Comments on the interpretation of deformation textures in rocks

In rocks that undergo ductile deformation, preferred orientation develops as a result of intracrystalline slip and mechanical twinning. The orientation distribution is a consequence of the microscopic mechanisms and of the strain path. It can be used to get some insight into the deformation history; however it is never unique. The interpretation relies largely on polycrystal plasticity theory. The concepts of stress equilibrium and strain compatibility, which are two extreme assumptions made to model deformation, are discussed. New approaches such as the viscoplastic self-consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy. Important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations. All these theories defy the popular notion which is becoming entrenched in the geological literature, that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress, and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction, an orientation referred to by geologists as ‘easy glide’. It is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition, in accordance with an old recommendation of Walter Schmidt.     

The enigma of post-perovskite anisotropy: deformation versus transformation textures

The D′′ region that lies just above the core mantle boundary exhibits complex anisotropy that this is likely due to preferred orientation (texturing) of the constituent minerals. (Mg,Fe)SiO₃ post-perovskite is widely thought to be the major mineral phase of the D′′. Texture development has been studied in various post-perovskite phases (MgSiO₃, MgGeO₃, and CaIrO₃), and different results were obtained. To clarify this controversy, we report on transformation and deformation textures in MgGeO₃ post-perovskite synthesized and deformed at room temperature in the diamond anvil cell. Transformed from the enstatite phase, MgGeO₃ post-perovskite exhibits a transformation texture characterized by (100) planes at high angles to the direction of compression. Upon subsequent deformation, this texture changes and (001) lattice planes become oriented nearly perpendicular to compression, consistent with dominant (001)[100] slip. When MgGeO₃ post-perovskite is synthesized from the perovskite phase, a different transformation texture is observed. This texture has (001) planes at high angle to compression and becomes slightly stronger upon compression. We also find that the yield strength of MgGeO₃ post-perovskite is dependent on grain size and texture. Finer-grained samples exhibit higher yield strength and are harder to induce plastic deformation. Strong textures also affect the yield strength and can result in higher differential stresses. The inferred dominant (001) slip for pPv is significant for geophysics, because, when applied to geodynamic convection models, it predicts the observed anisotropies of S-waves as well as an anti-correlation between P- and S-waves.     

Bismuthoan Galena—A New Variety of Galena

Bismuthoan galena is a variety of galena, resulting from the replacement of Bi for some Pb in galena.This mineral occurs in the No.11 orebody of the Lamo skarn-type Zn-Cu deposit in the Dachang ore field ,but only the No,11 orebody of the Lamo deposit is developed such bismuthoan galena.This is closely related to such a geochemical background that the No.11 orebody contains high Bi and Bi-sulfosalt minerals are well developed there.Eight electron microprobe analyses of five samples give 60.17-70.20%Pb(66.94% on average),10.00-16.06%Bi (12.47%),4.83-8.97%Ag(6.39%)and 13.25-13.98% S(13.65%).Its molecular formula is Pb0.76,Bi0.14,Ag0.13,1.03S.No galena so high in Bi has been reported in the literature and this is the first report in China.     

Dislocation etch pits in galena

The results of dislocation etch pit studies of naturally and experimentally deformed galena single and polycrystals are presented. The etch pitting is simple, inexpensive, rapid and reveals dislocation substructures similar to those found in metals and ceramics. Both {110} < 1 0 > and {100} < 110 > type slip systems have been found in naturally and experimentally deformed galena.     

Evolution of quartz microstructures and textures during polyphase deformation within the Tauern Window (Eastern Alps)

The interior of the Tauern Window exposes underplated Penninic continental lithosphere and the overlying obducted Penninic oceanic crust within a large antiformal dome in the internal zone of the Eastern Alps. These units have been affected by a polyphase deformation history. Generally, three deformation events are distinguished. D₁ is related to underplating of, and top-to-the-N nappe stacking within, the Penninic continental units of the Tauern Window. Deformation stage D₂ is interpreted to reflect the subsequent continent collision between the Penninic continental units and the European foreland, D₃ is related to the formation of the dome structure within the Tauern Window. During thickening of continental lithosphere and nappe stacking (D₁), and subsequent intracontinental shortening (D₂), these tectonic units have been ductilely deformed close to a plane strain geometry. Conditions for the plastic deformation of the main rock-forming mineral phases (quartz, feldspar, dolomite, calcite) have prevailed during all three phases of crustal deformation. Generally, two types of quartz microstructures that are related to D₁ are distinguished within the Tauern Window: (a) Equilibrated and annealed fabrics without crystallographic preferred orientations (CPO) have only been observed in the central part of the southeastern Tauern Window, corresponding with amphibolite-grade metamorphic conditions. (b) In the northeastern and central part of the Tauern Window microstructures are characterized by quartz grains that show equilibrated shape fabrics, but well preserved CPO with type-I cross girdle distributions, indicating a deformation geometry close to plane strain. During D₂, two types of quartz microstructures are distinguished, too: (a) Quartz grains that show equilibrated shape fabrics, but well-preserved CPO. The c-axes distributions generally are characterized by type-I cross girdles, locally by type-II cross girdles, and in places, oblique single girdle distributions. (b) A second type of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain-boundary migration, and strong CPO. This type is restricted to the southern sections of the western and eastern Tauern Window. The c-axis distributions show type-I cross girdles in the western part of the Tauern Window and single girdles in the southeastern part. In the western part of the Tauern Window, a continuous transition from type (b) microstructures in the south to type (a) microstructures in the north is documented. The microstructural evolution also documents that the dome formation in the southeastern and western Tauern Window has already started during D₂ and has continued subsequent to the equilibration during amphibolite to greenschist facies metamorphism. D₃ is restricted to distinct zones of localized deformation. D₃-related quartz fabrics are characterized by the formation of ribbon grains; the c-axes show small-circle distributions around the Z-axis of the finite-strain ellipsoid. During exhumation and doming (D₃), deformation occurred under continuously decreasing temperatures.     

Lead isotope data on three North Australian galena localities

Gas-source intercomparison data from three Precambrian locations, reproducing to within 0.1 to 0.2 percent in each ratio at the 95 percent level, have been normalized through the Broken Hill standard to the NBS lead, SRM 981. The bedded Pb-Zn deposits at Mount Isa and McArthur River show the marked isotopic uniformity long associated with major stratiform deposits. At Mount Isa this uniformity extends meridionally for at least 35 km, and over the full depth of the host formation. It encompasses samples of fault-fill sulphides within the mine area, thus emphasizing the dominance of the ore lead during subsequent remobilization. At McArthur River there is strong evidence that veins cutting the adjacent formations comprise a mixture of ore Pb with material of different provenance; the evidence for comparable mixtures at Mount Isa is not strong. At the intervening Law Hill field, the Pb in vein deposits is similar to the more extreme of the McArthur River vein leads. This could possibly indicate a geochemical and age similarity between the source rocks in these two widely-separated areas.     

Minor elements in sphalerite and galena from Binnatal

The trace element contents of 23 sphalerites and galenas from Binnatal, Switzerland, have been determined. Most of these samples have been previously studied in respect to lead and sulphur isotope abundances. Coloration of sphalerites — varying from yellow to black for nearly identical iron contents — seems to be strongly influenced by the manganese content. A linear relationship between sulphur isotope composition and copper content in sphalerites was found. Trace elements in galenas show a significant inverse relationship between silver and copper. With the determination of the bismuth content, it is possible to distinguish several galena types; a similar grouping has been found by lead isotope determinations. The results of the trace analyses are discussed in connection with the occurrence of a large number of very rare and special Pb-As-sulphosalt minerals in the Binnatal dolomites.     

岩体基本参数对岩质边坡变形的影响

文章利用有限差分程序FLAC,定量化讨论了边坡岩体(软岩)的变形模量、泊松比、密度、粘聚力、内摩擦角、边坡高度等参数对软岩边坡变形的影响。研究表明,边坡位移随变形模量E的增大而减小;泊松比μ的增加,则会引起x方向的位移的增加,y方向的位移减小;而岩体密度对岩石边坡位移的影响甚弱。边坡岩体的粘聚力c对边坡变形的影响程度与岩体的其他参数间存在很大关系。内摩擦角对边坡位移的影响大小,则取决于粘聚力c的取值。此外,随着边坡高度的增加边坡变形则会随之增加。其中x方向位移变化对边坡高度的变化更为敏感;同样,在边坡岩体其他参数不变的情况下,边坡倾角越大,边坡变形也越大。     

Depression of galena by slime gangue in an agitated pulp

The effect of gangue on mineral behaviour in a miniature batch version of an industrial flotation machine was examined. The flotation systems consisted of a sulphide, galena, with a specific collector, potassium ethyl xanthate, and a particular non-sulphide gangue in each case. Three gangue minerals were selected: corundum, fluorite and quartz. Flotation experiments were augmented with spectrophotometric adsorption and zeta potential measurements. Based on this data, the validity and significance of the slime-coating hypothesis in the context of the environment characteristic of industrial flotation processes were assessed.     

Formation of olivine-metal textures in pallasite meteorites

The olivine-metal textures of pallasites help elucidate the formation of these meteorites. Large sections of pallasites with angular olivines commonly contain olivine masses 5–30 cm in size, which appear to have been disintegrating into smaller fragments, like those dispersed through the metal. It is suggested that these masses were parts of olivine layers, which were intruded by underlying molten metal. Gravitational separation of olivine and metal was probably prevented by incipient solidification of metal.Pallasites with rounded olivines probably had a fairly similar history, but with additional annealing after metal solidification had started. Grain boundary energy was reduced by decreasing the area of metal-olivine interface and increasing the contact between touching olivines. Because of the vastly different nature of metal and olivine, their mutual interfacial energy is high and the degree of anisotropy of this energy is low. The occurrence of profuse, small faces on rounded olivines is therefore consistent with the ‘metamorphic’ origin which is proposed for rounded olivines. In many pallasites with angularolivine macrostructures, magnifications of 5–50 × reveal similar rounding effects, including aggregates of rounded olivines, and individual crystals which may be faceted.The high metal-troilite ratio of Phillips County suggests that this pallasite formed by mixing of olivine with S-rich liquid metal around 1000°C, near the Fe-FeS eutectic temperature.     

High-temperature stress relaxation of synthetic,polycrystalline galena

Stress relaxation experiments were performed on hot-pressed cylinders of synthetic, polycrystalline galena at temperatures of 500°, 600°, 700° and 800°C at atmospheric pressure after various levels of axial strain, in the range from 2% to 25%, had been imposed at a constant strain rate (?) of 3×10^?4 s^?1. For this study a new apparatus was built which can measure small changes in applied stress (σ), better than ±0.5 × 10⁵ Pa, and which incorporates a facility whereby the composition of the test specimen can be fixed through equilibration with a gas atmosphere of controlled composition. Deformation mechanism maps constructed largely on theoretical grounds predict that, under the conditions studied here, high-temperature dislocation creep, for which ?∝σ⁵, gives way on lowering the applied stress to diffusion creep, for which ?∝σ. The experimental results agree in large measure with these theoretical predictions. However, it is not possible in this preliminary study to conclusively identify the dominant deformation mechanisms.     

Observation of the oxidation of galena using Raman spectroscopy

Oxidation of galena (PbS) to oxysulfates, (PbO·PbSO₄, 3PbO·PbSO₄ and 4PbO·PbSO₄)_, has been observed using Raman spectroscopy. Peaks associated with the oxidation products have been assigned. The reaction appears to be a high temperature oxidation induced by the high laser (25 mW at 514.5 nm) power density at freshly cleaved galena surfaces. Damage to the galena surface was observed visually under the microscope. Moderate laser powers (5 mW at 514.5 nm) did not result in any damage. No Raman bands were observed or expected for freshly cleaved galena because it has the rock salt structure. Laser-induced production of these oxysulfates is dramatically different from high temperature methods previously employed. This procedure will permit easy identification of galena in complex mineral ore samples. Spontaneous air oxidation of freshly cleaved galena to oxides or polysulfides was not detected.     

Experimental modelling of corona textures

Formation of corona textures along olivine–plagioclase and orthopyroxene–plagioclase interfaces has been experimentally reproduced at 670 and 700 °C and 5 kbar with either a pure H₂O fluid phase or 0.1 and 37 m NaCl–H₂O solution fluid. In these experiments, we investigate the interaction of primary olivine and/or orthopyroxene and plagioclase in powders and polished crystals, and in small samples of a natural gabbro. The experiments result in the formation of corona textures with several layers of different assemblages (according to the experimental conditions) consisting of garnet (grossular), clinopyroxene, orthopyroxene, amphibole, chlorite and phlogopite. The experiments show major differences in the number of layers, the mineral assemblages and mineral composition, and in the trends of composition of plagioclase in coronas around olivine and orthopyroxene. The fluid phase composition influences the corona assemblages and the composition of the minerals in the experimental coronas; for example, garnet appears in the coronas in the second experiment where the NaCl–H₂O ratio is low. Experimental modelling of corona textures confirms a model of simultaneous growth of layers by the mechanism of diffusion metasomatism with participation of a fluid phase through which mass is transferred. Zoning in the experimental coronas shows opposing diffusion of Al and Ca from plagioclase and Mg and Fe from olivine/orthopyroxene; difference in the mobility of the components is inferred from observations in the coronas. The experimental corona textures are compared with natural coronas from the Belomorian belt (Baltic shield), developed at 670–690 °C and 7–8 kbar, and the Marun‐Keu complex (Polar Urals), developed at 670–700 °C and 14–16 kbar, where the corona textures correspond to a transitional stage of the gabbro‐to‐eclogite transformation.     

Galvanic interaction between galena and pyrite in an open system

1IntroductionMost sulfide minerals in nature have perfect con-ductivity and possess semiconducting properties.Gal-vanic corrosion may occur when two sulfide mineralswith different potentials are connected together in thesolution.The mineral with the highe…     

First principles studies of the surface of galena PbS

Ab initio calculations of the surface structure and properties of the (001) surface of galena (PbS) have been performed using two complementary approaches. Both Gaussian and numerical basis functions have been utilized, as embodied within the programs CRYSTAL98 and SIESTA, with their associated implementations, in order to determine to what extent computational factors are influencing the results. Relaxations are presented based on calculations employing the HF, B3LYP, LDA, and GGA Hamiltonians. Significant relaxations (up to about 0.2 Å) are found to occur at the surface which decay with increasing depth into the bulk. The magnitude of the relaxation oscillates between layers, leading to pairing, with Pb ions being displaced outwards (by approximately 0.05 Å) relative to the S ions. We find that the LDA, GGA, and B3LYP Hamiltonians give results in reasonable agreement with each other, but that the structures computed with HF theory are vastly different. The present calculations also predict a surface core level shift of 0.3 eV for the sulphur 2p level at the surface itself, in excellent agreement with a recent experimental determination.     

Discovery of galena in Tertiary pebbly coquina and its implication in mineralization

A suite of terrestrial-marine transitional carbonate beds occur in the Lower Tertiary of the Kuqa Basin, Xinjiang. The pebbly coquina, Paleocene in age, is distributed along the southern pediment of the Tianshan Mountains north of Baicheng, where Pb-mineralization has been observed for the first time. The pebbly coquina consists of well-rounded quartzitic pebbles, quartz sands, shell fossils, shell fragments and equigranular fecal pellets. Four types of carbonate minerals have been revealed: 1) euhedral dolomite (Mg₆₀Ca₄₀)CO₃, 2) anhedral dolomite (Mg₅₇Ca₄₃)CO₃, 3) dolomitic calcite (Mg₂₀Ca₈₀)CO₃ and magnesitic dolomite with a trace amount of Pb(Mg₇₃Ca₂₇)CO₃. Five types of galena filling the pebbly coquina have been found: 1) in the shell tower end, 2) on the inner shell wall, 3) on the outer shell wall, 4) around the shell wall, and 5) among shell fragments. The galena replacement took place mainly in the early stage of diagenesis. Stable isotope analyses show (δ¹³C values range from −0.8 to +2.3 and δ¹⁸O values from −0.1 to +0.5, indicating a marine environment under the influence of fresh water, which is consistent with the result of megascopic facies analysis in paleogeography. It is concluded that the discovery of galena in the Tertiary pebbly coquina in the north of Baicheng, Xinjiang may provide new clues to the searching for strata-bound Pb-Zn deposits in the vast area of northern Tarim Basin.     

冻融循环对土体强度变形参数的影响

在寒区工程建设中,冻融循环会引起土强度和变形特性发生改变。为了研究多次冻融循环后土的强度和变形参数的变化量,本文以冻融和非冻融的粉质粘土饱和试样为研究对象,通过固结排水三轴剪切试验、三轴等向压缩试验和弹性模量试验,比较了冻融循环对土样强度参数和变形参数的影响。试验结果显示：冻融和非冻融试样的应力—应变关系曲线均呈应变硬化型,对于原状试样冻融作用破坏了其结构性;冻融作用导致土体强度的降低,主要体现在粘聚力的降低,约降低了55.6%;冻融循环对变形参数有明显的影响,其中最明显的是弹性模量的降低,降低了31.2%。该研究结论可为工程冻害处理提供参考,也可为相关研究的数值计算提供试验参数。     

Arsenite adsorption on galena (PbS) and sphalerite (ZnS)

Arsenite, As(III), sorption on galena (PbS) and sphalerite (ZnS) was investigated as a function of solution composition and characterized using X-ray absorption spectroscopy (XAS). Adsorption conformed to a Langmuir isotherm except at the highest surface loadings, and it was not strongly affected by changes in ionic strength. Arsenite sorbed appreciably only at pH > ∼5 for PbS and pH ∼4.5 for ZnS, behavior distinct from its adsorption on other substrates. Arsenite adsorption on PbS and ZnS resulted in the conversion from As-O to As-S coordination. Arsenite does not adsorb through ligand-exchange of surface hydroxyl or sulfhydryl groups. Rather, it forms a polynuclear arsenic sulfide complex on ZnS and PbS consistent with the As₃S₃(SH)₃ trimer postulated by Helz et al. (1995) for sulfidic solutions. This complex was unstable in the presence of oxidizing agents and synchrotron light—it quickly converted to As(V), which was largely retained by the surface. These data illustrate the complexity of As(III) adsorption to even simple sulfide minerals.     

Microstructure and texture formation in extruded lead sulfide (galena)

Lead sulfide (galena) of different purity and grain size was extruded through a round and rectangular die at temperatures between 773 and 923 K. Global and local lattice preferred orientations (here referred to as textures) were measured by neutron and electron back-scattering diffraction. Tension leads to a <100> <111> double fibre texture. Pure shear deformation yields texture components near the ideal face-centered cubic metal brass, copper, Goss and cube positions. The intensity of the components depends on the purity and/or grain size. The microstructure is partially recrystallized. Electron back-scattering diffraction indicates that in tension the <100> and in pure shear the Goss and cube components are associated with dynamic recrystallization. The deformation texture can be qualitatively explained by the full and relaxed constraints Taylor model using slip on {100}<110>, {110}<110> and {111}<110> systems. The texture formation in lead sulfide compares well with that observed for other ionic crystals with the NaCl-structure as well as for face-centered cubic metals with a high stacking fault energy.     

Metamorphic textures and deformation: A new mechanism for the development of syntectonic porphyroblasts and its implications for interpreting timing relationships in metamorphic rocks

The formation and growth of porphyroblasts is treated in detail, with particular reference to garnet. Previous explanations for included fabrics in porphyroblasts, and mechanisms of syntectonic growth leading to sigmoidal inclusion patterns, are shown to be based on invalid assumptions and are unacceptable. The following new mechanism for garnet porphyroblast growth is proposed. During deformation, fluid filled veins open, particularly where micaceous foliae open in folds associated with crenulation cleavages and kink bands. Garnet nucleates in these veins as they develop. The geometry of the veins depends on variability of strain rates, fluid diffusivity and the sequence of progressive deformation. In areas of low strain rate and high diffusivity, long and widely spaced macroscopic garnet veins form. These may be buckled during the continuous deformation process. Conversely, where strain rates are high, and diffusivity is low, short and closely spaced microscopic garnet veins form. Buckling and shear may alter the vein shapes during their formation and sigmoidal veins can result. Veins join and coalesce to form single porphyroblasts, leaving a selvage of quartz and other minerals as an included fabric. The finite geometry of the included fabric ranges from helicitic to sigmoidal, which is interpreted to reflect the local deformation history during vein propagation. The fabric geometry is finalized when the veins fail to respond individually to the deformation due to their coalescence and formation into a single porphyroblast. Continuous vein propagation without significant deformation of the veins could lead to augen textures, whilst a process of continuous vein formation and rotation is shown to explain apparent syntectonic rotations of more than 180°. This new hypothesis has the merit of being very simple, but it does require a new approach. This approach necessarily questions a number of previously well established “facts”, such as pre-, syn-, and post-tectonic textures. It is argued that extreme care must be taken, when relating deformation episodes and metamorphic growth periods.