High-T phase transition of synthetic ANaB(LiMg)CMg5Si8O22(OH)2 amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study

The synthetic amphibole Na_0.95(Li_0.95Mg_1.05)Mg₅Si₈O₂₂(OH)₂ was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P2₁ /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm⁻¹, respectively, and two minor bands at 3,667 and 3,687 cm⁻¹. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site (^A□_0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T _c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P2₁ /m ⇔ C2/m phase transition at 255°C for synthetic amphibole ^ANa_0.8^B(Na_0.8Mg_1.2)^CMg₅Si₈O₂₂(OH)₂ indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T _c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.     

An infrared spectroscopic study of lawsonite to 20 GPa

The infrared spectrum of CaAl₂Si₂O₇ · H₂O-lawsonite, has been characterized to pressures of 20 GPa at 300 K. Our results constrain the response to compression of the silicate tetrahedra, hydroxyl units, and water molecules in this material. The asymmetric and symmetric stretching and bending vibrations of the Si₂O₇ groups (at zero pressure frequencies between 600 and 1000 cm⁻¹) increase in frequency with pressure at rates between 3.6 and 5.9 cm⁻¹/GPa. All silicate modes appear to shift continuously with pressure to 20 GPa, although the lowest frequency stretching vibration becomes unresolvable above 18 GPa, and a splitting of the main bending vibration is observed near this pressure. The O-H stretches of the hydroxyl units exhibit a discontinuity in their mode shifts at ∼8–9 GPa, which we interpret to be produced by a pressure-induced change in hydrogen bonding. The stretching and bending vibrations of the water molecule are relatively unaffected by compression to 20 GPa, thus demonstrating that the structural cavities in which water molecules reside are relatively rigid. Significant changes in the amplitude of the O-H stretches of the hydroxyl and water units are observed at this pressure as well; nevertheless, our results demonstrate that the dominant structural units in lawsonite persist metastably at 300 K with only modest structural modifications well beyond the known stability field of this phase. Received: 10 July 1998 / Revised, accepted: 23 October 1998     

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.     

且干布拉克蛭石的矿物学研究

本文对产于新疆且干布拉克的我国最大蛭石矿的组成、结构和谱学特征做了系统的阐述。研究表明:且干布拉克蛭石是由金云母风化而成。由于风化的不完全,该蛭石保留了许多金云母的特征。该蛭石不是个严格矿物学意义上的蛭石,它是由金云母、严格矿物学意义上的蛭石、以及两者组成的多种混层矿物的混合物。金云母风化过程中可能有杂质铁相生成。蛭石的颜色和多色性,可能起因于其中的O~(2-)→Fe~(2+23+)和Fe~(2+)→Fe~(3+)     

俯冲带温度条件下橄榄石中硅氧四面体的非谐振动:透射红外光谱研究

橄榄石既是上地幔的主要矿物,又是俯冲板块的主要矿物。在正常地幔温度条件下,橄榄石中原子之间的振动是非谐振动,这已成为学界共识;然而,对于俯冲带的较低温情况,橄榄石中原子之间的振动的非谐性是否重要,目前还没有比较一致的结论。本研究利用透射红外光谱的方法,系统研究了室温下不同厚度橄榄石中硅氧四面体的红外光谱特征,并着重对其在2100~1500cm-1区域形成的二倍频峰/合频峰进行了指认。在此基础上,本研究通过采集不同温度下的透射红外光谱数据(最高温度达约450℃),确立了橄榄石中硅氧四面体的相应基频峰及二倍频峰峰位对温度的依赖性,进而得出其相应红外振动的非谐系数(χ)随温度(T)的变化关系,具体为:χ838=6.37(2)×10-7×T+0.0014(1)及χ993=7.86(3)×10-7×T+0.0015(1),其中温度的单位为℃。外推到600℃时,有χ838=0.0018(1)及χ993=0.0020(2)。这些结果表明,在俯冲带的较低温条件下,橄榄石中硅氧四面体的内部振动的非谐性可能不重要。     

Raman Micro-Spectroscopic Study of Sulfate Ion in the System Na2SO4 – H2O

This work aims to quantify sulfate ion concentrations in the system Na₂SO₄-H₂O using Raman micro-spectroscopy.Raman spectra of sodium sulfate solutions with known concentrations were collected at ambient temperature（293 K） and in the 500 cm^₁-4000 cm^-1 spectral region.The results indicate that the intensity of the SO₄^2- band increases with increasing concentrations of sulfate ion.A linear correlation was found between the concentration of SO₄^2-（c） and parameter I₁,which represents the ratio of the area of the SO₄^2- band to that of the O-H stretching band of water（A_s/A_w）:I₁=-0.00102+0.01538 c.Furthermore,we deconvoluted the O-H stretching band of water(2800 cm^-1-3800 cm^-1) at 3232 and 3430 cm^-1 into two sub-Gaussian bands,and then defined Raman intensity of the two sub-bands as A_Bi(3232 cm^-1) and A_B2(3430 cm^-1),defined the full width of half maximum（FWHM） of the two sub-bands as W_B1(3232 cm^-1) and W_B2(3430 cm^-1).A linear correlation between the concentration of SO₄^2-（c） and parameter I₂,which represents the ratio of Raman intensity of SO₄^2-（A_s）(in 981 cm^-1) to(A_B1+A_B2),was also established:I₂=-0.0111+0.3653 c.However,no correlations were found between concentration of SO₄^2-（c） and FWHM ratios,which includes the ratio of FWHM of SO₄^2-（W_s） to W_B1 W_B2 and W_B1+B2(the sum of W_B1 and W_B2),suggesting that FWHM is not suitable for quantitative studies of sulfate solutions with Raman spectroscopy.A comparison of Raman spectroscopic studies of mixed Na₂SO₄ and NaCI solutions with a constant SO₄^2- concentration and variable CI^- concentrations suggest that the I\ parameter is affected by CI^-,whereas the I₂ parameter was not.Therefore,even if the solution is not purely Na₂SO₄-H₂O,SO₄^2- concentrations can still be calculated from the Raman spectra if the H₂O band is deconvoluted into two sub-bands,making this method potentially applicable to analysis of natural fluid inclusions.     

Diamond in metasedimentary crustal rocks from Pohorje,Eastern Alps: a window to deep continental subduction

We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp³‐bonded carbon, in most cases centred between 1332 and 1330 cm^?1, with a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered graphitic (sp²‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO₂ and CH₄ in polyphase inclusions_. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.     

Application of Geochemistry and VNIR Spectroscopy in Mapping Heavy Metal Pollution of Stream Sediments in the Takab Mining Area, NW of Iran

This study considered the possibility of using visible and near infrared (VNIR) spectral absorption feature parameters (SAFPs) in predicting the concentration and mapping the distribution of heavy metals in sediments of the Takab area. In total, 60 sediment samples were collected along main streams draining from the mining districts and tailing sites, in order to measure the concentration of As, Co, V, Cu, Cr, Ni, Hg, Ti, Pb and Zn and the reflectance spectra (350–2500 nm). The quantitative relationship between SAFPs (Depth500nm, R610/500nm, R1344/778nm, Area500nm, Depth2200nm, Area2200nm, Asym2200nm) and geochemical data were assessed using stepwise multiple linear regression (SMLR) and enter multiple linear regression (EMLR) methods. The results showed a strong negative correlation between Ni and Cr with Area2200nm, a significant positive correlation between As and Asym2200nm, Ni and Co with Depth2200nm, as well as Co, V and total values with Depth500nm. The EMLR method eventuated in a significant prediction result for Ni, Cr, Co and As concentrations based on spectral parameters, whereas the prediction for Zn, V and total value was relatively weak. The spatial distribution pattern of geochemical data showed that mining activities, along with the natural weathering of base metal occurrences and rock units, has caused high concentrations of heavy metals in sediments of the Sarough River tributaries.     

红土镍矿样品前处理方法和分析测定技术研究进展

随着常规镍来源的硫化镍矿资源的日益枯竭,可直接生产氧化镍、镍锍和镍铁等产品的红土镍矿倍受关注。对于红土镍矿中主量、次量、痕量元素的检测,相同的检测项目存在多种测试方法,且部分相同原理的测试方法存在细节上的差异,使得检测者选择合适的检测方法变得困难。本文综述了近年来红土镍矿中24种元素测定的样品前处理方式及分析技术研究进展。样品前处理方式依据目标元素及后续的分析方法进行选择,其中酸溶法和碱熔法用途最广。酸溶法引入的盐分少,操作简单,但是分解过程中易导致挥发元素As、Sb、Bi、Hg的损失,Cr易随高氯酸冒烟损失。碱熔法分解能力强,适合分析Cr、Si、全铁等项目,但会引入大量的盐类和因坩埚材料损耗而带入其他杂质,给后续分析带来困难。红土镍矿的分析技术依据实验室条件及目标元素的性质和浓度进行选择。电感耦合等离子体发射光谱法(ICP-AES)是主量、次量元素的主要分析方法,适合于分析含量为10-5~30%级别的金属元素;X射线荧光光谱法主要用于分析含量为10-3~1级别的元素,尤其适合于测定Al、Si、Ti、V和P,由于该方法的准确性依赖于一套高质量的标准样品,故更适合炉前检测或检测大批红土镍矿样品。电感耦合等离子体质谱法(ICP-MS)最适合于分析10-4含量以下的重元素,特别是稀土和贵金属元素。原子吸收光谱法(AAS)适合于分析10-4~10-2级别的Ca、Mg、Ni、Co、Zn、Cr、Mn等低沸点、易原子化元素。分光光度法主要用于分析Ni和P。原子荧光光谱法(AFS)主要用于分析As、Bi、Sb等易形成气态氢化物的元素。容量法主要用于分析Al、Fe、Mg和Si O2等主含量元素。尽管AAS、分光光度法、AFS法和容量法检测周期长,但所用仪器为实验室常规配置,可满足缺乏相应大型仪器实验室的日常检测。本文认为,针对各种检测方法的适用性及存在问题,应从开发微波消解法、固体进样直接测汞法、ICP-MS法以及Cr与其他元素同时分析的快速分析方法等方面开展研究,建立灵敏、准确的检测方法,从而更好地服务于红土镍矿的贸易、检验和综合利用。     

橄榄石中H的结合机制及扩散行为

名义上无水矿物(nominally anhydrous minerals,NAMs)中以点缺陷形式存在的结构水,因其对于矿物物理化学性质的显著影响而受到越来越广泛的关注。橄榄石是上地幔中含量最丰富的矿物,水在橄榄石中的存在形式、扩散机制和速率对上地幔的流变学和电导性质有着重要的影响。因此,对于橄榄石中H的结合机制及其扩散机制和速率的了解有助于理解地球深部的水循环,也有助于构建上地幔流变结构以及解释电导测量结果。本文总结了近几十年来学界对于H在橄榄石中存在的缺陷类型与OH红外光谱吸收峰之间的对应关系,以及H在矿物晶格中的扩散机制、扩散速率等重要问题的研究成果,并探讨了现有研究中依然存在的问题。