The influencing factors and mechanisms of the electromagnetic radiation during rock fracture

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Magnetomineralogy of the cordierite gneiss from the magnetic anomaly at Humpolec,Bohemian Moldanubicum (Czech Republic)

Magnetic properties as well as the magnetomineralogy were investigated in rocks underlying a 7 km long aeromagnetic anomaly situated in the Moldanubian crystalline unit of the Bohemian Massif. The anomaly is caused by highly magnetic cordierite gneiss forming a stripe of NE — SW direction east of the town of Humpolec, eastern Bohemia. Magnetic susceptibility and its anisotropy (AMS), natural remanent magnetization, field and temperature variations of susceptibility were measured. Optical study of thin sections, electron microprobe and Mössbauer studies were also used to reveal the carrier of the high susceptibility and the high NRM. There appear to be two major generations of Fe-Ti oxides: older hematite with ilmenite exsolutions (Ti-hematite) which is the dominant remanence phase, and younger magnetite, the dominant susceptibility phase, usually associated with rutile. This indicates a reaction Hematite + Ilmenite → Magnetite + + Rutile; the trace elements in magnetite, as well as texture and morphology of the oxide grains support this assertion. An additional minor portion of maghemite is revealed by Mössbauer and thermomagnetic results. The Ti-hematite belongs to the oldest mineral assemblage in the rock, despite its anhedral morphology. Inclusions in Ti-hematite, among which corundum and abundant paragonite occur, record a strongly peraluminous and probably disequilibrium association during the crystallization of the Ti-hematite.     

Crystal-field spectra of fassaite from the Angra dos Reis meteorite

Fassaitic pyroxene from the Angra dos Reis meteorite has striking spectral properties. The⁵⁷Fe Mössbauer spectra show no Fe^3&+;, and thus the absorption is thought to originate from a complex charge-transfer process. Intense absorption at 480 nm dominates the spectrum of the meteorite and may be important in the interpretation of telescope spectra of objects in space.     

Chemistry and mineralogy of pyrite-enriched sediments at a passive margin sulfide brine seep: abyssal Gulf of Mexico

Pyrite is rapidly accumulating at the contact between the Cretaceous limestones of the Florida Platform and the hemipelagic sediments of the abyssal Gulf of Mexico. Sediments sampled with the submersible “Alvin” in 3266 m of water are associated with a dense community of organisms that depend on chemosynthetic primary production as a food source. Analysis of the chemistry, mineralogy, and textural composition of these sediments indicate that iron sulfide mineralization is occurring at the seafloor within an anoxic micro-habitat sustained by the advection of hydrogen sulfide-charged saline brines from the adjacent platform. The chemosynthetic bacteria that directly overlie the sediments oxidize hydrogen sulfide for energy and provide elemental sulfur that reacts with iron monosulfide to form some of the pyrite. The sediments are mixtures of pyrite ( 30 wt.%), BaSr sulfates ( 4 wt.%), clays, and locally derived biogenic carbonates and are progressively being cemented by iron sulfides. Oxidation of hydrogen sulfide produces locally acidic conditions that corrode the adjacent limestones. Potential sources of S, H₂S, Fe, Ba, and Sr are discussed.     

Environmental magnetism of nanophase iron minerals: Testing the biomineralization pathway

Nanoparticles of iron oxides and iron oxyhydroxides are sensitive proxies of environmental change near the earth's surface. Our research programme aims to provide information about the environmental control variables (temperature, rainfall, pH, as well as microbial type and concentration) for the process of Fe(III) to Fe(II) reduction and solid phase precipitation. Such solid phases of environmental change proxies (e.g., magnetite and maghemite) are being produced in the laboratory for calibration of field data. Low temperature magnetometry and Mössbauer spectra are shown to be extremely useful for characterization and granulometry of such nanophases. In addition, it appears that magnetism may also be a sensitive probe for physical–chemical properties of the outermost ‘skins’ of such nanoparticles, confirming Louis Néel's predictions for the magnetic behavior of such outermost layers.     

An Estimation Method of Pore Structure and Mineral Moduli Based on Kuster-Toksöz (KT) Model and Biot’s Coefficient

Pore structure and mineral matrix elastic moduli are indispensable in rock physics models. We propose an estimation method of pore structure and mineral moduli based on Kuster-Toksöz model and Biot’s coefficient. In this technique, pore aspect ratios of five different scales from 100 to 10^?4 are considered, Biot’s coefficient is used to determine bounds of mineral moduli, and an estimation procedure combined with simulated annealing (SA) algorithm to handle real logs or laboratory measurements is developed. The proposed method is applied to parameter estimations on 28 sandstone samples, the properties of which have been measured in lab. The water saturated data are used for estimating pore structure and mineral moduli, and the oil saturated data are used for testing these estimated parameters through fluid substitution in Kuster-Toksöz model. We then compare fluid substitution results with lab measurements and find that relative errors of P-wave and S-wave velocities are all less than 5%, which indicates that the estimation results are accurate.     

Characterization of Saale River Sediments by IR Spectroscopy Charakterisierung von Saale-Sedimenten durch Infrarotspektroskopie

The main constituents of sediment samples of the river Saale were analysed by IR spectroscopy. The composition of the sediments is determined mainly by the geological conditions (increase of carbonates, decrease of clays along the stream). Anthropogenic influences on the main components were not recognized. IR spectroscopy connected with multivariate data treatment is a suitable method to determine the main components of sediments and soils presupposed that the particle size distribution of the samples is similar. A quantitative chemical analysis in terms of individual substances or substance groups is possible if a set of reference spectra can be compiled.     

Tetrahedral occupancy of ferric iron in (Mg,Fe)O: Implications for point defects in the Earth's lower mantle

We investigated the concentration and site occupation of ferric iron (Fe³⁺) in (Mg,Fe)O to understand the influence of point defects on transport properties such as atomic diffusion, electrical conductivity and viscosity. We conducted Mössbauer spectroscopy of (Mg_0.8Fe_0.2)O single crystals synthesized at temperatures from 1673 to 2273 K and pressures from 5 to 15 GPa with Re–ReO₂ and Mo–MoO₂ oxygen fugacity buffers. The isomer shift of the Mössbauer spectra suggests that Fe³⁺ occupies mostly the tetrahedral site at reduced conditions and both the octahedral and tetrahedral sites at oxidized conditions. We formulate a thermodynamic model of point defect dissolution in (Mg,Fe)O which suggests that unassociated tetrahedral Fe³⁺ is more stable than unassociated octahedral Fe³⁺ at high-pressure and low oxygen fugacity due to the effect of configurational entropy. The pressure dependence of Fe³⁺ concentration indicates a change in the dominant site occupancy of Fe³⁺: (1) Fe³⁺ in the tetrahedral site, (2) Fe³⁺ in the octahedral site, and (3) defect clusters of Fe³⁺ and cation vacancy, in the order of increasing oxygen fugacity and decreasing pressure. This is in reasonable agreement with previously reported experiments on Fe³⁺ concentration, Mg–Fe interdiffusivity and electrical conductivity. We consider it plausible that (Mg,Fe)O accommodates Fe³⁺ in the tetrahedral site down to the lower mantle. Based on our results and available experimental data, we discuss the solubility competition between Fe³⁺ and protons (H⁺), and its implications for transport properties in the lower mantle.     

Distribution of secondary minerals in crusts developed on sandstone exposures

Previous research on rock weathering crusts has revealed their large variability depending on the type of host rocks and development of weathering processes. The composition of crusts developed on natural sandstone exposures is less documented in the literature in comparison to those developed on architectonic stones. In both cases, previous research has focused mainly on the progress of salt weathering. This study considers the surfaces of sandstone tors in the Polish Outer Carpathians. The exposed parts of the rocks in this area are often covered by crust, which is up to several centimetres thick, and differs from the internal part in colour and composition. The crusts were characterized using light and electron microscopy, X‐ray diffractometry, thermal analyses, Mössbauer spectroscopy, bulk chemical analyses and sequential chemical extractions. Porosity was estimated by digital image processing. The following two hardened zones were observed: (1) thin (up to 30 µm), black, external layer, rich in carbon and composed of opal‐type silica, covered in places by sulphate incrustations and numerous spherical particles of anthropogenic origin; (2) thicker (up to several millimetres), internal part composed of a set of laminae of variable colouration, enriched in iron (oxyhydr)oxides (goethite and hematite) in comparison to the rock interior. Development of the crust results from silicon and iron redistribution during the sandstone alteration. The chief source of silica is hydrolysis of aluminosilicates, whilst that of iron is decomposition of aluminosilicates, carbonates and sulphides. Hematite is probably a result of goethite transformation. However, air pollutants may play an important role in the formation of sulphates. Silica and iron compounds affect the properties of the rock, hardening the surface and lowering porosity by formation of secondary cement. Crystallization of sulphate salts, in turn, may contribute to mechanical disintegration of the rock. Copyright © 2013 John Wiley & Sons, Ltd.     

Complex resistivity of mineral rocks in the context of the generalised effective‐medium theory of the induced polarisation effect

This paper develops the generalised effective‐medium theory of induced polarisation for rock models with elliptical grains and applies this theory to studying the complex resistivity of typical mineral rocks. We first demonstrate that the developed generalised effective‐medium theory of induced polarisation model can correctly represent the induced polarisation phenomenon in multiphase artificial rock samples manufactured using pyrite and magnetite particles. We have also collected representative rock samples from the Cu–Au deposit in Mongolia and subjected them to mineralogical analysis using Quantitative Evaluation of Minerals by Scanning Electron Microscopy technology. The electrical properties of the same samples were determined using laboratory complex resistivity measurements. As a result, we have established relationships between the mineral composition of the rocks, determined using Quantitative Evaluation of Minerals by Scanning Electron Microscopy analysis, and the parameters of the generalised effective‐medium theory of induced polarisation model defined from the laboratory measurements of the electrical properties of the rocks. These relationships open the possibility for remote estimation of types of mineralisation and for mineral discrimination using spectral induced polarization data.     

Shales in the Qiongzhusi and Wufeng–Longmaxi Formations: a rock-physics model and analysis of the effective pore aspect ratio

The shales of the Qiongzhusi Formation and Wufeng–Longmaxi Formations at Sichuan Basin and surrounding areas are presently the most important stratigraphic horizons for shale gas exploration and development in China. However, the regional characteristics of the seismic elastic properties need to be better determined. The ultrasonic velocities of shale samples were measured under dry conditions and the relations between elastic properties and petrology were systemically analyzed. The results suggest that 1) the effective porosity is positively correlated with clay content but negatively correlated with brittle minerals, 2) the dry shale matrix consists of clays, quartz, feldspars, and carbonates, and 3) organic matter and pyrite are in the pore spaces, weakly coupled with the shale matrix. Thus, by assuming that all connected pores are only present in the clay minerals and using the Gassmann substitution method to calculate the elastic effect of organic matter and pyrite in the pores, a relatively simple rock-physics model was constructed by combining the self-consistent approximation (SCA), the differential effective medium (DEM), and Gassmann’s equation. In addition, the effective pore aspect ratio was adopted from the sample averages or estimated from the carbonate content. The proposed model was used to predict the P-wave velocities and generally matched the ultrasonic measurements very well.     

Trace Element Behavior in Methane‐Rich and Methane‐Free Groundwater in North and East Texas

There is concern about adverse impacts of natural gas (primarily methane) production on groundwater quality; however, data on trace element concentrations are limited. The objective of this study was to compare the distribution of trace elements in groundwater samples with and without dissolved methane in aquifers overlying the Barnett Shale (Hood and Parker counties, 207 samples) and the Haynesville Shale (Panola County, 42 samples). Both shales have been subjected to intensive hydraulic fracturing for gas production. Well clusters with high dissolved methane were previously found in these counties and are thought to be of natural origin. Overall, groundwater in these counties is of excellent quality with typically low elemental concentrations. Several statistical analyses strongly suggest that most trace element concentrations, generally at low background levels, are no higher and even reduced when dissolved methane is present. In addition, trace element concentrations are not correlated with distance to gas wells. The reduction in trace element concentrations is attributed to anaerobic microbial degradation of methane, is associated with a higher pH (>8.5), and, likely, with precipitation of carbonates and pyrite and formation of clays. Trace and other elements are likely incorporated within the precipitating mineral crystalline network or sorbed. High pH values are found throughout these high‐methane clusters (e.g., Parker‐Hood cluster), even in subregions where methane is not present, which is consistent with a pervasive natural origin of dissolved methane rather than a limited gas well source.     

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.     

The hydrothermal karstification and its effect on Ordovician carbonate reservoir in Tazhong uplift of Tarim Basin,Northwest China

With a detailed study on petrology, mineralogy and geochemistry of some important Ordovician carbonate well core samples in Tazhong uplift of Tarim Basin, the distinguishing symbols of hydrothermal karstification are first put forward as the phenomena of rock hot depigmentation, hot cataclasm and the appearance of typical hydrothermal minerals such as fluorite, barite, pyrite, quartz and sphalerite. The main homogenization temperatures of primary fluid inclusions in fluorite are from 260 to 310°C, indicating the temperature of hydrothermal fluid. The fluid affected the dissolved rocks and showed typical geochemistry features with low contents of Na and Mg, and high contents of Fe, Mn and Si. The ratio of ³He/⁴He is 0.02R _a, indicating the fluid from the typical continental crust. The hydrothermal fluid karstification pattern may be described as follows: the hot fluid is from the Permian magma, containing dissolving ingredients of CO₂ and H₂S, and shifts along fault, ruptures and unconformity, and dissolves the surrounding carbonates while it flows. The mechanism of hydrothermal karstification is that the mixture of two or more fluids, which have different ion intensity and pH values, becomes a new unsaturated fluid to carbonates. The hydrothermal karstification is an important process to form hypo-dissolved pinholes in Ordovician carbonates of Tazhong uplift of Tarim Basin, and the forming of hydrothermal minerals also has favorable influence on carbonate reservoirs.     

The hydrothermal karstification and its effect on Ordovician carbonate reservoir in Tazhong uplift of Tarim Basin,Northwest China

With a detailed study on petrology, mineralogy and geochemistry of some important Ordovician carbonate well core samples in Tazhong uplift of Tarim Basin, the distinguishing symbols of hydrothermal karstification are first put forward as the phenomena of rock hot depigmentation, hot cataclasm and the appearance of typical hydrothermal minerals such as fluorite, barite, pyrite, quartz and sphalerite. The main homogenization temperatures of primary fluid inclusions in fluorite are from 260 to 310°C, indicating the temperature of hydrothermal fluid. The fluid affected the dissolved rocks and showed typical geochemistry features with low contents of Na and Mg, and high contents of Fe, Mn and Si. The ratio of ³He/⁴He is 0.02R _a, indicating the fluid from the typical continental crust. The hydrothermal fluid karstification pattern may be described as follows: the hot fluid is from the Permian magma, containing dissolving ingredients of CO₂ and H₂S, and shifts along fault, ruptures and unconformity, and dissolves the surrounding carbonates while it flows. The mechanism of hydrothermal karstification is that the mixture of two or more fluids, which have different ion intensity and pH values, becomes a new unsaturated fluid to carbonates. The hydrothermal karstification is an important process to form hypo-dissolved pinholes in Ordovician carbonates of Tazhong uplift of Tarim Basin, and the forming of hydrothermal minerals also has favorable influence on carbonate reservoirs.

     

Electrical properties and geochemistry of carbonate rocks from the Qasr El‐Sagha Formation,El‐Faiyum,Egypt

Understanding petrographical, geochemical and electrical properties of rocks is essential for investigating minerals. This paper presents a study of the petrographical, geochemical and A.C. electrical properties of carbonate rock samples. The samples collected show six lithostratigraphic rock units. Electrical properties were measured using a non‐polarizing electrode at room temperature (～20°C) and a relative atmospheric humidity of ～50% by weight in the frequency range from 42 Hz to 5 MHz. The difference in electrical properties between the samples was attributed to the change in composition and texture between the samples. Electrical properties generally change with many factors (grain size, chemical composition, grain shape and facies). The dielectric constant decreases with frequency and increases with conductor composition. The conductivity increases with the increase of conductor paths between electrodes. Many parameters can contribute to the same result of the electrical properties. The main objective of the present study is to shed more light on the relation between the texture and geochemical composition of measured samples (carbonates that contain clays and quartz grains) through electrical laboratory measurements (conductivity and dielectric constant as a function of frequency).     

Cation migration in hydrothermal clays: The problem of mineralization criteria in gas-hydrothermal fluids of hydrothermal fields in Southern Kamchatka

Based on a comprehensive study of the hydrothermal clay layer that occurs in geothermal fields, the conditions of formation of cation composition in argillitized rocks are discussed. Under the influence of gas-water fluids and pore solutions, micro- and nano-mineral mixtures are formed in hydrothermal clays; these mixtures include crystalline, amorphous, and transitional mineral phases. A considerable role in their composition belongs to cations of several metals (Fe, Al, Ti, Na, Mg, Ca, K, Mn, and Ba), as well as Si, C, N, S, and volatiles (F^? and Cl^?). The sources of cations and other elements are unaltered host rocks, newly formed hydrothermal-metasomatic rocks, hydrothermal clays, salt deposits, siliceous, carbonate, and other sediments, as well as deep fluids. In the structures of geothermal anomalies and deposits the ??hydrothermal metasomatic rock??gas-water fluid??newly formed mineral chemical compounds?? united system is formed. Each of the elements of this system takes part in the transportation, accumulation, and redistribution of metals. This approach to studies of the geochemistry of present-day geothermal systems may serve as a foundation for developing criteria for the presence of mineralization in metasomatites, gas-hydrothermal fluids, and new mineral associations.     

Magnetic particles in atmospheric particulate matter collected at sites with different level of air pollution

Magnetic measurements of deposited atmospehric dust can serve as an additional parameter in assessing environmental pollution. This method is based on the assumption that atmospherically deposited particles contain significant portion of ferrimagnetic iron oxides of anthropogenic origin, which can be easily detected. Aim of this paper is to identify clearly magnetic fraction of daily samples of particulate matter less than 10 μm (PM10), routinely used for air quality assessment and monitoring. We used combination of thermomagnetic analyses and other physical and chemical methods, including scanning electron microscopy (SEM) and Mössbauer spectroscopy. Our results show that daily samples of PM10, collected at sites with different degree of atmospheric pollution, contain magnetite of spherical shape, which is presumably of industrial origin. Thus, magnetic methods can be applied directly to the same substances, which are used routinely in air quality assessment and monitoring.     

A 3.5 Ga record of water-limited, acidic weathering conditions on Mars

The secondary mineral budget on Earth is dominated by clay minerals, Al-hydroxides, and Fe-oxides, which are formed under the moderate pH, high water-to-rock ratio conditions typical of Earth's near-surface environment. In contrast, geochemical analyses of rocks and soils from landed missions to Mars indicate that secondary mineralogy is dominated by Mg (± Fe, Ca)-sulfates and Fe-oxides. This discrepancy can be explained as resulting from differences in the chemical weathering environment of Earth and Mars. We suggest that chemical weathering processes on Mars are dominated by: (1) a low-pH, sulfuric acid-rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite (± Fe–Ti oxides) is promoted; and (2) relatively low water-to-rock ratio, such that other silicate phases with slower dissolution rates (e.g., plagioclase, pyroxene) do not contribute substantially to the secondary mineral budget at the Martian surface. Under these conditions, Al-mobilization is limited, and the formation of significant Al-bearing secondary phases (e.g., clays, Al-hydroxides, Al-sulfates) is inhibited. The antiquity of rock samples analyzed in-situ on Mars suggest that water-limited acidic weathering conditions have more than likely been the defining characteristic of the Martian aqueous environment for billions of years.     

地下煤层自燃区岩石磁性增强特征及机理研究——以内蒙古乌达和宁夏汝萁沟煤矿为例

采用室内加温实验和岩矿测试分析手段,对地下煤层自燃区采集的岩石标本,测定其在不同温度条件下岩石磁性、含铁氧化物含量及其结构的变化情况,分析其特征和变化规律,探讨温度变化引起岩石磁性变化的机理.实验研究表明,温度变化可引起岩石磁性变化,通常在加温过程中多数岩石样品在低于400℃时其磁性较弱且没有明显的变化,继续升温达到含铁氧化物居里点温度前磁性增加,高于居里点温度后磁性消失;再由居里点温度或以上降温过程中磁性显著增强,并在常温下获得较岩石加温前更强磁性,但磁性变化较大.磁性变化主要与岩石中的黄铁矿、赤铁矿、菱铁矿、褐铁矿等含铁氧化物含量及其结构变化有关,岩石样品加热至700℃再冷却到常温以后,其铁磁性矿物含量明显增加;矿物结构有所变化,加温前一般为不规则微粒状、短脉状,加温后多为规则微粒状,且颗粒大小比加温前略有减小,一般为10~150 μm,大多属于多畴(MD)结构,部分膺单畴(PSD)结构.磁性增强的主要原因是含有少量含铁矿物的岩石在温度升高时产生了新的铁磁性矿物(如磁铁矿、磁黄铁矿、磁铁矿等).上述研究成果对解释煤层自燃区磁异常的成因具有重要意义.