The quartz-coesite transition measured in the presence of a silicate liquid and calibration of piston-cylinder apparatus

Hydrothermal melting reactions in the system NaAlSiO₄-SiO₂-H₂O have been extended into the coesite stability field using piston-cylinder apparatus, and the quartz-coesite transition has been bracketed at 735° (28.0±0.9 Kb) and at 800° C (28.15±0.9 Kb) in the presence of a silicate liquid phase; this provided favorable kinetic conditions. The results agree closely with previous piston-cylinder determinations employing a correction for pressure loss due to friction. Consideration of previous calibration and hysteresis experiments leads to the conclusion that despite its potential utility, the quartz-coesite transition can not be confidently used as a calibrant for piston-cylinder apparatus until the question of whether or not to apply a friction correction has been resolved.     

Phase boundary between ilmenite and perovskite structures in MnGeO3 determined by in situ X-ray diffraction measurements

In situ X-ray observations of the phase transition from ilmenite to perovskite structure in MnGeO₃ were carried out in a Kawai-type high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between the ilmenite and perovskite structures in the temperature range of 700–1,400°C was determined to be P (GPa) = 16.5(±0.6) − 0.0034(±0.0006)T (°C) based on Anderson’s gold pressure scale. The Clapeyron slope, dP/dT, determined in this study is consistent with that for the transition boundary between the ilmenite and the perovskite structure in MgSiO₃.     

In situ X-ray observations of the coesite-stishovite transition: reversed phase boundary and kinetics

Using a DIA-type, cubic-anvil, high-pressure apparatus (SAM-85) in conjunction with in situ X-ray diffraction, we have investigated phase relations between coesite and stishovite up to 12 GPa and 1530 °C using synthetic powders of the two phases as the starting materials. The phase transition between coesite and stishovite was identified by observing the first appearance of a phase that did not already exist or by a change in the relative intensity of the two patterns. In most experiments, the diffraction patterns on samples were collected within 10 minutes after reaching a pressure and temperature condition. On this time scale, two phase boundaries associated with the coesite-stishovite transition have been determined: (1) for the stishovite-to-coesite transition, observations were made in the temperature range of 950–1530 °C, and (2) for the coesite-to-stishovite transition from 500 to 1300 °C. These observations reveal that there exists a critical temperature of about 1000 °C to constrain the coesite-stishovite equilibrium phase boundary. Above this temperature, both boundaries are linear, have positive dP/dT slopes, and lie within a pressure interval of 0.4 GPa. Below this temperature, the dP/dT slope for the stishovite-to-coesite phase boundary becomes significantly larger and that for the coesite-tostishovite phase boundary changes from positive to negative. As a result, an equilibrium phase boundary can only be determined from the results above 1000 °C and is described by a linear equation P (GPa)=6.1 (4)+ 0.0026 (2) T (°C). This dP/dT slope is in good agreement with that of Zhang et al. (1993) but more than twice that of Yagi and Akimoto (1976). For the kinetics of the phase transition, preliminary rate data were obtained for the stishovite-to-coesite transition at 1160 and 1430 °C and are in agreement with the simple geometric transformation model of Avrami and Cahn.     

原子荧光光谱法直接测定载金炭样品中的砷

建立了原子荧光光谱法直接测定载金炭样品中砷的分析方法。采用硝酸溶样,原子荧光光谱法直接测定载金炭样品中的砷。该方法具有准确度高、流程短、精密度好、速度快等优点。比对试验结果表明,该方法测定结果与全谱直读ICP法结果一致。     

TEM investigation of the crystal microstructures in a quartz-coesite assemblage of the western alps

Atransmission electron microscope (TEM) study of quartz-coesite inclusions in garnet in crustal rocks from the Western Alps is presented. Coesite shows a low dislocation density (<10⁷ cm^?2), and quartz a higher density of defects, Brasil twins (10⁴ cm^?1) and dislocations (10⁸ cm^?2). It is concluded that coesite has been not or only slightly plastically deformed and that the yield strength of coesite is higher than that of quartz. The large scale deformation implications are briefly discussed. TEM observations show no systematic topotactic relationship between the two polymorphs and their boundaries have a scalloped morphology which suggests that growth of quartz from coesite was controlled by a diffusion process.     

X-ray single-crystal study of spinels: in situ heating

 Two MgAl₂O₄ stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized, and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts, the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample, which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range, only partially reversible order–disorder processes occurred in the time span used for the experiments. Received: 16 July 2001 / Accepted: 8 January 2002     

Interpretation of in situ stress measurements in anisotropic rocks with the doorstopper method

Summary A correct interpretation of in situ stress measurements by means of the doorstopper technique in cross anisotropic rocks requires that 10 stress concentration factors at the bottom of the borehole be known instead of the 3 which are needed for isotropic rocks. In order to analyse the data obtained in a site investigation in a gneiss of the Alpine region, these factors were determined by a three-dimensional high-order finite-element model. The results are presented and compared with those obtained by means of first approximation methods. The feasibility of carrying out measurements in anisotropic rock by means of only 2 boreholes is discussed and it is shown that in practice 3 properly oriented boreholes are needed in this case too.     

A simple method for the in situ determination of alluvial densities

Alluvial density is an important parameter in hydrogeologic surveys. These density values are often difficult and expensive to obtain, especially in the large Basin and Range province of the southwestern United States. Various methods are available for obtaining densities; however, many are complex and involve expensive and cumbersome equipment. In most cases, in situ densities can be determined by the shallow core-barrel method, which can be used to obtain both wet and dry density values. Values obtained are well within the accuracy limitations demanded by exploratory hydrological or geophysical surveys. Equipment needs and sampling procedures are simple and inexpensive.     

Measuring quartz solubility by in situ weight-loss determination using a hydrothermal diamond cell

The solubility of quartz was determined using a hydrothermal diamond anvil cell (HDAC) within the temperature and pressure ranges of 126 to 490°C and up to 8.9 kbar, respectively. A novel approach has been used to measure the amount of dissolved silica. The quartz was abraded into spheres which have a diameter of ∼40 μm. The spheres were then placed in pure water inside the diamond anvil cell and heated externally. Because the transparency of the diamonds allows direct observation of the sample chamber during the experiment, we were able to estimate the amount of quartz dissolved in the water at various stages of the dissolution process by measuring the decrease in the sphere’s diameter over time. Experiments were performed along isochores between 0.92 and 0.99 g/cm³. The maximum solubility measured was 0.165 molal. The experimental solubility data were limited to 370°C because of overestimation of solubilities above this temperature. Reprecipitation of silica inside the HDAC sample chamber and the refaceting of the spheres to trigonal form at temperatures above 350°C are major contributors to the overestimation.     

Spinel-garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited: an in situ X-ray study

Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.     

Estimating the hydraulic properties of an aquitard from in situ pore pressure measurements

A workflow is described to estimate specific storage (S _s) and hydraulic conductivity (K) from a profile of vibrating wire piezometers embedded into a regional aquitard in Australia. The loading efficiency, compressibility and S _s were estimated from pore pressure response to atmospheric pressure changes, and K was estimated from the earliest part of the measurement record following grouting. Results indicate that S _s and K were, respectively, 8.8?×?10^?6 to 1.2?×?10^?5 m^?1 and 2?×?10^?12 m s^?1 for a claystone/siltstone, and 4.3?×?10^?6 to 9.6?×?10^?6 m^?1 and 1?×?10^?12 to 5?×?10^?12 m s^?1 for a thick mudstone. K estimates from the pore pressure response are within one order of magnitude when compared to direct measurement in a laboratory and inverse modelled flux rates determined from natural tracer profiles. Further analysis of the evolution and longevity of the properties of borehole grout (e.g. thermal and chemical effects) may help refine the estimation of formation hydraulic properties using this workflow. However, the convergence of K values illustrates the benefit of multiple lines of evidence to support aquitard characterization. An additional benefit of in situ pore pressure measurement is the generation of long-term data to constrain groundwater flow models, which provides a link between laboratory scale data and the formation scale.     

赤铁矿高温原位X射线衍射研究

在空气和真空条件下对α-Fe_2O_3粉末进行了从室温到1 000℃的高温原位X射线衍射研究,分别修正了空气和真空条件下赤铁矿在27~1 000℃范围内的晶胞参数,从而得到了晶胞参数随温度变化的关系和赤铁矿在空气和真空条件下的热膨胀系数,并得出了热膨胀系数与温度的关系,即赤铁矿的热膨胀系数不随温度变化。赤铁矿在空气气氛下的热膨胀系数为αa=9.603 16×10~(-6)/℃,αc=6.647 67×10~(-6)/℃,β=2.606 33/℃;真空气氛下的热膨胀系数为αa=9.006 79×10~(-6)/℃,αc=6.891 23×10~(-6)/℃,β=2.511 51/℃。     

Determination of the phase boundary between ilmenite and perovskite in MgSiO3 by in situ X-ray diffraction and quench experiments

Determination of the phase boundary between ilmenite and perovskite structures in MgSiO₃ has been made at pressures between 18 and 24 GPa and temperatures up to 2000 °C by in situ X-ray diffraction measurements using synchrotron radiation and quench experiments. It was difficult to precisely define the phase boundary by the present in situ X-ray observations, because the grain growth of ilmenite hindered the estimation of relative abundances of these phases. Moreover, the slow reaction kinetics between these two phases made it difficult to determine the phase boundary by changing pressure and temperature conditions during in situ X-ray diffraction measurements. Nevertheless, the phase boundary was well constrained by quench method with a pressure calibration based on the spinel-postspinel boundary of Mg₂SiO₄ determined by in situ X-ray experiments. This yielded the ilmenite-perovskite phase boundary of P (GPa) = 25.0 (±0.2) – 0.003 T (°C) for a temperature range of 1200–1800 °C, which is generally consistent with the results of the present in situ X-ray diffraction measurements within the uncertainty of ∼±0.5 GPa. The phase boundary thus determined between ilmenite and perovskite phases in MgSiO₃ is slightly (∼0.5 GPa) lower than that of the spinel-postspinel transformation in Mg₂SiO₄. Received: 19 May 1999 / Accepted: 21 March 2000