Mechanism and kinetics of hydrothermal replacement of magnetite by hematite

The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions(140 -220℃, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O_2(aq) environments, some of the replacement still occurred via Fe²⁺ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe²⁺ (and the oxidant) to and from the reaction front. Reaction rates are fast at ~200℃, and in nature transport properties of Fe and,in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ± 6 kJ mol^-1.This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions.     

ELECTROCHEMICAL STUDIES ON THE CATHODIC REACTION OF MARINE ATMOSPHERIC CORROSION

Electrochemical studies on the cathodic reaction of marine atmospheric corrosion using Kelvin probeas reference electrode showed that the rate of cathodic reaction-oxygen reduction first increases then de-creases, with the reaction maximizing at a certain thickness as the electrolyte film decreases duringevaporation. It was indicated that with decreasing electrolyte thickness by drying, the oxygen reduction ratewas accelerated by the faster oxygen diffusion due to the thinner electrolyte layer on the metal surface. Theresults also revealed that although the oxygen salting out effect has great influence on the rate of oxygenreduction, it is not the main causative factor for the decrease in cathodic limiting current in the case of avery thin electrolyte layer.     

IMAGES OF MAPS: A ROTATION EXPERIMENT*

Theories which explain mental imagery are discussed and are related to cognitive maps. An experiment was conducted to illustrate that people can code information about maps as visual images and use map images for a map-reading task. Subjects were shown maps which were rotated at various angles from north-at-the-top. The time needed to decide if a map was a correct or mirror representation was recorded. The results support the notion that people did this task by forming and rotating map images.     

大别山地区超高压变质岩的不平衡退变质反应及动力学

大别山区超高压变质岩的退变质作用表现为后成合晶和后成合晶冠状体的形成。这是薄片尺度不平衡反应的产物，岩石学和矿物微区化学研究表明，后成合晶结构是等化学出溶反应的结果，而后成合晶冠状体结构是通过扩散反应形成的，这两种反应结构都是由页片状和杆状矿物组成，矿物页的形态与超高压变质岩的ｐＴｔ轨迹相关。     

Genesis of the Kapuskasing (Ontario) migmatitic mafic granulites by dehydration melting of amphibolite: the importance of quartz to reaction progress

Migmatitic, granulite-grade mafic gneisses make up a significant part of the Kapuskasing Structural Zone (KSZ), Ontario. Although they contain a common mineral assemblage [hornblende (Hbl)+plagioclase (Pl)+diopside (Di)±garnet (Grt)+quartz (Qtz)±titanite (Ttn)], the mafic gneisses show wide variations in modal mineralogy from hornblende-rich to diopside+garnet-rich varieties and all gradations between. Up to 25 vol.% segregated plagioclase+quartz-rich (trondhjemitic) leucosome (Tdh) is intimately associated with the mafic gneiss, occurring in a continuum of patches, veins and transecting dykes at scales ranging from decimetres to micrometres. The texture and composition of the leucosome, combined with P-T estimates for the host rocks above the solidus, suggest it represents crystallized trondhjemitic melt. Quartz is mainly restricted to the segregated leucosomes but more rarely occurs in a variety of interstitial textures in the mafic gneiss, suggesting that it crystallized from a melt phase rather than having been present as a solid phase at peak metamorphic conditions. Modal and textural data indicate a reaction relationship of the form: Hbl+Pl(+Qtz?)=Grt+Di+Ttn+leucosome (Tdh), implying that the granulite-forming process involved dehydration melting of an amphibolite protolith. Pressure-temperature estimates from Grt+Di+Pl+Qtz geothermobarometry are 9 kbar and 685-735 °C; however, based on experimental studies of dehydration melting of amphibolite, we estimate that peak conditions were closer to 11 kbar, 850 °C. Mass balance analysis, using the technique of singular value decomposition, and reaction space analysis were used to quantify the reaction and to determine the controls on reaction progress. The following mass balance provides a model for the natural reaction:1.00 Hbl+0.92 Pl+3.76 Qtz=1.14 Grt+1.54 Di+0.21 Ttn+1.49 Tdh+0.14 ‘pg’+0.39 Fe_?1Mg+0.33 NaSiCa_?1Al_?1where ‘pg’ is a pargasite-like exchange. In all model mass balances tested, quartz is a reactant with a large coefficient. We argue that the abundance of quartz in the amphibolite protolith was the primary control on the differing extents of reaction observed. Mineral compositional variation exerted a secondary control on reaction progress, with Fe-richer layers containing An-richer plagioclase and more actinolitic amphibole reacting earliest (i.e. at lowest temperatures). Comparison of the calculated amount of melt produced in the gneisses with that now observed implies expulsion of 5–30% of the melt. These volumes are similar to those predicted from REE modelling of Archaean tonalities and trondhjemites from a garnet amphibolite source, suggesting that the KSZ mafic gneisses may be representative of partially depleted source rocks for trondhjemite-tonalite generation.     

怀安—大同一带基性麻粒岩变质反应序列与水活度

晋冀蒙三省交界的怀安－大同一带基性麻粒岩有保存完好的变质反应结构。依据岩相学特征，可在基性麻粒岩中区分出四个世代的矿物组合：①峰前阶段（Ｍ１）以包裹在石榴石或紫苏辉石中的Ｈｂ＋Ｐｌ±Ｑｚ、Ｃｐｘ＋Ｈｂ＋Ｐｌ±Ｑｚ组合为代表，Ｔ为６００℃～７００℃，Ｐ为０．７０～０．８２ＧＰａ，相应的ａＨ２Ｏ值为０．５０～０．６２；②峰期阶段（Ｍ２）以三联点结构的基质矿物为特征，组合为Ｏｐｘ＋Ｃｐｘ＋Ｈｂ＋Ｐｌ±Ｑｚ、Ｇｔ＋Ｏｐｘ＋Ｃｐｘ＋Ｈｂ＋Ｐｌ±Ｑｚ，Ｔ为７５０℃～８５０℃，Ｐ为０．８４～１．０５ＧＰａ，相应的水活度降至０．３９～０．５２；③峰后近等温减压阶段（Ｍ３）以石榴石普遍转变为Ｏｐｘ＋Ｐｌ±Ｍｔ及角闪石转变为Ｏｐｘ＋Ｃｐｘ后成合晶为特征，Ｔ为７００℃～７８０℃，Ｐ为０．５０～０．７０ＧＰａ，相应的ａＨ２Ｏ值降至０．０８～０．１１；④至降温阶段（Ｍ４），以无水矿物石榴石、辉石退变为Ｈｂ＋Ｐｌ组合为代表，Ｔ为５３０℃～６３０℃，Ｐ为０．３０～０．４２ＧＰａ，ａＨ２Ｏ值再次升至０．３７～０．４０。研究表明，矿物组合演化和所发生的（脱水）变质反应不仅受ＰＴ条件的控制，而且与体系水活度密切相关     

含水层中考虑水—岩间阳离子交换的溶质运移方程

导出了能反映含水层中水—岩间阳离子交换作用的溶质运移方程,并提出了一个新的海水入侵数学模型.模型以交换阳离子Na+和Ca2+作为模拟因子,考虑了海水入侵过程中水—岩间的Na+—Ca2+交换,模型用于模拟山东省龙口市黄河营地区的海水入侵,取得了令人满意的结果,从而证明导出的溶质运移方程是正确的,建立的模型是可靠的.     

快速制备肼—高岭石插层复合物的方法及意义

将一定浓度的肼溶液与高岭土混合，施以机械研磨，不仅加快了肼分子插入高岭石的速率(10分钟内可使插层率达96％)，而且超细效果明显。在加温120℃后，插入高岭石层间的肼分子挥发，产生的气体能进一步将高岭土剥离，最后得到—2μm＞90％的超细高岭土粉体，且片状晶形完好。这种技术为制备“双90”超细高岭土提供了新的思路和方法。     

Adsorption and reaction of trichlorofluoromethane on various particles

As a possible tropospheric sink of trichlorofluoromethane (CCl₃F), its adsorption and reaction on solid particles were studied with or without UV light longer than 310 nm. The adsorption and photodesorption occurred for most of the particles examined. The amounts depended not only on the BET surface area but also on the chemical property of the particles. The reaction rates were less than 0.1% h^-1. The surface induced degradation of CCl₃F in air was confirmed under photoirradiation at room temperature by the detection of Cl^- or F^- on some metal oxides. When the particles were pretreated with heat and evacuation, the reaction rate became larger and disproportionation of CCl₃F took place. The tropospheric lifetime of CCl₃F was reevaluated based on a reported model. It is suggested that the heterogeneous reaction of CCl₃F on particles may be a possible tropospheric sink.     

Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan

Abstract The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc. In Baltistan, this suture has been reactivated by the late 'break-back'Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low- P (2–4 kbar, c. 500° C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent south-verging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8–9-kbar rocks over 5–6-kbar rocks). There is little variation in temperature with structural level (550–625° C). These two different terranes have been juxtaposed after metamorphism by the late south-directed MKT.