Magnetocrystalline anisotropy of rocks and massive ores: A mathematical model study and its fabric implications

A simple mathematical model has been used to evaluate the influence of grain magnetocrystalline anisotropy and the scatter of crystallographic axes of grains on the magnetic anisotropy of rocks and massive ores whose carrier of magnetism is a magnetically uniaxial mineral of the type of pyrrhotite or hematite. The variation in magnetic anisotropy of rocks and ores whose carrier of magnetism displays the magnetocrystalline anisotropy greater than 100 is due to the variation in the preferred orientations of crystallographic axes, while the influence of the variation in the grain anisotropy on the rock (ore) anisotropy can be neglected.     

La mesure de l'anisotropie de susceptibilité magnétique dans les flyschs: Application à la recherche des directions des paléocourants

Studies made on the anisotropy of magnetic susceptibility of artificial sediments have shown that there are correlations between the orientation of anisotropy axes and of transport directions. Thus, the analysis of parameters of anisotropy of magnetic susceptibility can constitute a means to investigate transport directions in flysch. To test this method in a natural model, the authors have made some measurements on flysch beds belonging to the “grés d'Annot complex”. The sediments have been sampled in the Contes and Peira-Cava synclines where the directions of paleocurrents have been determinated by classical methods. Results obtained both in Contes and in Peira-Cava on fine-grained detrital beds show a good agreement between directions of paleo-transport deduced from orientation of anisotropy axes and those established by classical methods on the levels showing “sole-marks”. However, in some beds of the Peira-Cava syncline, which are characterized by coarser grain size and “sole-marks” of good quality, the scattering of the directions of the anisotropy axes is very great and in some cases the measurements are not reproducible in the same sample.It appears, therefore, that the measurement of the anisotropy of magnetic susceptibility can constitute a good approach to analysing emplacement modes of fine-grained flysch deposits in which the use of classical methods is often difficult. This method of paleocurrent analysis complements the other techniques used in this field of research.     

Magnetic susceptibility anisotropy in the Cambrian slate belt of North Wales and correlation with strain

The magnetic susceptibility anisotropy of 275 specimens comprising 38 sites from the Cambrian slate belt in North Wales was measured to determine the magnetic fabric of the slates. The susceptibility ellipsoid is oblate for all sites, and the maximum/intermediate susceptibility plane always coincides with the cleavage plane of the slates which has a Caledonian strike and is nearly vertical. The maximum axes align sub-vertically and the intermediate axes sub-horizontally, trending NE-SW. The minimum susceptibility axes are normal to this foliation plane and coincide with the poles to the slaty cleavage. The orientations of the principal susceptibility axes are found to be in excellent agreement with the orientations of the principal strain directions, determined by X-ray goniometry on one of the samples from almost all of the sites. Correlation of the magnetic susceptibility anisotropy with predicted March strains (March, 1932) shows that the principal magnitudes of susceptibility can be related to those of the strain by: (for i = 1, 2, 3. The orthogonal principal axes), where χ_f and χ₀ are the final and initial susceptibilities along a given axis i and l_f and l_i are final and initial axial dimensions in the same direction i of a principal strain axis. The exponent a for the North Wales slates was found to be 0.145 ± 0.005. Knowledge of such a relationship may permit rapid approximate determinations of a petrofabric in similar rocks from their magnetic fabrics. However, the exponent a will probably have to be recalibrated for each rock type.     

高温循环作用后大理岩裂纹扩展试验研究

利用偏光显微镜对经历不同高温循环作用后的细粒大理岩进行观测,通过统计微裂纹长度、开度、数量等参数,探讨不同加热循环次数后岩样内微裂纹的扩展规律。结果表明,（1）经历不同热循环次数作用的岩样,微裂纹的数量和长度均不同,未经热处理岩样内颗粒致密,胶结较好,内部无明显的微裂纹。经历4次热循环作用后岩样内微裂纹发育明显,以晶界裂纹为主,开度和长度均显著增加。岩样经历16次热循环作用后开度持续增大,为55 µm,是4次热循环的4倍;（2）对不同热循环处理后岩样内部发育微裂纹的方向进行了统计,发现微裂纹无明显的方向性,还表明细观切片的取样位置对微裂纹的发育影响不大;（3）线性裂纹密度随热循环次数的增加而增大,经历16次热循环作用后岩样的线性裂纹密度是没有经历热处理岩样的21倍。     

The sheath folds in the Tananao Group between Tienshiang and Tailuko, east-west cross-island highway, Taiwan

Sheath folds or “eye” folds on decimetric to metric scales are well-developed in the metachert-marble-green rock interlayers of the Changchun Formation and in the marble lens of the Tienhsiang Formation, within the Tananao Group between Tienhsiang and Tailuko, along E-W cross-island highway of Taiwan. Closely associated with the sheath folds are the tight to isoclinal folds with rectilinear axes which are parallel to the hinge line of the “eyes”, and the directions of these folds range from N-S to N30°E with gentle plunges to the north or south.The sheath folds are believed to have been formed during the second phase of deformation in this region. The traces of the earlier folding can generally be found at the hinges or limbs of these sheath folds.The explanation presented here is that the sheath fold might be generated episodically during the F₂ deformational phase throughout the entire history of progressive shearing as a result of episodic instability of the flow with successive refolding of metamorphic fabric, during Plio-Pleistocene deformation of Taiwan.     

The magnetic fabrics of some slates from the borrowdale volcanic group in the English Lake District and their correlations with strains

The magnetic fabrics of 150 samples from 25 sites from the English Lake District were determined using the magnetic susceptibility anisotropy technique. The susceptibility ellipsoid for 22 of the sites is oblate and for the remaining 3 prolate. The maximum intermediate susceptibility planes of the ellipsoids always coincide with the recorded cleavages which have a Caledonian strike and are sub-vertical. The orientations of the principal susceptibility axes are found to be in excellent agreement with those of principal strains determined by researchers using petrofabric methods on samples from near the author's sites. A correlation of the magnetic susceptibility anisotropy with the recorded strains shows that the magnetic anisotropy parameters can be related to those of strain by the empirical relationship: (for i = 1, 2, 3; j = 1, 2, 3 and i ≠ j), where χ_i and χ_j are orthogonal principal axes and li and l_j are the corresponding orthogonal principal strain axes. The exponent a for the correlated sites from the andesitic tuffs of the Borrowdale volcanic group in the English Lake District is 0.142 ± 0.001. This index corresponds closely to that (0.145 ± 0.005) found for the Cambrian slate belt of North Wales (Rathore, 1979).     

Preliminary investigation results on fabrics and related physical properties of an anisotropic gneiss

In order to evaluate the fabric-dependent anisotropy of a particular gneiss type, we assessed the quartz lattice and shape preferred orientations as well as the microcrack pattern statistics. In the rock mechanics laboratory, several strength and deformability tests on drilled rock samples were run and the seismic wave propagation properties along the rock’s principal strain axes were determined. In the mechanical tests it turned out that despite a distinct stretching lineation with initially extreme grain elongation, the rock performs for the larger part as a transversely isotropic material with the schistosity as plane of isotropy. The anisotropy in seismic wave propagation within the plane of schistosity is attributed primarily to a highly anisotropic microcrack pattern, the distinct gneissose banding and to a lower degree to the LPO of quartz.     

Microwave response of green vegetation

The cavity perturbation technique has been used to measure the dielectric constant of vegetation, such as soyabean, grass, corn and green spruce at 9.5 GHz. The dielectric properties of these vegetations are highly dependent upon the moisture content. For freshly plucked specimen the dielectric constant was found to be maximum which decreased gradually as the specimen dried up. The role of these basic physical parameters in microwave scatterometer experiment has been discussed. Some parameters of remote sensing application have been estimated from these measurements and their practical role has been discussed     

The relationships of thrust and shear deformations in the southern part of the Polar Urals as indicated by petromagnetic data

Based on the petromagnetic characteristics of rocks that crop out within the Main Ural Fault zone (MUF) and Voykar–Synya ophiolites the multistage pattern of deformation processes in the study areas has been established. An analysis of the anisotropy of magnetic susceptibility of rock minerals detected the orientation, which allows us to reveal magnetic orientations in minerals recorded as a results of thrust (overthrust) and strike-slip deformations at an early stage of the evolution of the Ural orogenic belt. The main axes of the ellipsoid of anisotropy of the magnetic susceptibility (AMS) associated with regional thrusting, namely, the main stage of formation of the structure of the Urals, have rarely been revealed. This is evidence that thrust petrofabrics are almost completely veiled by superimposed shear deformations.     

Soft rock pillars

Summary Soft rock pillars can be designed by several methods available in the mining literature. All of these methods include the effect of shape, or geometry, on the average strength of specimens and pillars. All of the pillar design methods include some measurement of the strength of specimens of the pillar rock. The most common rock specimen strength property measured is the unconfined compressive strength. However, the average strength of triaxially confined rock specimens is much greater than the unconfined specimen strength, which can be more important to pillar strength. The estimation of the strength of a pillar is complicated by the decrease in rock specimen strength with increase in specimen size.Editor's note: In common with North American engineering practise, the paper uses English units throughout, where feasible conversions are included in the text. Where not, the following factors may be used: 1 inch=25.4 mm; 1 ft=0.3048 m; 1 lbf/in.^–2=6.895 kn/m^–2; 1Tonf.=8.896 kN.     

Variability of East Asian Winter Monsoon in Quaternary Climatic Extremes in North China

In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess–paleosol sequences in the Chinese Loess Plateau were investigated. Sandy layers in the loess deposits, the “Upper sand” and “Lower sand” (layers L9 and L15, respectively), which represent a high-resolution record of paleomonsoon changes, have been sampled at intervals of 5–6 cm from sections at Luochuan and Xifeng. The grain size and magnetic susceptibility was measured for all samples. The grain-size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. However, the winter monsoon was relatively stable during warm periods. The magnetic susceptibility signal (a proxy of summer monsoon intensity) is practically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr. The detection of this oscillation in older glacial stages strongly suggests that it may be a pervasive cycle of the cold climatic phases of the Quaternary. Millennial-scale variations of the winter monsoon may be caused by instability of the westerly jet, which is determined by temperature differences between the polar and the equatorial regions.     

冻融损伤与围压对层状岩石强度各向异性的影响北大核心CSCD

层状岩石具有显著的层理面结构,层理面结构的方向决定了岩石的各向异性,相邻层面的岩石物理力学性质差异和连接强度决定了各向异性的强弱,此外,层状岩石的层理结构使其易受冻融循环(FT)等风化作用的影响。为了研究冻融损伤与围压对层状岩石强度各向异性的影响,分别在0、10、20、30和40次的冻融循环和0、5、15、25和40MPa的围压条件下,使用岩石三轴试验系统(GCTS)对五种不同层理倾角的层状砂岩试件进行试验。试验结果显示,冻融循环会使岩石层理面产生裂隙,更易发生沿层理面的剪切滑移破坏,冻融40次后,β=0°的试件单轴抗压强度降低14.5%,而β=67.5°的试件单轴抗压强度降低57.9%,使层状岩石的固有各向异性增强。围压对岩石的各向异性影响与其固有各向异性有关,表现为随着冻融次数的累积,围压对岩石的应力诱导各向异性由起初的增强逐渐转变为抑制。本研究可为寒区层状岩石工程的设计、施工和安全运行提供理论支持。     

岩石各向异性实验研究及对工程勘探的影响

以不同的砂岩和大理岩为标本,对岩石的各向异性在应力应变关系、波速、动静杨氏模量、动静泊松比、抗压强度等方面的各向异性问题进行了测试和研究。通过实验得到岩石标本在平行层理和垂直层理方向上述物理性质存在着的差异情况和变化规律,分析和探讨了岩层的各向异性对工程勘探工作带来的影响情况。指出地壳介质的岩体各向异性对地球物理勘探来说是不能忽视的,尤其是各向异性会导致横波分裂,影响横波勘探的精度,但同时也有助于了解岩体中裂纹结构的信息,做出工程事故的准确预报。     

Magnetic anisotropy of hematite natural crystals: increasing low-field strength experiments

Hematite is a very abundant mineral in natural rock samples. Despite being one of the most important carriers of remanent magnetization, its magnetic anisotropy is not well understood partially due to its high coercivity and complex behavior. In particular, the field intensity beyond which the Rayleigh relation no longer holds varies from one crystal to another. This field threshold is usually less than the field used in most commercial instruments. The nonlinear behavior of low-field susceptibility may thus hinder the magnetic fabric analysis. We have carried out an intensive study of the low-field bulk susceptibility and anisotropy of magnetic susceptibility (AMS) at increasing low fields in the range of 2–450 A/m (effective value) in a collection of hematite natural crystals. Standard rock magnetic properties, X-ray diffraction, and mass spectrometry have also been determined in order to discover the parameters influencing the low-field susceptibility variations with field. The AMS principal directions, the shape of the AMS ellipsoid, and the degree of anisotropy are the parameters that can vary with different applied fields. It has been found that there is no correlation between magnetic properties like coercivity or saturation magnetization and the range in which the Rayleigh approximation is valid. However, there seems to be a correlation with the peak width determined from X-ray diffraction, suggesting that the Rayleigh region in hematite crystals is related to the spatial orientation of the physical domains within the basal plane.     

Magnetic anisotropy of hematite natural crystals: high field experiments

Hematite is one of the most important carriers of remanent magnetization in natural samples. Its strong magnetocrystalline anisotropy makes it difficult to determine one single value for the magnetic anisotropy constant and other magnetic properties. In particular, the anisotropy of hematite within its basal plane is controversial because an assumed triaxial anisotropy compatible with the crystallographic structure has not always been detected. This study presents a comparative analysis of rock magnetic properties, compositional analysis and determination of the magnetic anisotropy constant. Different models with anisotropy constant within the basal plane ranging from 0 to 13 (J m⁻³) are considered in the evaluation of the factors that control the presence or absence of a triaxial anisotropy. A linear relationship between saturation magnetization (M _s ) and coercitivity (B _c ) in measurements at increasing temperatures is observed in samples where anisotropy is either uniaxial or biaxial while those with significant triaxial anisotropies have a power–law relationship between M _s and B _c .     

Assessment of rock geomechanical quality by quantitative rock fabric coefficients: Limitations and possible source of misinterpretations

Rock microfabrics were quantified by image measurement system (petrographic image analysis) of thin sections. Along with common measured parameters (grain area and size, length of major and minor axes and their orientation), some quantitative parameters (aspect ratio, grain boundary smoothness) and fabric coefficients (micropetrographic quality index (K), “texture” coefficient (TC), indexes of interlocking (t) and grain size homogeneity (g)) were determined. The reliability of these parameters for the evaluation of geomechanical properties was tested on the set of granitic rocks showing pronounced variability in rock mechanical properties. None of the tested rock fabric coefficients or parameters proved a close correlation to rock mechanical properties except grain size. Based on experimental findings, the grain size is, along with porosity, the controlling factor of rock mechanical properties of genetically and mineralogically similar rocks. Obliteration of the real effects of rock fabric parameters on the mechanical properties of rocks by application of complex fabric coefficients disables their practical use in petrography applied to geomechanics.     

有明黏土各向异性固结特性的试验研究

由于大多数黏土矿物是片状的,在沉积和随后的固结过程中黏土颗粒和组构单元趋向水平定向排列,使天然沉积黏土呈现各向异性的固结特性。通过垂直切和水平切试样的恒应变速率固结试验研究了日本有明黏土的各向异性固结特性。试验结果表明,水平切试样的先期固结压力pch与垂直切试样的先期固结压力pcv的比值在0.5～1.0之间,主要是因为天然沉积黏土各向异性的屈服轨迹以及垂直切和水平切试样所经历的应力路径不同;在正常固结阶段,水平切试样的固结系数ch与垂直切试样的固结系数cv之比约为1.43,水平切试样的渗透系数kh与垂直切试样的渗透系数kv之比约为1.40,水平切试样的体积压缩系数mh与垂直切试样的体积压缩系数mv基本相等。因此,有明黏土固结系数的各向异性主要是因为渗透系数的各向异性,而渗透系数的各向异性本质上是因为其微观结构的各向异性。     

Magnetic fabric study across the Ailao Shan–Red River shear zone

The anisotropy of magnetic susceptibility (AMS) of 351 specimens from 51 sites across the Ailao Shan–Red River shear zone (ASRR) was measured to determine its magnetic fabric. Rocks range westward from core schistose gneiss, through low-grade schist, to Triassic sediment. Magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric. P_J value drops by 22.4% in the low-grade schist and by 27.4% in the Triassic sediment on average with respect to the gneiss, suggesting a rapid decrease of deformational intensity. The directions of principal susceptibilities are closely related to the deformation of the Ailao Shan–Red River shear zone. The susceptibility plane always coincides with the schistosity or cleavage plane. Most of the maximum susceptibility axes trend NW–SE. In the shear zone, the maximum susceptibility axes (K_max) are parallel to the lineation within the foliation plane. With increasing distance from the shear zone, there is a trend that they become parallel to the down-dip of reverse faults or cleavage. This indicates changes in deformation mode, inside and outside the shear zone. Within the shear zone, horizontal movement is dominant. Outside, shortening prevails. The overall minimum magnetic axes align NE–SW with subhorizontal to low dip angles, suggesting that the dominant shortening is NE–SW directed. Caution should be exercised when AMS is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis (K_min) and pole of foliation is small, and also because the attitude of foliation varies from place to place. They result in unreliable or even wrong shear sense. Another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section. With these data, it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset.     

辉绿岩裂缝对其磁组构的影响

磁组构是由岩石中磁性矿物定向分布而产生的组构特征,因此磁组构分析是研究岩石组构常用的技术手段,它具有 见效快、灵敏度高、无损样品等特点,近年来得到广泛应用。然而在磁组构研究工作中钻取定向样品或定向手标本时,常 常会遇到天然样品含有裂缝且裂缝中充填大量近地表沉积物和自生矿物。本文针对辉绿岩裂缝充填物对其磁组构结果的影 响程度做了详细研究,选用160个含裂缝的辉绿岩岩芯定向样品,通过对比去除裂缝充填物前后的磁组构变化,发现实验 前后所有样品磁化率大小变化率平均值<1%,磁组构方向变化平均值<1°,因此认为辉绿岩裂缝物质对其磁组构影响甚微。 矿物学和岩石磁学分析表明裂隙充填物主要为石英、长石,以及少量赤铁矿、黄铁矿和绿泥石等表生矿物。而辉绿岩的携 磁矿物主要是亚铁磁性的磁铁矿,其磁化率强度约为同等质量裂缝填充物的30倍,致使辉绿岩中的裂隙充填物对磁组构的 影响很小。     

Comparison between “magnetic” and sedimentary fabric in graded and cross-laminated sand layers,Southern California

Paleocurrent directions can be reconstructed by determining optically the preferred orientation of longest grain axes of sediments; this can be estimated by measuring the anisotropy of magnetic susceptibility (Rees, 1965). In order to check the applicability of both methods, the orientation of large- and small-scale sedimentary structures (cross-bedding, sole markings, grain orientation) was compared with the “magnetic fabric” of some sand and silt layers of different ages and origins:

1. Rosario Formation (Late Cretaceous), La Jolla: thin siltstone layers with small-scale current-ripple cross-lamination;
2. Monterey and Topanga Formations (Miocene, Los Angeles Basin): graded and laminated sandstone beds (“turbidites”);
3. Holocene submarine La Jolla Fan: graded and cross-laminated sand layers (Rees, v.Rad &Shepard, 1968).

Most of the investigated sand layers have a “primary-style” magnetic fabric, characterized by well-grouped, horizontal maximum susceptibility axes, sometimes slightly imbricated upcurrent, minimum susceptibilities normal to the bedding, and intermediate values of the factor q (about 0.1–0.5), suggesting moderately strong current conditions during deposition. The orientation of maximum susceptibility axes correlates with the directions of sand transport, inferred from cross-lamination foresets and sole markings (a). Also conventional optical grain orientation measurements give very similar directions as maximum susceptibilities (b). In Recent deep-water sand layers (c) cross-lamination foresets and maximum susceptibility axes are parallel, and influenced by the local morphology (e. g. fan-valley axis). Since current-parallel orientation of the longest grain axes and maximum susceptibility axes is prevailing in deep-water sands, the magnetic method is a useful tool for palcocurrent analysis. Characteristic differences of the magnetic fabrics from different depositional environments may also give clues to the mechanisms of sediment transport.