Chemical transformations of n-hexane and cyclohexane under the upper mantle conditions

Alkanes are an important part of petroleum,the stability of alkanes under extreme conditions is of great significance to explore the origin of petroleum and the carbon cycle in the deep Earth.Here,we performed Raman and infrared(IR)spectroscopy studies of n-hexane and cyclohexane under high pressure up to~42 GPa at room temperature(RT)and high temperature(HT).n-Hexane and cyclohexane undergo several phase transitions at RT around 1.8,8.5,18 GPa and 1.1,2.1,4.6,13,30 GPa,respectively,without any chemical reaction.By using resistive heating combined with diamond anvil cell at pressure up to 20 GPa and temperature up to 1000 K,both n-hexane and cyclohexane decompose to hydrogenated graphitic carbon and n-hexane exhibits higher stability than cyclohexane.Our results indicate that hydrocarbons tend to dehydrogenate in the upper mantle,and the extension of carbon chains may lead to the formation of some unsaturated compounds and eventually transfer into graphitic products.     

The electrical conductivity of olivine under highly reducing conditions

The electrical conductivity of San Carlos olivine has been measured at 1100 °C under reducing conditions at controlled oxygen fugacity, inside and outside the olivine stability field, in order to study the kinetics of olivine destabilization. Electrical conductivity increases along the direction [010] and decreases along [001]. as oxygen fugacity decreases. To understand these dependences, electrical conductivity transitory regimes were studied. In response to decreases in oxygen fugacity, two transient regimes with different time scales have been observed. A fast (≈1–2 min) increase of electrical conductivity is first observed, followed by a slower decrease (1–10 h, depending on the crystal orientation). After a few hours of annealing, precipitation of metallic iron and nickel and formation of amorphous silica can be observed at the crystal surface. The fast conductivity increase in the first transient regime is ascribed to an increase in the population of electrons at the olivine surface. Two effects: (1) equilibration of surface defects with the bulk of the crystal, and (2) iron loss from the olivine due to metal precipitation, could explain the subsequent decrease of electrical conductivity. Anisotropic diffusion of surface defects to the bulk of the crystal, by a process faster than atomic diffusion is the most likely. Received: 3 September 1997 / Revised, accepted: 16 April 1998     

Silica phase transformations in the petroliferous sequences

The joint evolution of organic matter and silica in petroliferous sequences is considered in the terms of the laws of transformation of dispersion systems. The dispersion systems are transformed under conditions of low-temperature solid-phase processes accompanied by the silica phase transition and dehydration that favors the evolution of organic matter.     

High pressure phase transformations in aragonite-type carbonates

High-temperature and high-pressure experiments conducted in a diamond-anvil cell revealed phase transformations in the aragonite-type carbonates of strontianite (SrCO₃), cerussite (PbCO₃), and witherite (BaCO₃) at pressures below 4 GPa and ～1000?°C. The powder X-ray diffraction patterns of these high-pressure phases can be reasonably indexed with the same type of orthorhombic cell having a space group of P2₁22 (17). By assuming 16 MCO₃ (M=Sr, Ba or Pb) molecules in a unit cell, the transition from the aragonite form to a new phase was concomitant with a volume contraction of 4.23, 2.38, and 2.34% for SrCO₃, PbCO₃, and BaCO₃, respectively. If the same phase transition were to occur in CaCO₃, it has been estimated that the transition would accompany a 7% volume contraction.     

Investigating the swelling pressure of highly compacted bentonite/sand mixtures under constant-volume conditions

Acta Geotechnica - Compacted bentonite/sand mixtures are often considered as sealing/backfilling materials in deep geological disposal for radioactive waste. This study investigates the swelling...     

Electron microscope study of phase transformations in cubanite

Direct observations of the phase transformations in cubanite both on heating and cooling have been made in a transmission electron microscope. Low temperature orthorhombic cubanite undergoes a cation disordering process at about 200° C resulting in a hexagonal wurtzite type structure. The reordering process takes place by the growth of independently nucleated regions of short range order. Long range order is difficult to attain in short term experiments due to the lack of order correlation between regions. At a slightly higher temperature a transformation from hexagonal close-packing to cubic close-packing takes place within the sulphur structure, by the propagation of partial dislocations along alternate close-packing planes. This transformation is irreversible. On cooling the disordered cubic structure, cation ordering takes place as an alternative to the transformation back to hexagonal close-packing, but in the absence of an ordering scheme at this composition based on the cubic subcell, the ordering process results in the exsolution of chalcopyrite. This transformation is interpreted in terms of metastable, kinetically controlled behaviour.     

Trace element partitioning between vapor, brine and halite under extreme phase separation conditions

Experiments were conducted to investigate the partitioning of Li, Br, Rb, Cs and B between vapor, brine and halite during subcritical and supercritical phase separation in the NaCl-H₂O system (388-550 °C, 250-350 bars). Results indicate that Li and Br partition preferentially into the low-salinity vapor fluids, while Rb and Cs become more enriched in the coexisting brines. Under more extreme conditions of pressure and temperature in the two-phase region, especially near the vapor-brine-halite boundary, strong salting-out effects imposed on neutral aqueous species enhance significantly partitioning of all trace elements into the low-salinity fluid. Dissolved boron is strongly affected by this and a particularly strong enrichment into vapors is observed, a trend that can be effectively correlated with changes in reduced density. Exclusion of Li, Br, Rb, Cs and B from halite, when precipitated, further increases the solubility of these species in the coexisting Cl-poor fluid. In general, the lack of distortion in the partitioning behavior of trace elements between vapor, brine and/or halite with the transition from subcritical to supercritical conditions in the NaCl-H₂O system precludes the need for special reference to the critical point of seawater when interpreting phase relations in submarine hydrothermal systems. The combination of experimentally determined trace element partitioning data with constraints imposed by mineral solubility provides a means to better understand the origin and evolution of hot spring vent fluids. For example, in Brandon hydrothermal system (21°S EPR) supercritical phase separation and subseafloor mixing appear to be the main heat and mass transport mechanisms fueled by a shallow magmatic intrusion, with boron systematics ruling out major contributions from magmatic degassing processes accompanying the near-seafloor volcanism.     

Effect of water on tholeiitic basalt phase equilibria: an experimental study under oxidizing conditions

To investigate the effect of water on phase relations and compositions in a basaltic system, we performed crystallization experiments at pressures of 100, 200 and 500 MPa in a temperature range of 940 to 1,220°C using four different water contents. Depending on the water activity, the oxygen fugacity varied between 1 and 4 log units above the quartz-magnetite-fayalite buffer. Addition of water to the dry system shifts the solidus > 250°C to lower temperatures and increases the amount of melt drastically. For instance, at 1,100°C and 200 MPa, the melt fraction increases from 12.5 wt% at a water content of 1.6 wt% to 96.3% at a water content of 5 wt% in the melt. The compositions of the experimental phases also show a strong effect of water. Plagioclase is shifted to higher anorthite contents by the addition of water. Olivine and clinopyroxene show generally higher MgO/FeO ratios with added water, which could also be related to the increasing oxygen fugacity with water. Moreover, water affects the partitioning of certain elements between minerals and melts, e.g., the Ca partitioning between olivine and melt. Plagioclase shows a characteristic change in the order of crystallization with water that may help to explain the formation of wehrlites intruding the lower oceanic crust (e.g., in Oman, Macquarie Island). At 100 MPa, plagioclase crystallizes before clinopyroxene at all water contents. At pressures > 100 MPa, plagioclase crystallizes before clinopyroxene at low water contents (e.g. < 3 wt%), but after clinopyroxene at H₂O in the melt > 3 wt%. This change in crystallization order indicates that a paragenesis typical for wehrlites (olivine–clinopyroxene–without plagioclase) is stabilized at low pressures typical of the oceanic crust only at high water contents. This opens the possibility that typical wehrlites in the oceanic crust can be formed by the fractionation and accumulation of olivine and clinopyroxene at 1,060°C and > 100 MPa in a primitive tholeiitic basaltic system containing more than 3 wt% water. The comparison of the experimental results with evolution trends calculated by the thermodynamic models “MELTS” and “Comagmat” shows that neither model predicts the experimental phase relations with sufficient accuracy.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

In situ X-ray diffraction study of an aluminous phase in MORB under lower mantle conditions

In situ X-ray diffraction study was conducted to identify the crystal structure of the “Al-phase”, which was previously reported to form in basaltic compositions at pressures and temperatures of the uppermost part of the lower mantle. Le Bail whole-pattern fitting method was adopted to investigate the structure of the Al-phase under high pressure and temperature as well as ambient conditions. Observed patterns were satisfactorily fitted using the “hexagonal phase” with space group P6₃/m (plus minor amount of garnet) under both of these conditions. On the other hand, the calcium ferrite structure model proposed in some earlier studies based on quench experiments yielded profile-fitting results at significantly lower confidence levels, particularly at simultaneous high pressure and high temperature conditions, suggesting that this phase may not form in oceanic crust materials subducted in the uppermost lower mantle. The difference in densities of hexagonal and calcium ferrite phases, however, is only ~1% under pressures and temperatures of the uppermost part of the lower mantle conditions, which yields a negligible effect on the bulk density of the subducted oceanic crust.     

常压下硫酸盐与基性硅酸盐熔体不混溶的实验研究

自然界中广泛存在硅酸盐与硫酸盐之间的熔体不混溶现象,这种不混溶过程控制了岩浆演化过程的氧逸度变化和岩浆中S的含量,同时也对金属元素的富集具有重要意义。前人对硫酸盐与硅酸盐的熔体不混溶过程及稀土元素在不混溶相分配行为的研究主要集中在碱性硅酸盐与硫酸盐体系,但对基性硅酸盐与硫酸盐的熔体不混溶行为及稀土元素在此过程中的分配规律研究仍相对薄弱。本文选择基性硅酸盐样品与实验用Na₂SO₄按质量比1∶1制成混合实验样品粉末,并添加少量H₃BO₃作为助熔剂,通过马弗炉加热至1 200℃,使粉末完全熔融,并在1 200℃恒温12 h后在马弗炉中快速冷却至常温。对加温-冷却后的样品进行详细的岩矿相、SEM/EDS和不同相态的原位LA-ICP-MS分析。研究结果表明,在1 200℃的条件下,硫酸盐与基性硅酸盐熔体可以大比例混溶,且降温会造成两者的不混溶,在不混溶过程中Na、Ca、K、REEs等趋向于进入硫酸盐熔体。不混溶形成的硫酸盐熔体中,稀土元素含量明显高于残余硅酸盐熔体,但轻重稀土元素没有明显的分异。不混溶硅酸盐熔...     

Functioning of a karst aquifer from S Spain under highly variable climate conditions,deduced from hydrochemical records

Over a period of three hydrological years during which precipitation was highly variable, measurements of discharge, electrical conductivity, temperature and chemical components of the water at the Cañamero spring (southern Spain) were taken in order to characterise the hydrogeological functioning of the karst aquifer that it drains. The results obtained from analysis of the frequency distribution of electrical conductivity data revealed histograms with a plurimodal morphology (up to 5 modes) and a range of variation that increased with greater pluviometry during the study period. The discharge from this spring increased sharply a few days after major rainfall episodes and, when the discharge reached a value of approximately 3.73 m³/s, an overflow spring started to function. During high-water periods, there were sudden and large falls in EC, temperature and all the chemical components dissolved in the water. This high variability reflects the predominance of groundwater flow through karst drains, a typical feature of conduit flow systems. The considerable hydrochemical heterogeneity observed is caused by the mobilisation of important volumes of recharge waters from shallow areas of the aquifer (soil, epikarst) towards the discharge point, with greater participation of the unsaturated zone during high-water conditions, and a more active role played by the saturated zone during low-water conditions. In karst systems such as the one drained by the Cañamero spring, it is important to monitor several responses (hydrodynamic, hydrothermal, hydrochemical, etc.), with an appropriate sampling frequency and under different hydrometeorological conditions, in order to achieve an adequate hydrological characterisation. This is fundamental for planning, management and protection of groundwater in karst aquifers.     

水合物地层中考虑相变的轴对称热传导问题

沉积层中水合物热分解是一个含有水合物相变的热传导过程,相变界面的演化过程是水合物分解规律及相关灾害分析的最基本问题。文中在试验观测和分析实际物理过程的基础上,建立了水合物沉积层中混相多相变阵面多区域的热传导理论模型,并针对轴对称问题进行了系列数值模拟;模型试验测定了四氢呋喃水合物沉积物中不同热源温度下水合物相变阵面的演化过程。通过对比,水合物分解相变阵面扩展的数值与试验结果吻合较好。     

Determination of the enthalpies of formation of some platinum antimonides and their phase diagrams under standard conditions

Using the method of direct synthesis calorimetry, we determined the standard enthalpy of formation of PtSb (stumpflite), Δ _f H°_298.15 (PtSb, cr) =–105.16 ± 0.84 kJ/mol and PdSb₂ (geversite), Δ _f H°_298.15 (PtSb₂,cr) =–160.92 ± 0.84 kJ/mol. Isothermal (298.15 K, p = 1 bar) phase diagrams were computed for the Pt–Sb–S and Pt–Sb–O ternary systems in the coordinates composition of the Pt–Sb binary system versus fugacity of a gaseous volatile component (O₂, S₂).     

Phase transformations in ABO 4 type compounds under high pressure

For ABO ₄ type ternary oxides, high pressure phase transformations known up to the present are reviewed, and an attempt is made to explain and predict crystal structures of their high pressure phases. When ABO ₄ type compounds are plotted based on the two variables, k=r _A /r _B and t=(r _A +r _B )/2r _O, where r _A , r _B , and r _O are the ionic radii of A and B cations and divalent oxygen, they can be classified into the major structure types. It is found empirically that a compound basically transforms to the structure type isostructural with a compound lying in a classified area with the same k and larger t values in the diagram.     

The high-pressure phase transformations of PbO2: An in-situ X-ray diffraction study

Using a diamond-anvil high-pressure cell, an in situ X-ray diffraction study of PbO₂ to about 240 kbar at room temperature has revealed the following phase transformations: rutile → αPbO₂ → tetragonal fluorite → cubic fluorite with increasing pressure. The volume change for the transition rutile → αPbO₂ is about ?2% and for the transition αPbO₂ → tetragonal fluorite is about ?6%, nearly constant within the pressure range of investigation. The volume difference between the tetragonal and the cubic fluorite-type phases is negligibly small. Both the tetragonal and the cubic fluorite-type phases cannot be preserved after removal of the pressure, even after heating by the laser. Both phases have been found to revert to αPbO₂ at one atmospheric pressure. Optically, the colour of αPbO₂ changes from an opaque black to transparent red with increasing pressure. The tetragonal fluorite-type phase, which exists in the pressure range between about 90 and 180 kbar at toom temperature, is slightly transparent with very dark red or brown colour, and the cubic fluorite-type phase is dark red and transparent.     

Cordierite breakdown under high-pressure hydrous conditions

Green and Vernon (1974) have experimentally determined a divariant field of cordierite-orthoamphibole-kyanite-quartz with a slope of 12±4 bars per °C occurring between 8.0 and 10.4 kbar at 750° C and between 9.5 and 11.3 kbar at 850° C. However, the writers' thermodynamic calculation of the divariant band indicates a more limited range of 0.65 and 0.7 kbar at 750° C and 850° C respectively, with an estimated error of ±0.2 kbar.