A mathematical model of spheroidal weathering

A mathematical model is developed to explain the geometrical patterns of spheroidal weathering. The model is then analyzed, and results of computer simulations for the weathering of spherical and ellipsoidal surfaces are presented. Ellipsoids weather initially into ellipsoids of greater or lesser eccentricity, depending on boundary conditions, and finally into spheres. This is in qualitative agreement with the geometry of observed weathering patterns. Some of these features would be difficult to explain by a diffusion model. The weathering of rectangles also is simulated, and they weather into ellipses or circles. These are also in qualitative agreement with observed weathering patterns.     

剪节理发育地区球状风化物的成因分析

在系统总结国内外关于球状风化物成因研究已有认识的基础上, 针对其中存在的一些值得商榷的问题,结合大量野外图片以及标本分析,对剪节理发育地区球状风化物的成因展开重点研究。提出由于应力作用使岩石内部产生球状细微剪切变动,之后受风化作用的影响形成了球状风化产物的新观点。以此为基础,分析了其他类型球状风化物的形成与力作用的关系。     

Arc-shaped and spheroidal stellar complexes

Complexes of young clusters and high-luminosity stars in the shape of regular, circular arcs have been found in a number of galaxies, first and foremost the LMC, NGC 6946, and M83. These shapes are found even in strongly inclined galaxies, suggesting that the observed arcs are projections of partial spherical shells. Obviously, these stellar shells must have formed from gaseous shells swept up by some source of central pressure and become gravitationally unstable. The power of this source corresponds to several dozen supernova explosions; however, its nature remains unclear. A central cluster providing a source of O stars and supernovae is usually absent. The presence of multiple arcs located close to each other can be explained by the fall of a swarm of fragments or by the progenitor stars originating in a single peculiar star cluster, implying the existence of stellar objects capable of giving rise to explosions with energies an order of magnitude higher than those of individual supernovae. The same objects may be responsible for gamma-ray bursts. It may be that only the most massive clusters with frequent or especially powerful supernova explosions are capable of producing HI supershells. Otherwise, it is impossible to explain why no supershells have been found around numerous clusters that should be capable of producing them according to current theories. The presence of star clusters in shell-like structures provides extremely important information about the physical conditions in and the ages of the initial gaseous shells, making stellar arcs the best available laboratory for studies of triggered star formation.     

Intergrowth and twinning in opal-CT lepispheres

Under conditions of unobstructed diagenetic growth, opal-CT usually forms spherical aggregates, so-called lepispheres. From a scanning electron microscope study of deep-sea porcelanites, the development of these structures is described, from isolated opal-CT platelets via complete lepispheres to large coalescent aggregates. The conspicuous symmetric ultrastructure of lepispheres—reflected by the constancy of angles between interpenetrating crystal blades—is crystallographically determined: The lepispheres consist of groups of opal-CT blades intergrown according to the (30–34) and (10–16) twinning laws of tridymite. The same rules of intergrowth can be applied to the morphology of hydrothermally synthesized opal-CT lepispheres.     

Determining mineral weathering rates based on solid and solute weathering gradients and velocities: application to biotite weathering in saprolites

Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.

Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10⁻¹⁷ mol m⁻² s⁻¹ for biotite. Faster weathering rates of 1.8 to 3.6×10⁻¹⁶ mol m⁻² s⁻¹ are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.     

The twinning microstructure and growth of amethyst quartz

The characteristic lamellar-twinning of the right-handed (R) and left-handed (L) structures in the major rhombohedral growth sectors of amethyst quartz has been studied by optical techniques, X-ray topography and transmission electron microscopy (TEM). The TEM observations show that the region of each Brewster fringe consists of fine-scale Brazil twin lamellae parallel to one of the r, z{10 \(\overline {\text{1}} \) 1} planes, and structural considerations suggest that it is one of the r-planes. The twin boundary corresponding to a Brewster fringe has the form of a zig-zag structure consisting of Brazil twin boundaries on two r{10 \(\overline {\text{1}} \) 1} planes, with one predominating. The Brewster fringes appear black between crossed polarizers because light travelling along the optic axis [001] passes through almost equal distances of R and L quartz, giving essentially zero optical rotation. From the visibility of the Brazil twin boundaries in electron micrographs and the visibility of the Brewster fringes in X-ray topographs, the fault vector R and the corresponding composition plane of the major Brazil twin associated with each Brewster fringe has been determined. The streaking of the Brewster fringes observed optically and in the X-ray topographs appears to be due to the stair-rod dislocations at the intersections of the Brazil twin boundaries. Experiments in which synthetic quartz was grown hydrothermally on untwinned seeds and on twinned amethyst seeds showed that the initiation of Brazil twins and the development of Brewster fringes was dependent upon the presence of iron in the growth solution.     

锂同位素与大陆风化

气候变化与大气二氧化碳浓度息息相关.大陆岩石圈风化是影响大气二氧化碳浓度的重要过程.通过还原陆壳古风化信息,我们可以有效地了解地球气候条件的演化历史.传统方法上,前人曾使用锶同位素示踪大陆风化,但其解释尚有不足.例如,海水锶同位素比值会受到海洋热液的影响,而河流锶同位素比值则易受风化岩石类型的影响.此外,只有硅酸盐风化被认为在长时间尺度控制着大气碳汇,但锶的碳酸盐风化却与硅酸盐风化很难分辨.因此,我们需要一种更理想的同位素体系作为示踪大陆风化历史的介质.锂,作为微量元素,主要集中在岩石圈的硅酸盐矿物中,在碳酸盐岩含量较少.所以,硅酸盐风化可以使用锂同位素予以记录.同时,锂同位素受生物分馏效应影响较小,可以在海相碳酸盐岩中保存良好.这些优势为海相碳酸盐岩的锂同位素信号示踪大陆风化历史提供了有力支撑,但我们仍需对风化、迁移和结晶等过程的锂同位素地球化学行为有清晰的认识.为此,本文回顾不同储库的锂同位素组成以及各物相间锂元素配分和同位素分馏特征,总结了近年来锂同位素在重建大陆风化历史方面的进展,并详述了有待解决的关键问题.     

Investigation of the weathering layer using seismic refraction and high-resolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16?m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18?m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.     

Submicroscopical twinning in lunar and experimentally deformed pyroxenes

During deformation experiments at high pressures (10 kb) and medium temperatures (500–650°) diopside became twinned on {100} in small optically invisible domains. If host and twin occur with equal frequency, the X-ray single-crystal photograph would show orthorhombic symmetry due to the special geometry of the diopside lattice. A crystal of pyroxene from Mare Tranquillitatis has been found to give just such an X-ray photograph with orthorhombic symmetry; this is explained as a possible deformation product.     

Mechanical twinning and slip in experimentally deformed plagioclases

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)₄O₈, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An₁, An₅₉). Three (An₃₂, An₃₉, and An₅₃) were slightly disordered, and two (An₇₆ and An₉₅) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An₁, An₂₀), feldspars in the peristerite range (An₁₁ An₁₄) or in a low structural state labradorite (An₅₁). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning.Work performed under the auspices of the U. S. Atomic Energy Commission.     

Mechanical twinning in small quartz crystals

Quartz is known to be ferrobielastic; that is, quartz crystals have domain states (Dauphiné twins) which differ in their elastic compliance values and which can be switched by an appropriately oriented stress. Polycrystalline quartz has also been reported (Tullis 1970) to show preferential orientation of these domains following application of large uniaxial stresses. These experiments were designed to study twinning of synthetic quartz “grains” (minimum size 0.07×0.07×0.02 cm) in specially-constructed composites and of grains in three natural quartz aggregates — a quartzite, a novaculite, and a jasper. Backreflection X-ray techniques were used to verify twinning in the composite grains, while special electroding and electrical detection allowed the twinning processes to be examined in “real time.” Small synthetic quartz crystals were found to behave identically to the massive samples previously studied. Electrical pulses due to the reversal of piezoelectric coefficient d ₁₁ in twinned quartz were detected from quartzite and from the man-made composites. Novaculite also gave electrical pulses which were probably from twinning (evidenced by the correlation of expected and observed pulse sizes and shapes), while no pulses from the jaspers indicative of twinning were detected. Grain size distribution differences are considered the main structural reason for the different behaviors.     

The effect of weathering on the distribution of strontium isotopes in weathering profiles

The strontium isotope ratio in sea water has varied through geologic time owing to the input of strontium from rock weathering. To evaluate the possibility that $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratios might be altered during weathering, seven weathering profiles developed on Mesozoic arkoses located along the length of New Zealand were investigated. The rubidium-strontium-strontium isotope relations in these profiles give ‘isochron’ ages less than the ages of deposition of the arkoses. These ages appear to result from the weathering of a homogeneous source rock. The age calculated from the rubidium-strontium system (t′) is related to the original age (t) by the equation t′/t = (n ? 1)/n, where n is the ratio of the amounts of common and radiogenic strontium leached from the rock. Shales formed by the accumulation of these residual solids may inherit misleading isochron relationships which may not be erased during deposition or early diagenesis. The strontium which goes into solution and is transported to the sea is slightly less radiogenic than the strontium in the unweathered rock, while the residual clays may be much more radiogenic.     

Domain twinning of Laihunite and refinement of its crystal structure

Domain twinning of laihunite has been investigated based on diffracton phenomena, and its crystal structure has then been refined. Space group with respect to the domain isP2₁/c, and cell parametersa=5.813,b=4,812,c=10.211(A), β=90.87°. Atomic coordinate and bond length have been recalculated. Discussions are made of the Fe²⁺ distribution, lattice distortion, degree of order of laihunite and the relationship of this mineral with fayalite and ferrifayalite. The authors still hold that laih unite should be considered as a new silicate mineral with dominant Fe³⁺ and less amount of Fe²⁺.     

Bacterially-mediated weathering of crystalline and amorphous Cu-slags

Two types of Cu-slags (CS: crystalline massive slag and GS: granulated amorphous slag) exhibiting a different chemical and mineral phase composition were compared with respect to their susceptibility to bacterial weathering using Pseudomonas aeruginosa (n° CIP 105094). Abiotic conditions e.g. sterile growth medium and ultrapure water were used for comparison. The experiments were extended up to 112 days with a systematic liquid phase renewal every 14 days. The results revealed significant release of elements in the bacterially mediated weathering experiments. Concentrations of elements (Si, Fe, Cu, Zn and Pb) in the biotic solutions were increased at least by 20% up to 99% compared to abiotic ones. From 3 to 77% of the leached elements were associated to the fraction >0.22 μm. Scanning electron microscope observations demonstrated greater weathering of mineral phases in biotic experiments than in abiotic ones which is in accordance with the solution chemistry exhibiting higher concentrations of elements leached in biotic set-ups. In the case of CS, glass and sulfides weathering was yet observed in abiotic experiment, whereas partial dissolution of fayalite (Fe₂SiO₄) was solely affected by the presence of bacteria. GS having a higher bulk content of metallic elements was found to be more stable than sulfide-bearing CS, while its (GS) glass matrix was found to weather easier under biotic conditions.     

Microlensing by a noncompact spheroidal gravitational lens

In the process of their evolution, small-scale clusters of weakly interacting massive particles (WIMPs) that formed in the early Universe can acquire rotational momentum and spheroidal shape. Even small oblateness of a cluster similar to that of the critical Roche surface can lead to the appearance of caustics in the plane of a source lensed by the object. The multiple source images that form in this case cannot be resolved in modern observations, and the cluster behaves like a noncompact spheroidal lens. The caustic crossing that occurs in the case of relative motion of the observer, the cluster of particles, and the lensed star can produce a large variety of flux curves, including those such as have been observed during microlensing events and interpreted as manifestations of binary gravitational lenses. Therefore, we cannot rule out the possibility that at least some of these events might actually be associated with clusters of WIMPs.     

Space weathering simulation and spectrum decoding

Visible and near-infrared spectra are routinely used to achieve mineral abundances and mineral chemistry of the global surfaces of the Moon and asteroids.However,these spectra can be significantly modified by space weathering,including micrometeorite impacting,solar wind implanting and cosmic ray irradiation.In this paper we report results of laser-bombarding experiments on the Jilin ordinary chondrite,simulating micrometeorite impacting on the surface of asteroids.After laser bombardment,the spectra became significantly redder and moderately darker. With the Modified Gaussian Model(MGM)method,the absorption band positions of olivine can be decoded from the modified spectra,which are correlated with their fayalite contents.In addition,a continuum of the modified spectra can be decoded,and its slope may be used to depict the degree of space weathering.However,relative strengths of the absorption sub-bands of olivine and pyroxenes show significant variant after the bombardment, hence they cannot be used to estimate the relative abundances of high-Ca to low-Ca pyroxenes of the lunar surface and other matured surfaces of asteroids.     

Dauphiné twinning and texture memory in polycrystalline quartz

Mechanical twinning in polycrystalline quartz was investigated in situ with time-of-flight neutron diffraction and a strain diffractometer. Dauphiné twinning is highly temperature sensitive. It initiates at a macroscopic differential stress of 50–100 MPa and, at 500°C, saturates at 400 MPa. From normalized diffraction intensities the patterns of preferred orientation (or texture) can be inferred. They indicate a partial reversal of twinning during unloading. The remaining twins impose residual stresses corresponding to elastic strains of 300–400 microstrain. Progressive twinning on loading and reversal during unloading, as well as the temperature dependence, can be reproduced with finite element model simulations.