Cover Picture: Minerals as Molecules—Use of Aqueous Oxide and Hydroxide Clusters to Understand Geochemical Reactions (Chem. Eur. J. 18/2009)

A model nanoparticle respresenting goethite (α‐FeOOH), an important soil mineral, is depicted in the center of the cover picture. The solvent‐accessible surface is represented by the translucent material, and the surface oxygen atoms are represented with spheres. The colors of the spheres give their average protonation states over the course of a molecular dynamics simulation, with red denoting Fe? OH₂ groups and blue denoting FeOH groups. Iron or aluminum Baker–Figgis–Keggin isomers are shown in the corners. For more details on the chemistry of minerals see the Concept article by W. H. Casey, J. R. Rustad and L. Spiccia on page 4496 ff .

     

Minerals as Molecules—Use of Aqueous Oxide and Hydroxide Clusters to Understand Geochemical Reactions

Large aqueous oxide ions as minerals! Minerals dissolve by repeated ligand exchange reactions and geochemists use polyoxometalate ions to establish structure–reactivity relations for environmentally important functional groups. Here, for example, are plotted the dissolution rates of two classes of minerals against rates of solvent exchanges around the corresponding aquo ions.

     

57Fe Mössbauer Spectroscopy Studies of Meteorites: Implications for Weathering Rates,Meteorite Flux,and Early Solar System Processes

Ordinary chondrite finds, terrestrial age dated using ¹⁴C analyses, from different meteorite accumulation sites, have been examined by Mössbauer spectroscopy to quantitatively determine terrestrial oxidation. We observe differences in weathering rates between sites, and also between different chondrite groups. A comparison of weathering over time, and its effect in eroding meteorites, together with the number and mass distribution of meteorites in each region, enables us to derive estimates of the number of meteorite falls over a given mass per year. Studies of how the oxygen isotopic composition of samples varies with weathering indicate that incipient alteration may occur without a pronounced isotopic effect, possibly due to weathering of silicates to topotactically oriented smectite confined spaces where the water volume is limited. This finding has profound implications for the use of oxygen isotopes as a tool in understanding water–rock interaction. It also may reconcile previously contradictory data regarding the nebular or asteroidal location of pre-terrestrial aqueous alteration. Finally, Mössbauer spectroscopy is also found to be a useful tool in determining mineral abundance in carbonaceous chondrites, where a fine-grained matrix makes traditional approaches inapplicable. Again, the results have implications for the modification of chondritic materials in the early solar system.     

BPMA矿物元素分析工具在含金、含铌钽矿样品分析中的应用

工艺矿物学分析采用自动系统处理后,通常仍有部分精细分析数据需要人工采集,重复劳动工作量大。采用基于扫描电镜(SEM)-X射线能谱(EDS)的BPMA(BGRIMM Process Mineralogy Analyzing System)系统矿物元素分析工具针对目标矿物进行多点EDS元素分析,通过图像特征信息校正保障目标位置精准,通过高计数谱图信息尽量保障后续元素分析的精度。使用工具软件对含金样品进行位置回溯和元素分析统计的测试,并利用工具软件及BPMA系统对某含铌钽矿样品进行工艺矿物学分析测试,实验表明：样品中含量极少的铌钽矿物主要分布在2-5μm,粒度极细,且60%以上被其他矿物包裹,其中,90%的铌分布于褐钇铌矿、铌钙矿;超过80%的钽分布于铌钙矿和铌易解石。工具软件实现的精度可以代替手动操作,批量回找和自动统计显著提高效率;且工具软件对重要矿物元素含量的分析数据则可进一步提高BPMA最终结果数据的精确度。     

Mineralogical,geochemical and radiological characterisation of Selmo Formation in Batman area,Turkey

This work deals with the mineralogical, geochemical and radiological characterisations of Selmo Formation in Batman neighbourhood. The upper Miocene–Pliocene Selmo Formation is common in the centre of Batman and composed of carbonated sandy claystones and silty–sandy stone lenses. The common whole minerals of the samples are quartz, feldspars, calcite and dolomite. The clay minerals are smectite, illite, chlorite and mixed-layer clay (chlorite–smectite). The geochemical mean values of the samples are 51.7% SiO₂; 12.6% Al₂O₃; 6.2% Fe₂O₃; 3.6% MgO; 6.3% CaO; 1.1% Na₂O; 1.7% K₂O; 0.8% TiO₂; 0.2% P₂O₅; 0.1% MnO; and 0.03% Cr₂O₃. In addition, baseline maps for the concentrations of each radionuclide, the radium equivalent activity and the outdoor gamma dose rate distributions have been plotted for the study area. The mean activity concentrations of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs were determined to be 32, 24, 210 and 9 Bq kg^?1, respectively. The assessments of the radiological hazard indices, such as radium equivalent activity, absorbed dose rate in air, annual effective dose equivalent, excess lifetime cancer risk, external hazard index and internal hazard index, were calculated and compared with the internationally accepted reference values. This study shows that the concentrations of radioactivities in the measured samples were within the recommended safety limits and did not pose to be any significant source of radiation hazard.     

Assessing the size-dependent chemical speciation of soil particles using electron probe X-ray microanalysis

Soils of the Pereemnaya river catchment, East Siberia, Russia, characterized by high percentage of particles under 10 μm size and the almost total absence of clay minerals, were studied using ultra-thin window electron probe X-ray microanalysis (EPMA). Three subfractions – 10–5 μm, 1–5 μm and <1 μm were distinguished within <10 μm fraction on the basis of mineralogical composition and chemistry peculiarities. Besides the single particles characterization, the bulk chemistry of specified subfractions and their contributions to the bulk soil chemistry were evaluated. It was shown that concentrations of alkali and alkaline-earth metals within <10 μm fraction increased in the row (5–10 μm) < (1–5 μm) < (<1 μm). The presence of two mica types – low-Fe and rich-Fe – was detected in all the subfractions. Being compared with the soil bulk chemistry, the data obtained for fraction <10 μm show that this fraction is the major source of alkali and alkaline-earth metals and consequently the main contributor to element release due to weathering.     

Two earth years of Mössbauer studies of the surface of Mars with MIMOS II

The element iron plays a crucial role in the study of the evolution of matter from an interstellar cloud to the formation and evolution of the planets. In the Solar System iron is the most abundant metallic element. It occurs in at least three different oxidation states: Fe(0) (metallic iron), Fe(II) and Fe(III). Fe(IV) and Fe(VI) compounds are well known on Earth, and there is a possibility for their occurrence on Mars. In January 2004 the USA space agency NASA landed two rovers on the surface of Mars, both carrying the Mainz Mössbauer spectrometer MIMOS II. They performed for the first time in-situ measurements of the mineralogy of the Martian surface, at two different places on Mars, Meridiani Planum and Gusev crater, respectively, the landing sites of the Mars-Exploration-Rovers (MER) Opportunity and Spirit. After about two Earth years or one Martian year of operation the Mössbauer (MB) spectrometers on both rovers have acquired data from more than 150 targets (and more than thousand MB spectra) at each landing site. The scientific measurement objectives of the Mössbauer investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron-bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these iron-bearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite and magnetite in a basalt), (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe²⁺, Fe³⁺, and Fe⁶⁺), and (4) the characterization of the size distribution of magnetic particles. Special geologic targets of the Mössbauer investigation are dust collected by the Athena magnets and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels. The Mössbauer spectrometer on Opportunity at Meridiani Planum, identified eight Fe-bearing phases: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and a metallic Fe–Ni alloy (kamacite) in a Fe–Ni-meteorite. Outcrop rocks consist of hematite-rich spherules dispersed throughout S-rich rock that has nearly constant proportions of Fe³⁺ from jarosite, hematite, and npOx (28%, 35%, and 19% of total Fe). Jarosite is mineralogical evidence for aqueous processes under acid–sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite-rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite-rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe³⁺/Fe_total ～0.2?0.4), with the fine-grained and bright aeolian deposits being the most oxidized. Basaltic soil at Meridiani Planum and Gusev crater have similar Fe-mineralogical compositions. At Gusev crater, the Mössbauer spectrometer on the MER Spirit rover has identified 8 Fe-bearing phases. Two are Fe²⁺ silicates (olivine and pyroxene), one is a Fe²⁺ oxide (ilmenite), one is a mixed Fe²⁺ and Fe³⁺ oxide (magnetite), two are Fe³⁺ oxides (hematite and goethite), one is a Fe³⁺ sulfate (mineralogically not constrained), and one is a Fe³⁺ alteration product (npOx). The surface material in the plains have a olivine basaltic signature (Morris, et al., Science, 305: 833, 2004; Morris, et al., J. Geophys. Res., 111, 2006, Ming, et al., J. Geophys. Res., 111, 2006) suggesting physical rather than chemical weathering processes present in the plains. The Mössbauer signature for the Columbia Hills surface material is very different ranging from nearly unaltered material to highly altered material. Some of the rocks, in particular a rock named Clovis, contain a significant amount of the Fe oxyhydroxide goethite, α-FeOOH, which is mineralogical evidence for aqueous processes because it is formed only under aqueous conditions.     

First principle investigation of the thermomechanical properties of type A carbonated apatite

Calcium apatites with the general chemical formula (Ca, X)₁₀(PO₄, Y)₆Z₂ represent a mineral family of utmost importance in several fields, for example, bone biology, biomaterials, and mineralogy. However, few works have focused on the mechanical properties of these phases, and in particular, no data are available on the thermomechanical and thermodynamic properties of carbonate-bearing (hydroxyl)apatites. In the present work, the equation of state of type A carbonated apatite (CAp, Ca₁₀(PO₄)₆CO₃, space group P1) was calculated by ab initio quantum mechanical methods within the density functional theory (DFT) framework. Starting from athermal results (at 0 K), the combined effect of temperature and pressure was investigated through the quasiharmonic approximation (QHA). In athermal conditions, the equation of state of the CAp unit cell volume can be described by a third-order Birch-Murnaghan formulation, with parameters V₀ = 538.14(5) ų, K₀ = 106.2(7) GPa, and K′ = 4.6(4). The QHA well described the temperature and pressure dependence of the thermodynamics and mechanical properties of the mineral. For instance, the bulk modulus at 0 GPa and ambient temperature (300 K) is K _T0 = 102.95 GPa, which is lower than that of stoichiometric apatite by about 6%. The unit cell thermal expansion coefficient between 0 and 600 K was also calculated and reported. The results are in line with the few available experimental data reported in literature on type AB carbonated hydroxylapatite. The reported findings further extend the knowledge of the mechanical and thermal behaviors of this important mineral found in biological environments, results that are useful for biotechnological and other applications of the (C)OHAp phases.     

基于扫描电子显微镜-X射线能谱的矿物自动分析系统(BPMA)测定高纯石英砂中杂质矿物

杂质矿物作为高纯石英中杂质元素主要载体之一,是高纯石英砂产品中严格限定的成分,也是影响高纯石英提纯方案的关键因素,对其组成、含量及赋存特征进行准确分析测定对高纯石英砂产品质量评价、原料选择合理性判断及提纯方案制定均有重要意义。采用基于扫描电子显微镜（SEM）和X射线能谱（EDS）的工艺矿物学参数自动分析系统（BPMA）测定了SiO2纯度≥99.998 %（4N8）的高纯石英砂中杂质矿物的组成、含量及嵌布特征,建立了高纯石英砂中杂质矿物的分析测定方法。结果表明：样品中石英的重量百分比为99.9988 %,与高纯石英砂产品质量指标对应的SiO2纯度化学分析结果基本一致;杂质矿物主要为辉石和锆石,偶见极其微量的方解石、磷钇矿、独居石和方铅矿;石英粒度均匀,主要分布在0.020～0.208 mm之间;杂质矿物嵌布粒度较细,绝大多数杂质矿物粒度在43 μm以下,主要以微细粒矿物包裹体的形式嵌布在石英颗粒内。方法简捷高效、测量结果准确可靠,适用于高纯石英砂中杂质矿物组成、含量及赋存特征分析测定,也可以为其它高纯物料中微量杂质矿物检测提供技术借鉴。