Electronic and magnetic structure of pyroxenes I. Hedenbergite

The electronic and magnetic structure of the chain silicate hedenbergite (CaFe²⁺Si₂O₆) has been investigated by a number of experimental methods (neutron diffraction, Mössbauer spectroscopy, low temperature magnetic measurements), as well as by electronic structure calculations for clusters of different size in the local spin density approximation. The calculated size-converged spectroscopic data (d-d excitation energies, hyperfine parameters) are in quantitative agreement with the respective experimental values. The calculated magnetic coupling constants are about +25 cm^?1 and ?4 cm^?1 for intra-chain and inter-chain coupling, respectively. The latter value shows that weak superexchange via edges of silicon tetrahedra is well reproduced by the calculations, and it is in qualitative agreement with an observed metamagnetic transition at 4.2 K in an external magnetic field with an onset around 4 T but saturation is not achieved in fields up to 14.5 T. The large ferromagnetic intra-chain coupling is attributed to a nearly degenerate ground state. The ratio between the two magnetic coupling constants agrees with earlier estimates on similar compounds. Finally, it is demonstrated how the detailed discussion of the various exchange pathways contributes to an improved understanding of the connection between magnetic properties and the geometrical structure.     

The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H

Phase relations for the bulk compositions 3CaO·2FeO_x·3SiO₂+excessH₂O and CaO·FeO_x·2SiO₂+excess H₂O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO₂. Andradite, Ca₃Fe³⁺₂Si₃O₁₂, synthesized above 550 °C hasan average unit cell edge, a_o, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, a_o increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P_fluid andradite is stable above an f_O2of 10¹⁵ bar at 800 °C and 10-³² bar at 400 °C. At lowerf_O2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe²⁺SiO₄)+ wollastonite and kirschsteinite+xonotlite (Ca₆Si₆O₁₇(OH)₂). Synthetic hedenbergite, CaFe²⁺Si₂O₆, has average unit cell dimensionsof a_o = 9.857± 0.004 Å, b_o = 9.033±0.002Å, c_o = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P_fiuid, hedenbergite is stable below an f_O2 of10-¹³ bar at 800 °C and 10-²⁸ bar at 400 °C. Above thesef_O2 values, hedenbergite+O₂ breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the f_O2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H₂O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences.     

碳酸盐岩溶蚀机制的实验探讨:表面溶蚀与内部溶蚀对比

通过表面溶蚀和内部溶蚀两种实验方式,分别对六种类型碳酸盐岩用0.2%乙酸溶液进行溶蚀实验,对比研究了从表生环境到深埋藏环境下有机酸与不同类型碳酸盐岩的溶蚀作用机制。对比研究表明:(1)在近地表环境下(25℃,1.0 MPa),泥灰岩和含生物碎屑泥晶灰岩的溶解速率大于细晶白云岩,前两者约为后者的2³倍;随着温压的增加,泥灰岩、含生物碎屑泥晶灰岩和细晶白云岩的溶解速率均相应增加,但细晶白云岩的溶解速率增加幅度更大;在深埋藏环境下(180℃,45 MPa到210℃,52.5 MPa),细晶白云岩的溶解速率逐渐与泥灰岩和含生物碎屑泥晶灰岩的溶解速率趋于一致。(2)在60℃,10 MPa时,白云质颗粒灰岩在乙酸溶液中的溶解能力大于粉细晶白云岩和亮晶鲕粒白云岩;随着温压的增加,有机酸溶液对石灰岩与白云岩的溶解能力均相应降低,且石灰岩溶蚀作用下降幅度更大,当温压达到或超过90℃、20 MPa时,粉细晶白云岩和亮晶鲕粒白云岩在乙酸溶液中的溶解能力大于白云质颗粒灰岩。根据实验结果推测:表生和相对浅埋藏的温压条件下,石灰岩的溶解速率和溶解能力大于白云岩,石灰岩的溶蚀作用较白云岩发育;但在深埋藏阶段,由于白云岩的溶解能力大于石灰岩,因此白云岩溶蚀产生的次生孔隙较石灰岩更为发育,这或许是深部的碳酸盐岩储层中多见白云岩储层的重要原因。     

On a Layer of Grossularite,Epidote and Calcite,Occurring at Kumush,Hoso,Sinkiang

正 In March,1944,while carrying on the geological reconnaissance in theHoso~1-Tokson~2 area,the writers had an opportunity of spending a fewdays in the vicinity of a small village named Kumush~3 (Fig.1) to study amoderately metamorphosed rock series.This region is characterized by     

Dissolution and deposition in fractures

Fractures are relatively planar discontinuities in rocks induced by the huge internal stresses which are created by the slow but constant motions of the underground masses. Deposition and/or dissolution of a single solute in a single fracture is addressed in the limit where the geometrical changes are very slow compared with the average fluid velocity. The deposition or dissolution fluxes are calculated by means of an efficient finite-difference scheme. Examples of Gaussian and self-affine fractures which undergo dissolution and also successive cycles of deposition-dissolution are studied for four different values of the Péclet and the Péclet-Damköhler numbers. Some general trends are discussed.     

Calcite Dissolution Kinetics

We compare the canonical treatment of calcite’s dissolution rate from the literature in a closed system, particle batch reactor, with the alternative approach suggested by Truesdale (Aquat Geochem, 2015). We show that the decay of rate over time can be understood in terms of the evolution and distribution of reactive sites on the surface of these particles. We also emphasize that interpretation of observed rates must not exclude the fundamental role of crystal defects, whose importance is already implicitly reflected in the common form of rate laws in geochemistry. The empirical behavior of overall rate in closed systems, such as those described by Truesdale, may thus reflect relationships between defect centers and the generation of steps over the calcite surface (previously documented for silicates), such that below a critical free energy limit, there is insufficient driving force to open hollow cores and thus a loss of reaction mechanism. Dissolution in this very-near-equilibrium regime will be dependent on the distribution of extant steps and the energetics of new kink site nucleation. However, these sensitivities are complicated in the case of particle systems by grain boundaries, edges, corners, and other terminations. Such discontinuities constitute a defect class whose overall kinetic importance will be strongly tied to particle diameter and which can act independently of the internal strain field imposed by screw and edge dislocations.     

Dissolution Enthalpies of Magnesian Calcites

A series of synthetic and biogenicmagnesian calcites was dissolved in weak acetic acidsolutions to measure the enthalpies of dissolution at25°C. For the synthetic phases, heat releasedwas 33.5 kJ/mol for calcite, decreasing to 33 kJ/molfor a phase of 2 mol % MgCO3, and increasing to35 kJ/mol for a phase of 15 mol %. Values of excessenthalpies, ΔHxs, calculated using calciteand magnesite end-members, average about -1 kJ/mol forthe synthetic phases. Total entropies of solidsolution formation, ΔSSS, also werecalculated using available data on Gibbs free energiesof formation and these excess enthalpies. Values ofΔSSS range from -2 J/(mol-K) at 2 mol % to-5 J/(mol-K) at 15 mol % MgCO3. These negativevalues of ΔHxs and ΔSSSsuggest that some form of ordering (cation?) isobtained in the synthetic phases, and that vibrationalentropies of the solid solution are diminished incomparison to the end-members. In contrast, biogenic samples generally have positivevalues of ΔHxs, increasing from +1 kJ/molat 5 mol % to +3 kJ/mol at 20 mol % MgCO3. Mostvalues of ΔSSS are equal (within errors)to values expected from configurational enthalpyalone. Thus, in most biogenic materials cationordering probably is not obtained, and most phases aremore typical of equivalent-site solid solutions.     

Dissolution kinetics and the weathering of mafic minerals

Fayalite, hypersthene, basalt, and obsidian were dissolved in buffered solutions (25°C; pH 4.5 and 5.5) under air, N₂ or O₂ atmospheres, in order to follow the kinetics of dissolution. Each dissolved more rapidly at lower pH values, dissolving most rapidly in the initial few days, followed by slower dissolution for periods up to six months. Dissolution was more rapid when air was excluded. In oxygen atmospheres an Fe(OH)₃ precipitate armors mineral surfaces, thus inhibiting further dissolution, and further affects the solution by scavenging dissolved silica and cations. Dissolution reactions include initial exchange between cations and H⁺, incongruent dissolution of silicate structures, oxidation of Fe²⁺ in solution, precipitation of Fe(OH)₃, and scavenging of dissolved silica and cations by Fe(OH)₃. Dissolution kinetics may explain weathering of mafic rocks and minerals at the Earth's surface, the formation of Fe-oxide coatings on mineral grains, weathering of submarine mafic rocks and intrastratal solution of mafic minerals in buried sandstones. Early Precambrian weathering would have been more rapid before the appearance of large amounts of oxygen in the atmosphere, and continental denudation rates may have been higher than at present because of this effect and the predominance of mafic igneous rocks at an early stage of continent formation and growth.     

石英溶解机理的研究进展

鉴于石英溶解对全球变化和环境影响的重要性,其机理的研究成为近来地学领域比较活跃的课题.许多学者从物理、化学,甚至生物学的角度研究了石英溶解的机理.本文从pH值、温度、离子强度和表面形态四个方面综述石英溶解机理的研究进展.前人的研究成功地解释了石英-水在不同条件下的溶解机理,成功地解决了溶解动力学相应的单个参数问题.但是,石英溶解的条件非常复杂,往往是多个因素共同作用的结果,因此只研究单个参数的影响无法真正了解矿物溶解的动力学机理.石英溶解机理需要探索实验和理论方面更深层次的问题.     

Dissolution rates of earthworm-secreted calcium carbonate

The dissolution of CaCO₃ granules secreted by earthworms in soil leaching columns was governed by soil pH and exchange sites available for Ca. Results indicate that granules could last for significant periods of time in soils and that, therefore, granules could be an important source of soil calcite.     

长石溶解模拟实验研究综述

在综合分析前人研究成果基础上,对长石溶解模拟实验研究进行了综述.研究表明,长石溶解与其成分、结构、反应的温压条件以及流体性质等有关.在相同的温压条件下,3种长石的稳定性依次为:钾长石>钠长石>钙长石,且温度升高可加强长石的溶解能力,促进长石的溶解,而压力的变化对长石的溶解影响不大.长石的溶解速率在酸性区域随pH值增大而减小、在中性区域溶解速率低且受影响小、在碱性区域随pH值增大而增大.有机酸通过提供H+、络合金属元素来提高长石的溶解度.长石的溶蚀速率与颗粒的总表面积大小以及颗粒表面粗糙度有关,总表面积越大,表面越粗糙,则反应速率越快,而且溶液的矿化度越低越有利于长石的溶解.长石的溶解过程由表面反应和扩散反应所控制,描述长石溶蚀机理的模型主要包括:表面反应模型和淋滤层扩散模型.     

Dissolution of olivine during natural weathering

Naturally weathered olivine occurring as phenocrysts in Hawai’ian volcanic rocks from several volcanic centers and regolith/outcrop settings, and as tectonized olivines from several metadunite bodies in the southern Appalachian Blue Ridge, are all similarly corroded by natural weathering. Conical (funnel-shaped) etch pits occur as individual pits, base-to-base pairs of cone-shaped pits, or en echelon arrays. Etch-pit shapes and orientations in the smallest etch-pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering. Etch pits occur in samples with chemical and/or mineralogical evidence of weathering, and/or are associated with, or proximal or directly connected to, fractures or exposed outcrop surface, and therefore are formed by weathering and not inherited from pre-weathering aqueous alteration (e.g., serpentinization, iddingsitization) of these parent rocks. Many etch pits are devoid of weathering products. Natural weathering of olivine is surface-reaction-limited. Similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent-rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization/recrystallization history, eruption/weathering/exposure ages of the olivine’s host rock, and the local regolith history.     

Sucrose Dissolution Studies Leading to a Generic Shrinking Object Model for Batch Dissolution of Regular-Shaped Particles

The dissolution of sieved sucrose crystals has been studied spectrophotometrically by observing the increase in dissolved sucrose concentration with time. Equations recently derived from the shrinking sphere model for the batch dissolution of a solid in under-saturated conditions tested successfully on both single crystal-size and mixtures of two sizes of sucrose crystals. Single-sized crystals provided a straight line for the plot of the fraction of un-dissolved solid to the power one-third, versus time ( vs. t). The dissolution of mixtures of two crystal sizes fitted the non-linear equation tested earlier on sodium chloride in water-propanone mixtures. Together, these two sets of tests on ionic and covalent substances verify that many simple dissolutions will be easily modelled using this physical model based on shrinkage, where the chemical composition of the solids is very much of secondary importance. Consequently, there is an increased chance that the equations will describe the dissolution of biogenic silica in seawater, the problem which originally inspired this study. More than this, though, the equations are discovered to be mathematically generic; very many geometries other than the sphere satisfy the same equations, and the “shrinking object dissolution model” is thereby defined. The approach should also apply even to non-aqueous dissolutions. A prototype plot of shrinking object rate constant (obtained from numerical fitting of the model to sucrose) versus particle size is presented, and it is shown how analogous treatments for other substances will be central to collection and use of much dissolution data in the future. The study is placed in context with much earlier solid phase decomposition studies, concluding that the key characteristic of the simplest of all dissolutions is that the interface between solid and liquid should advance at a uniform linear rate. It is shown how this approach leads to equations of the same mathematical forms already discussed above.     

鲕粒灰岩的溶解动力学特征和微观形貌的发育演化

鲕粒灰岩是重要的碳酸盐岩油气储层。通过常温常压下,北京西山鲕粒灰岩的溶解动力学实验发现,鲕粒灰岩的溶解主要受岩石和矿物的结构和化学成分控制。结构上表现为构造裂缝、鲕粒结合纹、颗粒晶体结合带等结构薄弱带优先发生溶蚀。成分上表现为白云岩化组分不仅自身较易溶蚀,而且能促进鲕粒灰岩的整体溶解;粘土矿物、石英等杂质组分常与鲕粒和白云岩化颗粒伴生,其对两者的阻溶效应更大。溶解过程中,鲕粒灰岩的构造裂隙、鲕粒放射结合纹和同心圈层等首先发生溶蚀,产生溶隙、粒内溶孔和粒间溶孔;其次为白云质组分以及白云质组分与方解石结合处而产生晶间溶孔、粒内溶孔;随着溶蚀强度的加强,围绕鲕粒形成环状溶蚀沟,在鲕粒溶蚀脱落后而形成鲕模孔。     

鲕粒白云岩内部溶蚀及孔隙非均质性演化研究

在地表和埋藏条件下溶蚀作用形成的次生孔隙是碳酸盐岩油气储层重要的储集空间。为探究孔隙的溶蚀改造机理及其控制因素,通过柱塞样,开展了0.2%的乙酸环境中鲕粒白云岩的溶蚀实验,实验温度和压力范围分别为40~160 ℃和10~50 MPa。实验采取连续取样获得溶液中Ca²⁺与Mg²⁺离子含量,通过CT 成像技术获取溶蚀反应前后孔隙图像并运用分形与多重分形方法定量分析孔隙在二维和三维空间上的非均质性。研究表明,溶蚀过程中,鲕粒白云岩溶蚀液中[Ca²⁺+Mg²⁺]浓度在实验开始时最低,之后稳步上升并在120 ℃、40 MPa时达到最大值,而后缓慢下降,溶蚀窗范围出现在70 ℃(20 MPa)~120 ℃(40 MPa)之间。二维和三维微观孔隙结构在空间上的分布具有显著的多重分形特征,溶蚀作用使得孔隙的不规则性减少、孔隙的奇异范围缩减。这些结果对于认识近地表和深埋藏条件下溶蚀作用对碳酸盐岩储层孔隙的改造强度及孔隙的动态演化规律具有重要的理论及实际意义。     

灰岩和白云岩溶解速率控制机理的比较

碳酸盐岩溶解的速率控制过程包括:(1)岩石表面上的非均相化学反应;(2)离子从岩石表面通过扩散向溶液中的传输;(3)CO2向H 和HCO-3的转换.通常是这3个过程中的最慢过程决定着碳酸盐岩的溶解速率.然而,实验和理论分析发现,在条件相似的情况下,白云岩的初始溶解速率不仅只有灰岩的1/3～1/60,而且灰岩和白云岩的溶解呈现出不同的速率控制机理.如对灰岩而言,在实验中加入能催化CO2转换反应的生物碳酸酐酶(CA)后,其溶解速率增加出现在CO2分压>100Pa的区域,最高可达10倍;而对白云岩,其溶解速率增加出现在CO2分压<10000Pa的区域,且增加仅3倍左右.此外,虽然2类岩石的溶解也均受水动力条件(旋速或流速)的控制,且主要出现在CO2分压<1000Pa的区域,但灰岩的溶解对水动力条件的变化比白云岩溶解更敏感.这些发现在解释和揭示自然界白云岩和灰岩岩溶发育及其相关资源环境问题的差异方面具有重要意义.     

硅灰石在氨基酸水溶液中的电化学及溶解作用研究

在模拟人体温度３７℃条件下,进行硅灰石纤维粉尘与氨基酸作用的实验研究,测定在７２ｈ内溶解过程中ｐＨ值和电导率（к）的变化。结果表明,硅灰石纤维矿物在氨基酸中发生的溶解作用与酸的性质有关,酸性氨基酸对矿物的溶解能力最强,中性次之,碱性最弱。硅灰石在酸性和碱性氨基酸中８ｈ左右出现溶解饱和点,而在中性氨基酸中７２ｈ内无溶解饱和点,溶解度具有随时间的增加而呈线性增长的趋势。硅灰石的这种溶解特征表明,硅灰石     

地应力对碳酸盐岩溶解和岩溶发育的影响

地应力对碳酸盐岩溶解和岩溶发育的影响研究是一个值得注意但又长期被忽视的问题, 结合地球化学、地质热力学、矿物岩石学、岩石弹塑性力学方面的知识, 在理论上全面地分析地应力的影响.研究表明, 在应力作用下, 碳酸盐岩岩体内应变能提高、溶解反应自由能增加、裂隙发育特征与水流运动条件发生变化, 使得碳酸盐岩固体表面处溶解物饱和浓度增大, 改变了岩体内水流的厚度、水流流态以及水中碳酸盐岩溶解物浓度, 从而影响了碳酸盐岩的溶解速率; 且应力的存在改变了有效的水岩相互作用面积; 应力作用下碳酸盐岩溶解存在的"应力-溶解"耦合竞争循环机制使得岩溶发育出现混沌现象和自组织行为, 初始应力介入所导致的一个很微小的影响因可被迅速放大至成百上千倍而不可忽略, 自然界中碳酸盐岩岩体内溶解和结晶并存、串珠状溶洞以及孤立溶洞的形成发育机制一定程度上可归结为应力溶解自组织行为的结果.