Kinetics of Hydrothermal Reactions of Minerals in Near-critical and Supercritical Water

This work presents new experimental results on the kinetics of mineral dissolution in near-critical and supercritical water in a temperature range (T) from 25 to 400℃ and a constant pressure of 23 MPa. Kinetic experiments were carried out by using a flow reactor (packed bed reactor) of an open system. The dissolution rates of albite and magnetite were measured under these experimental conditions. Na, Al and Si release rates for albite dissolution in water were measured as a function of the temperature and flow velocity in the reaction system. The maximum release rates of Na, AI and Si of albite dissolution in the hydrothermal flow systems under different flow velocities were always obtained at 300℃, that is to say, the maximum albite dissolution rates in the flow systems, regardless of different flow rates, were repeatedly measured at 300℃. Results indicate a wide fluctuation in albite dissolution rates occurring close to the critical point of water. The dissolution rates increased when the temperatu     

Deformation of natural gypsum rock: Mechanisms and questions

The deformation mechanisms of a natural gypsum rock were investigated in instantaneous and long-term laboratory tests. The mechanical behaviour of this rock is strongly influenced by the relative humidity, but this influence seems to be more important on the long-term behaviour. Apart from classical plastic mechanisms, we identify a damage-like mechanism assisted by the variation in relative humidity. Even though this mechanism contributes to the increase of creep strain rate, laboratory evidence shows that it is unrelated to oriented cracking, as usually is the case with stress-induced damage. This mechanism is thought to be due to the migration of water molecules from their sites in the crystalline structure of gypsum (CaSO₄·2H₂O) while under stress. By entering the capillary space, these molecules contribute to the formation of adsorbed water layers and the increased thickness of these layers, thus reducing the interaction between neighbouring faces of gypsum crystals. So, more than effective stress, a complex mechanism reducing the creep activation energy, assisted by relative humidity and stress, is considered as the principal mechanism controlling the long-term behaviour of natural gypsum rock. Such a mechanism would also explain the traces of dissolution observed in pillars of underground gypsum quarries, where, because of the absence of water table, a dissolution-precipitation creep seems improbable.     

Hydrochemistry of formation water with implication to diagenetic reactions in Sanzhao depression and Qijia-gulong depression of Songliao Basin, China

The Sanzhao depression and Qijia-gulong depression are two of the most important hydrocarbon-bearing sub-depressions in the Songliao basin, dominated by Cretaceous fluvial and lacustrine strata with volcanic and volcaniclastic rocks. In this paper, the pressure distributions and hydrochemistry variations in those two depressions are investigated. It is found that the Qijia-gulong depression is dominated by overpressure, but the Sanzhao depression is dominated by underpressure, especially in the Quantou formation. The salinity of formation water ranges from 1.0 g/L to 20 g/L, with a mean of 4 g/L in most formations. Water composition is dominated by NaHCO₃ water type but varies greatly in salinity and ionic ratios in the different depressions. The results of mass balance calculation reveal that, fluid chemistry in the Qijia-gulong depression is dominated by dissolution of silicate minerals and halite, whereas in the Sanzhao depression it is dominated by silicate mineral dissolution, evaporite dissolution (halite and gypsum), carbonate mineral dissolution/precipitation, and cation exchange.     

高铝粉煤灰合成莫来石的SEM和XRD研究

以Al2O3含量为52.72%的高铝粉煤灰和工业氧化铝为原料,在1300～1500℃保温2～4h合成了平均含量超过70%的M50、M60和M70莫来石。用SEM和XRD研究了合成莫来石的显微结构和物相组成。结果表明,未经盐酸处理的粉煤灰比用盐酸处理过的粉煤灰合成的莫来石的晶体和长径比大;随粉煤灰中Al2O3含量的增高,合成的莫来石的晶体尺寸和长径比变小,合成莫来石的温度需相应提高至1500℃;恒温时间长短对合成莫来石的影响远小于烧结温度的影响,高温下缩短恒温时间比低温下延长恒温时间有利于莫来石的生成。以高铝粉煤灰为原料合成M50和M60莫来石较为适宜,Al2O3与SiO2的质量比应控制在1.5～2.0。     

Chemical reaction diversity of geofluids revealed by hydrothermal experiments under sub- and supercritical states

Abstract   Hydrothermal experiments of dissolution of granite and quartz with pure water up to 600°C and 60 MPa were carried out in order to evaluate chemical reaction under sub- and supercritical conditions. The supercritical region beyond the critical point for water has been inferred to be a homogeneous state, which does not correspond to either a true liquid phase, or a true vapor phase. Results of dissolution experiments of granite and quartz in a supercritical state (artificially defined as higher pressures and temperatures than the critical point), show this fluid can be subdivided into two apparent phases comprising a 'liquid-like' region and a 'vapor-like' region. Chemical phenomena with respect to dissolution reactions in the 'liquid-like' region are more similar to those of subcritical water, whilst those reactions in the 'vapor-like' region even in the supercritical state are considered to be weak. The critical point of various kinds of geofluids, composed of solutions in the H₂O-CO₂-NaCl system was experimentally determined using a visible type autoclave. Chemical reaction diversity, depending on location of the critical point for a given solution and apparent phase boundary in terms of chemical reaction within the supercritical state, was recognized by hydrothermal experiments.     

Chemical mass balance in a reaction zone between serpentinite and metapelites in the Nishisonogi metamorphic rocks, Kyushu, Japan: Implications for devolatilization

Abstract   Reaction zones of 0.5–10.0 m thick are commonly observed between serpentinite and pelitic schist in the Nishisonogi metamorphic rocks, Kyushu, Japan. Each reaction zone consists of almost monomineralic or bimineralic layers of talc + carbonates, actinolite (or carbonates + quartz), chlorite, muscovite and albite from serpentinite to pelitic schist. Magnesite + quartz veins extend into the serpentinite from the talc + carbonates layer, while dolomite veins extend into the pelitic schist from the muscovite layer. These veins are filled by subhedral minerals with oriented growth features. Primary fluid inclusions yield the same homogenization temperatures (145–150°C) both in the reaction zone and in the veins, suggesting their simultaneous formation. Mass-balance calculations using the isocon method indicate that SiO₂, MgO, H₂O and K₂O are depleted in the reaction zone relative to the protoliths. These components were probably extracted from the reaction zone as fluids during the formation of the reaction zone.     

五氯苯酚在矿物表面吸附——实验、表面反应模式及其环境意义

利用批量平衡技术研究了石英、高岭石、伊利石、蒙脱石和铁氧化物对五氯苯酚(PCP)吸附的pH关系等温线和浓度关系等温线,发现所有矿物的pH关系等温线都表现出典型的峰形曲线特征,峰位在pH=5～6之间,依矿物不同而不同。基于矿物表面羟基位化合态和PCP的化合态考虑,提出一种包含表面络合反应和表面静电吸附反应的模式,对pH关系等温线计算拟合发现有很好的相关性。模式计算还表明,石英和层状硅酸盐矿物对PCP吸附以表面络合反应为主,而氧化铁矿物则包含表面络合反应和表面静电吸附反应,但以后者占主导,其反应平衡常数比前者大1～3个数量级。高岭石和氧化铁矿物的浓度吸附等温线可用Langmuir方程很好拟合,最大吸附量的大小顺序是赤铁矿>纤铁矿>针铁矿>高岭石>石英>蒙脱石≈伊利石,并可以用矿物表面羟基位浓度和反应机制加以解释。PCP在矿物表面可观的吸附量说明矿物表面吸附对憎水性可离解有机化合物(HIOCs)在天然水相体系和沉积中的迁移转化过程起着相当重要的作用。     

Pressure, temperature and cooling rates of granulite facies migmatitic pelites from the southern Adirondack Highlands, New York

Detailed petrographic analysis was performed on samples from five localities within the southern Adirondacks. Textures and zoning patterns in garnet from all samples provide evidence for dehydration melting of biotite. Zoning of grossular in garnet – providing a record of prograde growth – shows both increasing and decreasing trends in garnet from the same sample. However, Ca concentrations at the garnet rims of most samples are identical (grossular = 3.4%). These observations have been interpreted as evidence for the differential timing of garnet nucleation and growth. All Fe/(Fe + Mg) and some spessartine distributions are consistent between samples, displaying diffusive profiles established largely upon cooling. Only one sample, in which retrogression was minimal, contains garnet with flat Fe/(Fe + Mg) profiles. A general pelitic pseudosection constructed in the system MnNCKFMASH reveals a maximum for Ca in garnet where the plagioclase‐out isopleth intersects the solidus (muscovite = 0). The pseudosection predicts bell‐shaped core‐to‐rim profiles of grossular during anatexis, similar to those observed in the rocks. Garnet–biotite thermometry and GASP barometry indicate peak temperatures of at least 790 °C at about 7–9 kbar, similar to conditions determined for the central Adirondacks. Cooling rates determined from finite difference modelling of spessartine and Fe/(Fe + Mg) diffusional profiles indicate a multi‐stage cooling history in which some period of rapid cooling (>200 °C Myr^?1) is required.     

Transport of low-aH2O dehydration products to melt sites via reaction-zone networks, Milford Sound, New Zealand

The gabbroic/dioritic Pembroke Hornblende Granulite (PHG) of Milford Sound displays a geometrically simple mesoscopic network of sub‐planar garnet reaction zones (GRZ) in which the meta‐igneous hornblende granulite has been depleted of Na, Si, and H₂O, and c. 25 vol.% almandine‐rich garnet has formed. Some studies postulate the initial presence of melt along the centres of all GRZ, explaining the frequent absence of feldspathic veins by selective melt loss. A more parsimonious model is necessitated by structural evidence and, together with chemical data, suggests a relationship between mid‐range metasomatic transport and anatexis. The Pembroke outcrops show a process of incipient melting of gabbro/diorite in an environment of relatively low aH₂O in lithologies that have limited free quartz. A non‐equilibrium steady state is proposed, in which a sodic dehydration fluid moves some distance via the GRZ network towards areas of partial melting. Only in these areas are Na and Si reconstituted as albite, with more garnet as byproduct, having avoided the need for melt percolation. The combined structural and chemical evidence directs a focus on mass transport in low‐aH₂O gabbroic environments. In subsequent events of shearing and complete transposition, both sets of garnet – the atypical GRZ residue and partial melt melanosomes – were inherited by the Milford Gneiss ‘facies’ of the PHG.