Al hematites prepared by homogeneous precipitation of oxinates: material characterization and determination of the Morin transition

A novel method for synthesis of aluminium hematites, based upon the homogeneous precipitation of Fe and Al oxinates in various proportions, is presented. The precursor precipitates are heated in air at 700?°C. X-ray diffraction, thermal analyses, BET, FTIR, optical reflection analysis, TEM and Mössbauer spectroscopy at room temperature and 80?K of the resulting products indicate that single-phase hematites are formed with structural Al substitution of up to 10 at%. Interestingly, the particle size (>100?nm) is not substantially reduced by the Al content. Although it remains difficult to obtain a homogeneously distributed Al substitution in the final hematite, this processing line offers a unique opportunity to separate the effects of grain size and Al substitution on the Morin transition temperature (T _M) of Al hematite. From the comparison between the present hematites and a series of Al-substituted hematites with lepidocrocite as precursor, it could be shown that the effect on T _M, associated with a change of a factor 10 in grain size, is about 1/3 of the effect caused by a change of 10 in the degree of substitution. Finally, it is suggested that proper thermal treatments under different conditions of the same precursors are likely to produce spinel phases.     

Spectroscopic evidence for pressure-induced phase transitions in diopside

 Raman spectra of diopside were collected from atmospheric pressure to 71 GPa. The pressure dependences of 22 modes were determined. Changes occurred in the spectra at three different pressures. First, at approximately 10 GPa, the two Raman modes at 356 and 875 cm⁻¹ disappeared, while the mode at 324 cm⁻¹ split into two modes, diverging at this pressure with significantly different pressure shifts; second, at approximately 15 GPa, a small (1 to 2 cm⁻¹) drop in several of the frequencies was observed accompanied by changes in the pressure dependency of some of the modes; and third, above 55 GPa, the modes characteristic of chains of tetrahedrally coordinated silicon disappeared, while those for octahedrally coordinated silicon appeared. The first change at 10 GPa appears to be a C2/c to C2/c transition involving a change in the Ca coordination. The third change above 55 GPa appears to be a change in the silicon coordination. At 15 GPa, it is suggested that a change in compressional mechanism takes place. Received: 14 November 2000 / Accepted: 9 January 2002     

Premelting and calcium mobility in gehlenite (Ca2Al2SiO7) and pseudowollastonite (CaSiO3)

 Premelting effects in gehlenite (Ca₂Al₂SiO₇) have been studied by Raman spectroscopy and calorimetry, and in gehlenite and pseudowollastonite (CaSiO₃) by electrical conductivity. The enthalpy of premelting of gehlenite is 17.3 kJ mol⁻¹ and represents 9% of the reported enthalpy of fusion, which is in the range of the reported fraction of other minerals. The Raman and electrical conductivity experiments at high temperatures, for gehlenite and pseudowollastonite, show that the premelting effects of both compositions are associated with enhanced dynamics of calcium atoms near the melting point. This conclusion agrees with the results obtained for other minerals like diopside, but contrasts with those found for sodium metasilicate in which the weaker bonding of sodium allows the silicate framework to distort near the melting temperature and deform in such a way to prefigure the silicate entities present in the melt. Received: 30 April 2002 / Accepted: 7 August 2002 Acknowledgements We thank Y. Linard for help with DSC measurements and two anonymous reviewers for their constructive comments. This work has been partly supported by the EU Marie-Curie fellowship contract no. HPMF-CT-1999-00329, the CNRS-Carnegie Institution of Washington program PICS no.192, and the NSF grants EAR-9614432 and EAR-9901886 to B.O.M.     

厚层覆盖区综合物化探方法找金试验研究——以广西兴安金石金矿为例

在兴安金石金矿外围 4号分散流异常区 ,通过地电化学提取金测量法 ,查明测区存在 5个走向规模小于 5 0 m的金异常区 ,W( Au) 最大值为 72 .4× 10 - 9,高出背景值十几倍 ,并藉综合土壤吸附汞测量 ,高精度磁测和激发极化法等多种物化探方法来确定这些异常区的地质、地球化学和地球物理特征 ,提出金矿成矿的有利部位在金的地电提取高异常区、高汞量异常区、高磁性异常区、低视电阻率异常区和中等视极化率异常区的“三高一低一中”的找矿模式。确定测区的北北东角 (右下角 )一带具备有利的金矿成矿条件     

电磁波CT技术在探测堤坝工程中的应用

应用电磁波CT探测技术对流溪何竹料段大堤和丰顺县虎局水库大坝渗水通道进行了探测研究，由于电磁波的吸收系数和地层结构有密切关系，因此，电磁波CT是通过能量的变化观测反演吸收系数的分布，进而推测地下构造的分布，利用电磁波的吸收系数为层析物理量，在钻孔中进行透射观测，以优于0.5m的分辨率清楚地揭示了测区内渗水层位的分布情况。     

有限弹性变形地壳中的地震波

地壳由半无限大的基岩上一层厚度为H^-的表土层组成，入射地震波为垂直的SH波，产生水平地面运动。当浅源大地震发生时，在极震区以外行波传播产生地面运动将使地壳介质有非线性的有限弹性变形。用小参数摄动法使非线性控制方程为线性化的小参数各阶控制方程，得出头两阶线性控制方程的解析解。     

火焰原子吸收法测定化探样品中的银铜铅锌

样品经硝酸、氢氟酸、高氯酸分解，制成硝酸溶液，经巯基棉层析柱过滤富集Ag，并用2mol／LHBr溶液洗脱，用火焰原子吸收法测定洗脱液中的Ag和滤液中的Cu，Pb，Zn。用该法测定化探样品中银和铜铅锌，检出限低，结果准确度和精密度均令人满意。     

醋酸丁酯萃取-无火焰原子吸收法测定化探样品中痕量金

在KBr存在下,用小体积醋酸丁酯萃取金后,用无火焰原子吸收法测定化探样品中痕量金的分析方法较化学富集光谱法具灵敏度高(检出下限可达0.05×10-9)、精度好、简便快速、成本低等优点.通过批量化探样的检验,证明该方法足已满足化探样品对金的测定要求.