetic properties.     

Titanomagnetites and ilmenites from the Early Cenozoic basalts and limburgites of the Northern Tien Shan

Based on X-ray spectrometry and thermomagnetic analysis, the chemical composition is studied and the Curie points are determined for the Early Cenozoic basalts and limburgites of the Northern Tien Shan. The microprobe analysis used in combination with the thermomagnetic analysis unambiguously identified a series of homogeneous primarily magmatic titanomagnetites in the studied samples against the broad variety of grains of titanomagnetite that underwent single-phase heterophase oxidation, which are often undetectable by a microprobe alone. The titanium content or TiO₂/FeO ratio in titanomagnetites and, correspondingly, the Curie points reflect the depth of the most recent equilibrium state of the magma, i.e., the depth of the magma sources. According to the dependence of the content of primarily magmatic titanomagnetite on the source depth, the latest equilibrium state of the magmatic melt of the Northern Tien Shan occurred at a depth of 40 ± 5 km. The obtained results agree with the reduction in the seismic velocities by a few percent below the Moho revealed by seismic tomography.     

Magnetic properties of some tertiary basalts from West Greenland

Summary Measurements of bulk magnetic properties, including the natural remanent magnetization (NRM), susceptibility and the Königsberger ratio, on over 250 samples of Tertiary basalts from Disko and Nûgssuaq, West Greenland are reported.The NRM intensities in basalts (geometric mean value 3.3 A/m in SI units) were on average three to four times as large as the induced magnetization intensities. The susceptibilities (geometric mean value 2.1×10^–2 SI units) were much more uniform than the NRM intensities. In the majority of samples, the NRM was predominantly of reverse (R) polarity, but samples from a few sites showed a remanence of normal (N) polarity.The NRM of both polarity classes (N, R) was very stable against alternating field (AF) demagnetization with median destructive fields of the order of 20,000–30,000 A/m (250–350 Oe), comparable to those for many stable continental and oceanic basalts. The viscous remanence intensity, as studied by storage tests on some specimens, was found to be an insignificant fraction of the original NRM, except in few cases.The low field hysteresis loops (Rayleigh loops) were studied for some specimens. A qualitative association was noted between wide hysteresis loop and relatively low AF stability, but no correlation was apparent between the loop type and the Königsberger ratio (Q _n) of a specimen.Contribution no. 6 Institute of Geophysics, University of Copenhagen.     

Magnetic properties of the Tertiary red clay from Gansu

The study on magnetic properties of the red clay indicates that the red clay and loesspaleosol sequence have a common magnetic mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pedogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation in the red clay thus also provides an eolian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition is similar to the climatic shift from stage 5 (S1) to stage 2–4 (L1). This may suggest that during the Quaternary there is an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important environmental change from the underlying Cretaceous red sandstone. The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition.     

Magnetic properties of olivine basalt: Application to Mars

The natural remanent magnetisation (NRM) of basalt lava containing (oxidised) magnetic phases is usually assumed to be proportional to the weight percent of magnetic phases. It is shown here that olivine basalt has a different behaviour. The NRM intensity increases at an increasing rate with the amount of magnetic phases. This is attributed to the oxidation of olivine during cooling of the basalt that leads to the exsolution of magnetite in a single domain state. In this way, olivine basalt is found to become an order of magnitude more magnetic than basalt that does not contain olivine. A simple explanation for the magnetic anomalies on Mars is offered, based upon these findings and Mössbauer spectroscopy data from Gusev crater on Mars.     

Magnetic properties of the Cunene Anorthosite Complex,Angola

The Cunene Complex is the largest known anorthosite body and outcrops across the border between Angola and South West Africa. Palaeomagnetic results are reported from a traverse across the dark troctolitic facies of the anorthosite in Angola which yielded fifteen sites with two additional sites in gabbro bodies. Fourteen sites are stable to a.f. demagnetisation and a single site in the cumulative border zone of the anorthosite is reversed with respect to the remainder. Twelve sites combine to give a mean direction of D = 259°, I = ?46° (k = 7) with a virtual geomagnetic pole at 255°E and 3°S. The low overall precision is probably due to apparent polar movement during cooling of the Complex. Radiometric data are currently conflicting and imply that the anorthosite has an age between 1100 and 2600 m.y.; the only clear feature to emerge from age studies is a thermal overprinting at ca. 1100 m.y. The directions of magnetisation are shown to be most consistent with an age of ca. 2100 m.y. with cooling through the Curie point continuing to ca. 2000 m.y.A variety of magnetic tests demonstrate that magnetite is the principal remanence carrier in the dark troctolitic anorthosite where it occurs both as discrete grains and as fine rods in plagioclase. Lowrie-Fuller tests suggest that both these components include single domains but results from separated mineral fractions demonstrate that the bulk of the high coercivity remanence resides in magnetite rods within the feldspar.     

Magnetic properties and emplacement of the Bishop tuff,California

 Anisotropy of magnetic susceptibility (AMS) and characteristic remanence were measured for 45 sites in the 0.76 Ma Bishop tuff, eastern California. Thirty-three sites were sampled in three stratigraphic sections, two in Owens gorge south of Long Valley caldera, and the third in the Adobe lobe north of Long Valley. The remaining 12 sites are widely distributed, but of limited stratigraphic extent. Weakly indurated, highly porous to dense, welded ash-flow tuffs were sampled. Saturation magnetization vs temperature experiments indicate two principal iron oxide phases: low Ti magnetites with 525–570  °C Curie temperatures, and maghemite with 610°–640  °C Curie temperatures. AF demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite/maghemite predominantly in the multidomain to pseudo-single domain size ranges. Remeasurement of AMS after application of saturating direct fields indicates that randomly oriented single-domain grains are also present. The degree of anisotropy is only a few percent, typical of tuffs. The AMS ellipsoids are oblate with K_min axes normal to subhorizontal foliation and K_max axes regionally aligned with published source vents. For 12 of 16 locality means, K_max axes plunge sourceward, confirming previous observations regarding flow sense. Topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from K_max axes. Deposition east of the Benton range occurred by flow around the south end of the range and through two gaps (Benton notch and Chidago gap). Flow down Mammoth pass of the Sierra Nevada is also evident. At least some of the Adobe lobe in the northeast flowed around the west end of Glass mountain. Eastward flow directions in the upper Owens gorge and southeast directions in the lower Owens gorge are parallel to the present canyon, suggesting that the present drainage has been established along the pre-Bishop paleodrainage. Characteristic remanence directions from 45 sites (267 samples) yield an overall mean of D=348°, I=53° for the Bishop tuff. A correlation is found in two of the three profiles between density and remanence inclination. A mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash-fall tuff at Lake Tecopa is: D=353°, I=54°, k=172, α₉₅=2.9°, N=15. Received: 11 July 1995 / Accepted: 29 February 1996     

Magnetic properties,self-reversal remanence and thermal alteration products of smythite

Summary This study is a follow up of the investigation of some magnetic properties and metastability of greigite in samples obtained from Miocene claystones in the Kruné hory (Erzgebirge) Piedmont basins (Bohemia). Three different methods of upgrading the smythite were applied; the magnetic properties of the concentrates are compared. The thermal conversion of smythite sets in at 200°C while greigite converts at 250°C. The first intermediate products to be formed are iron sulphides, marcasite clearly dominating over pyrite and pyrrhotite. Apart from a Fe³⁺ sulphate with a composition of Fe₂(SO₄)₃, oxidation of these sulphides gives rise to -Fe₂O₃. The result of the subsequent decomposition of the mentioned sulphate is the formation of -Fe₂O₃, which retains the sulphate structure. The final product of the thermal decomposition at 800°C is -Fe₂O₃. In the smythite concentrate the conversion to Fe³⁺ sulphate and -Fe₂O₃ is about twice as intensive as in greigite. No direct conversion to -Fe₂O₃ was found. During the thermal process self-reversals of remanence were observed, in various samples as many as four reversals in the temperature interval from 340 to 590°C. The occurrences of self-reversals of remanence were only observed at high degrees of thermal demagnetization, of the order of 10^–2 down to 10^–3 in the temperature interval of sulphide origin (below 400°C), and of the order of 10^–4 down to 10^–6 in the temperature interval of Fe-oxides origin (above 400°C).Presented at the 3rd Conference on New Trends in Geomagnetism, Castle of Smolenice, Czechoslovakia, June 22–29, 1992