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1.
EMP determinations of Fe, Co and Ni in the metal phases of ordinary chondrites confirm the report of Sears and Axon that kamacite Co contents show restricted, nonoverlapping ranges in the three groups; ranges are 3.3–4.8 mg/g in H, 6.7–8.2 mg/g in L and 15–110 mg/g in LL. Experimental data by Widge and Goldstein show that the Ni concentration of the boundary increases with increasing Co concentration: unexpectedly, we find lower kamacite Ni concentrations in unequilibrated LL chondrites (44–55 mg/g) than in H and L chondrites (57–69 mg/g). We infer that, at temperatures below 550° C increasing Co causes a decrease in the equilibrium kamacite Ni concentration of an α-γ system. Although some evidence indicates that the equilibrated L chondrites Barratta, Knyahinya and Shaw have siderophile concentrations lower than the normal L-group range, they have kamacite and taenite Co concentrations in the L-group range.Metal-phase studies of petrologic type-3 ordinary chondrites having highly unequilibrated silicates showed a wide range in the degree of matrix kamacite equilibration ranging from nearly equilibrated in Mezö-Madaras to highly unequilibrated in Bishunpur, Ngawi and Semarkona. Kamacite in chondrule interiors is highly unequilibrated in all 9 chondrites, and in each setting taenite data are consistent with the expectation that it should be less equilibrated than kamacite. Our kamacite Co data confirm that Sharps is H and Hallingeberg. Khohar and Mezö-Madaras are L chondrites. Chainpur and Parnallee have kamacite Co concentrations between the L and LL ranges: we present evidence indicating that they are truly intermediate, i.e. neither L nor LL. Highly unequilibrated Ngawi is either LL or, less likely, still more oxidized. Bishunpur and Semarkona have mean kamacite Co concentrations in the H range but too unequilibrated to be used for classification. The highly heterogeneous compositions of the metal in Bishunpur, Ngawi and Semarkona indicate that their metal partially preserves properties established during nebular processes. Most of the taenite in these chondrites has high Ni contents (>470 mg/g) and is essentially unzoned; much of the kamacite is polycrystalline with crystals ?5μm across. Metamorphism causes tiny grains to disappear, increases the grain size of both kamacite and taenite, tends to equilibrate metallic minerals and, during cooling, can produce zoned taenite.A petrologic type-5 clast in the Ngawi LL3 chondrite has 3 coexisting metal phases, clear taenite (540 mg/g Ni, 21 mg/g Co), kamacite (30 mg/g Ni, 120 mg/g Co) and a phase tentatively identified as ordered FeCo (8.5 mg/g Ni, 370 mg/g Co). 相似文献
2.
In this study kamacite was experimentally grown in taenite grains of Fe-Ni-P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P. Both isothermal heat treatments and non-isothermal heat treatments at cooling rates of 2 to 5°C/day were carried out. Analytical electron microscopy was used to examine the orientation and chemical composition of the kamacite and the surrounding taenite matrix. The kamacite so produced is spindle or rod shaped and has a Widmanstätten pattern orientation. The presence of heterogeneous sites such as phosphides is necessary for the nucleation of the intergranular kamacite. During kamacite growth both Ni and P partition between kamacite and taenite with chemical equilibrium at the two phase interface. The growth kinetics are limited by the diffusion of Ni in taenite. Additional diffusion experiments showed that the volume diffusion coefficient of Ni in taenite is raised by a factor of 10 at 750°C in the presence of only 0.15 wt% P.A numerical model to simulate the growth of kamacite in Fe-Ni-P alloys, based on our experimental results, was developed and applied to estimate the cooling rates of the iron meteorites. The cooling rates predicted by the new model are two orders of magnitude greater than those of previous studies. For example the cooling rates of chemical groups I, IIIAB and IVA are 400–4000°C/106years, 150–1400°C/ 106 years and 750–6000°C/106years respectively. Previous models gave 1–4°C/106 years, 1–10°C/106 years and 3–200°C/106 years. Such fast cooling rates can be interpreted to indicate that meteorite parent bodies need only be a few kilometers in diameter or that iron meteorites can be formed near the surface of larger asteroidal bodies. 相似文献
3.
Edward R.D. Scott 《Geochimica et cosmochimica acta》1973,37(10):2283-2294
Taenite fields when etched develop a cloudy brown rim with approximate compositional limits of 25 and 40 per cent Ni. In iron meteorites this cloudy zone is only a few microns wide, with a sharp, high-Ni edge about 1 μm from the kamaciteinterface and a diffuse edge several microns from the central plessite. It is always present in irons unless the meteorite has been cosmically or terrestrially reheated.X-Ray and electron diffraction of grains scratched from exceptionally large areas of cloudy taenite in the mesosiderite Estherville show that this etching zone contains a fine exsolution of kamacite. Electron microscopy reveals a cellular structure with kamacite walls surrounding taenite volumes about 1000 Å in diameter; about one-third of the total volume is kamacite. Electron diffraction from a thin foil of Tazewell indicates that for several microns the cloudy border consists of a single crystal of kamacite interpenetrating a single crystal of taenite.Detailed electron-probe investigations of taenite in Estherville show that there is a step in the M-shaped Ni profile at the sharp, high-Ni edge of the cloudy region, the Ni dropping suddenly from approximately 45 to 42 per cent. It is proposed that exsolution in the cloudy region effectively froze in the Ni profile at that temperature. On subsequent cooling only the clear outer taenite continued to equilibrate with the kamacite matrix producing the kink in the M profile.Cloudy taenite is therefore a variety of plessite differing from the usual varieties in that it forms at lower temperatures in areas much richer in Ni, and the morphology is not crystallographically oriented. Its absence can provide a sensitive indication of reheating. 相似文献
4.
Andrew J. Campbell 《Geochimica et cosmochimica acta》2003,67(13):2481-2495
Siderophile element abundances in individual metal grains in the ungrouped chondrite Grosvenor Mountains (GRO) 95551 and in the ordinary chondrites Tieschitz H3.6, Soko-Banja LL4, and Allegan H5 were measured with laser ablation-inductively coupled plasma mass spectrometry. Matrix metal in GRO 95551 falls into two distinct compositional groups, a high-Ni group with 7.2 ± 0.4 wt% Ni and a low-Ni group with 3.7 ± 0.1 wt% Ni, indicating that kamacite/taenite equilibration at ∼1020 K was followed by rapid cooling. The nonrefractory siderophile elements P, Co, Cu, Ga, Ge, As, Pd, and Au also partition between the high-Ni and low-Ni metal in a manner consistent with kamacite/taenite fractionation, but the refractory siderophiles Ru, Re, Os, Ir, and Pt show correlated variations that are unrelated to kamacite/taenite partitioning and indicate that variations in refractory components of the metal were not completely erased during equilibration at ∼1020 K. The Ni-normalized bulk metal composition of GRO 95551 is refractory depleted and volatile rich relative to Bencubbin and related metal-rich chondrites but bears strong similarities to equilibrated ordinary chondrite metal. GRO 95551 represents a new chondrite type with chemical affinity to the ordinary chondrites. Individual metal grains in unequlibrated ordinary chondrites also have correlated variations in refractory siderophile contents that cannot be produced by redox processes alone; these variations span three orders of magnitude and diminish with increasing metamorphic grade of the ordinary chondrites. 相似文献
5.
The Pingchuan iron deposit, located in the Yanyuan region of Sichuan Province, SW China, has an ore reserve of 40 Mt with ~ 60 wt% Fe. Its genesis is still poorly understood. The Pingchuan iron deposit has a paragenetic sequence of an early Fe-oxide–Pyrite stage (I) and a late Fe-oxide–pyrrhotite stage (II). Stage I magnetite grains are generally fragmented, euhedral–subhedral, large-sized crystals accompanying with slightly postdated pyrite. Stage II magnetite grains are mostly unfragmented, anhedral, relatively small-sized grains that co-exist with pyrrhotite. Combined with micro-textural features and previously-obtained geochronological data, we consider that these two stages of iron mineralization in the Pingchuan deposit correspond to the Permian ELIP magmatism and Cenozoic fault activity event. Both the Stage I and II magnetites are characterized with overall lower contents of trace elements (including Cr, Ti, V, and Ni) than the ELIP magmatic magnetite, which suggests a hydrothermal origin for them. “Skarn-like” enrichment in Sn, Mn, and Zn in the Stage I magnetite grains indicate significant material contributions from carbonate wall-rocks due to water–rock interaction in ore-forming processes. Stage II magnetite grains contain higher Mn concentrations than Stage I magnetite grains, which possibly implies more contribution from carbonate rocks. In multiple-element diagrams, the Stage I magnetite shows systematic similarities to Kiruna-type magnetite rather than those from other types of deposits. Combined with geological features and previous studies on oxygen isotopes, we conclude that hydrothermal fluids have played a key role in the generation of the Pingchuan low-Ti iron deposit.
相似文献6.
7.
We have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes, using reflected-light microscopy and electron-probe analysis. In Weston and Fayetteville, which both contain solar-flare tracks and solar-wind gases, kamacite, taenite and tetrataenite (ordered FeNi) and troilite show a variety of textures. On a Wood plot of central Ni content vs dimension, taenite analyses scatter as if metal grains cooled at rates of 10–1000 and 1–100 K/Myr respectively through 700 K, although metal in an H6 clast in Fayetteville plots coherently with a cooling rate of 50 K/Myr. We propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted, and were not subsequently heated above 650 K. We predict a similar history for all gas-rich ordinary chondrites.By contrast, metallic minerals throughout Bhola and Mezö-Madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to ~600 K of 0.1 and 1 K/Myr, respectively. We conclude that host and xenoliths in both chondrites were slowly cooled after compaction. Thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 K in different environments.According to the conventional onion-shell model for H, L or LL chondrite parent bodies, material of petrologic types 3–5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately. But if peak metamorphic temperatures were reached during, not after accretion, as seems plausible, maximum metamorphism may have occurred in planetesimals <10 km in radius. Cooling through 700 K may then have occurred in larger bodies that accreted from these planetesimals. Iron meteorites, mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies. 相似文献
8.
Alan E Rubin∗ 《Geochimica et cosmochimica acta》2002,66(4):699-711
Smyer is an H-chondrite impact-melt breccia containing ∼20 vol% 0.5- to 13-mm-thick silicate-rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension. At the interface between some of the melt veins and chondritic clasts, there are troilite-rich regions consisting of unmelted, crushed 0.2- to 140-μm-size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins. Troilite fills every available fracture in the silicates, including some as thin as 0.1 μm. Little metallic Fe-Ni is present in these regions: the FeS/Fe modal ratio ranges from ∼25:1 to ∼500:1, far higher than the eutectic weight ratio of 7.5:1. The texture of these regions indicates that the sulfide formed from a fluid of very low viscosity. The moderately high viscosity (0.2 poise) and large surface tension of liquid FeS, its inability to wet silicate grain surfaces at low oxygen fugacities, and the supereutectic FeS/Fe ratios in the troilite-rich regions indicate that the fluid was a vapor. It seems likely that during the shock event that melted Smyer, many silicates adjacent to the melt veins were crushed. Upon release of shock pressure, some of the troilite evaporated and dissociated. Molecules of S2 were transported and condensed into fractures and around tiny silicate grains; there, they combined with Fe from small adjacent metallic Fe-Ni grains to form troilite. The Ni content at the edges of some of these metal grains increased significantly; Co from these Ni-rich grains diffused into nearby kamacite. Impact-induced S volatilization may have played a major role in depleting the surface of 433 Eros (and other chondritic asteroids) in S. 相似文献
9.
A. N. Kolesnik O. N. Kolesnik A. A. Karabtsov N. V. Bondarchuk 《Doklady Earth Sciences》2018,481(1):887-892
Data on electron microprobe X-ray analysis of mineral grains of base (Cu, Zn, Sn, Pb, Ni, Bi, and Mo) and precious metals (Ag, Au, Pd, and Pt) detected for the first time in the sediments of the Chukchi Sea are presented. The peculiarities of the morphology and chemical composition of the grains are considered along with their sources and distribution in the sediments. 相似文献
10.
Wallaby is a major gold deposit of the St Ives Gold field of Western Australia, with an estimated resource of 8 million ounces of gold. It has a well-established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time; from hematite to magnetite. Micro-analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions. The sulfur isotope composition has a δ34S range of −7.7 to +9.8‰ using 3 μm spots on the Sensitive High Resolution Iron Micro Probe-Stable Isotope (SHRIMP-SI). Negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite (which coexists with hematite) that also contains high concentrations of As, Ni, Zn, Ag, Sb, Cu and Pb. Conversely, positive values representative of reduced sulfur are found in later generations of pyrite, with lower concentrations of As, Ni, Sb, Cu, Zn and Pb. These later pyrite crystals display higher As/Ni, As/Sb, and As/Bi, and lower Cu/Te. These geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure. Previous studies suggested a single orogenic event formed the Wallaby Deposit. This is not supported by the present study. Trace element ratios such as As/Ni clearly delineate the high Au generations and could be used as an exploration tool. It is suggested that pyrite from the Wallaby Gold Deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated. Utilizing pyrite to link the entire fluid history of Wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold-bearing fluid systems. 相似文献
11.
The Emery mesosiderite contains large Ni-rich grains of the phosphide schreibersite, which have exsolved from kamacite. Computer simulation of diffusion-controlled growth of this schreibersite indicates that exsolution occurred during cooling at the rate of 0.1°C/Myr. This determination supports the cooling rate estimated for mesosiderites by Powell (Geochim. Cosmochim. Acta33, 789–810), using taenite-kamacite data. 相似文献
12.
吉林省塔东群以含大型磁铁矿床而著称,塔东铁矿是吉黑成矿省内最大的火山喷流沉积-变质改造型铁矿.由于塔东群地质特征复杂,多种地质作用复合叠加,使其形成时代归属一直存在争议.通过LA-ICP-MS锆石定年,确定了吉林省塔东群黑云斜长片麻岩和磁铁透辉斜长变粒岩中锆石年代.黑云斜长片麻岩中锆石呈长柱状,Th/U值为0.17~0.65,206Pb/238U加权平均年龄为517.6±2.7 Ma,代表了其原岩火山岩形成的年龄.磁铁透辉斜长变粒岩中共有两组锆石年龄,其中一组短柱状,Th/U值介于0.78~1.00,加权平均年龄为249.8±4.0 Ma,代表后期热事件年龄;另一组锆石半自形短柱状晶形,异常高的U、Th, Th最高含量可达17 422 μg/g,206Pb/238U加权平均年龄为518.3±3.8 Ma,表示热液活动年龄,显示该区518.3 Ma左右经历了一次热液作用,可能与该区的岩浆作用以及塔东铁矿的成矿有关.据此,塔东群内塔东铁矿可能存在一期早寒武世 (±520 Ma) 的成矿作用. 相似文献
13.
We have measured diffusion coefficients for P, Cr, Co, Ni, Cu, Ga, Ge, Ru, Pd, Ir, and Au in Fe metal from 1150 to 1400°C and at 1 bar and 10 kbar. Diffusion couples were prepared from high-purity Fe metal and metal from the IIA iron meteorite Coahuila (single crystal kamacite) or the pallasite Springwater (polycrystalline kamacite) and held at run conditions for 3.5 to 123 h. Diffusion profiles were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or the electron microprobe. Many elements were measured from the same experimental runs so interelemental comparisons are improved over other data sets in which data for different elements come from different experiments. Some literature diffusion coefficients (D) for Ni and Co in taenite can be up to a factor of 3 higher for Ni than Co, yet our results show no difference (e.g., DNi and DCo ∼ 2.2 × 10-15 m2/s at 1150°C). Thus, diffusion of Ni and Co in single crystal taenite will not measurably fractionate the Ni/Co ratio. On the other hand, the large difference in DNi and DIr (DIr is ∼5 times lower) and the similarity of DNi and DRu at all temperatures investigated indicates that Ni/Ir and Ni/Ru ratios in zoned metal grains will be useful discriminators of processes controlled by diffusion vs. volatility. In zoned metal grains in primitive chondrites, deviations of the Ni/Ru and Ni/Ir ratios from a condensation curve are opposite to a diffusion-controlled process, but consistent with a volatility-controlled process. The new multielement diffusion coefficients will also be useful in evaluating a variety of other processes in planetary science. 相似文献
14.
条带状铁建造(BIFs)中含有大量的亚铁磁性矿物,其组成及来源是认识BIF成因的重要依据。本文研究了南非巴伯顿绿岩带无花果树群(距今约32亿年)恩圭尼亚组的BIFs样品的磁学和矿物学特征。通过测量富铁层与富硅层的磁滞回线、等温剩磁获得曲线与退磁曲线、矫顽力谱分析、一阶反转曲线(FORC)、低温(20～300K)有场/无场冷却曲线以及k-T曲线、Lowrie三轴热退磁曲线,结合扫描电镜观测,揭示出研究样品中磁性矿物主要为赤铁矿和磁铁矿。基于矫顽力谱分析,富铁层中磁铁矿主要是多畴及假单畴颗粒,相对含量平均为2. 1%;赤铁矿的相对含量平均为97. 9%。富硅层中磁铁矿主要为假单畴及超顺磁性颗粒,相对含量平均为4. 6%;赤铁矿相对含量平均为95. 4%。测试样品具有Morin转变特征,转变温度介于250～260K,说明BIFs中主要为赤铁矿(0. 5～6mm)。富硅层样品出现～107K、～125K两个Verwey转变温度,表明其中可能存在生物成因和非生物成因两种类型磁铁矿。 相似文献
15.
内蒙古双尖子山银多金属矿床是中国乃至亚洲规模最大的银多金属矿床,本研究在回顾矿床发现史的基础上对双尖子山矿床区域地球化学数据进行再分析。采用19级累积频率法制作研究区39种元素或氧化物的地球化学图,结果发现在双尖子山矿区范围内出现明显高值的有MgO、CaO、Fe2O3、Pb、Zn、Ag、V、Cr、Ni等多达22种氧化物或元素,其中主量成分的明显富集特征应受控于研究区的岩性分布情况。采用变值七级异常法制作研究区29种微量元素的地球化学异常图,结果发现在双尖子山矿区及其附近存在W、Sn、Mo、Bi、Pb、Zn、Ag、As、Sb计9种热液成矿元素的异常区,而其他微量元素在双尖子山矿区的明显富集情况可能是由双尖子山矿区及其附近母岩岩性及其风化作用所致。基于地球化学基因技术制作了研究区金矿化基因和钨矿化基因的矿化相似度地球化学图,结果发现在双尖子山矿区及其附近存在金矿化相似度和钨矿化相似度的异常区,表明矿化相似度可以作为该区金属矿产勘查的一种有效指标,认为区域化探工作在该区金属矿产勘查中具有很好的参考价值。 相似文献
16.
论文给出了中国浙江火山岩区金矿床中黄铁矿的微量元素、形态和物理性质找矿标型特征.例如.(在许多)浙江火山岩区重要金-银矿床中黄铁矿相对富含铅、锌、钼、锡、砷、锑、铋而贫钴,镍、硒、碲:并且S/Se、Ag/Au、Pb/Ni、Se/Te、(As+sb+Bi)/(Se+Te)比值较高,Co/Nj、Ag/Pb、Ag/Zn、Cu/Zn和(Co+Ni)/(Pb+Zn)比值较低,再如含金黄铁矿比不含金黄铁矿的反射率低.总之,黄铁矿的标型性研究对于寻找金矿具有重大的理论意义和实际意义. 相似文献
17.
V. S. Antipin M. I. Kuz’min D. M. Pecherskii V. A. Tsel’movich S. A. Yazev 《Doklady Earth Sciences》2014,458(1):1082-1085
The fragments of the Chelyabinsk meteorite studied are represented by light-gray granular rock of chondritic structure. The chondrules and their cementing matter are mainly constituted by olivine and orthopyroxene. The matrix consists of a pyroxene-olivine aggregate with plagioclase, apatite, melted glass, and the inclusions of ore minerals: taenite, kamacite, troilite, pyrrhotite and pentlandite (more rarely), and individual grains of chromite and ilmenite. The comparison of the composition of the Chelyabinsk meteorite to the average composition of LL chondrites had shown their complete convergence. The concentrations of sidero- and chalcophile rare elements in the meteorite, normalized to CI chondrites, are much close to the values for LL chondrites and almost reproduce the character of their distribution in the spider diagram. However, some high-charged and lithophile elements (Nb, Zr, Hf, Sr, Ba, Th, and U) not belonging to the mentioned groups are characterized by somewhat increased contents. The enrichment of the samples of the Chelyabinsk meteorite in rare-earth elements compared to LL chondrite (5.18 against 3.58 ppm) is also revealed. This is related to the higher concentrations of light lanthanides in the meteorite samples, which is seen from the increased La/Yb ratio compared to the value for LL chondrite (1.9–2.3 and 1.4, respectively). Iron-nickel alloys are the main magnetism carriers in the Chelyabinsk meteorite. The compositions of kamacite, taenite, chromite, and Fe-sulfides are not much different. The optical and microprobe data are confirmed by the thermomagnetic parameters as well: (1) The specific magnetization of 4–6 Am2/kg points to small variations in the concentrations of magnetic minerals. (2) The M(T) curves for all the samples nearly repeat each other, and the Curie temperatures of 490–520 and 740–770°C are registered in the curves of the first and second heating, hence, these curves correspond to kamacite of various composition, right up to pure iron. (3) The monocline ferrimagnetic pyrrhotite of TC = 320–340°C is registered in the treated fragments in both the M(T) curves of heating and cooling. (4) The concentrations by thermomagnetic analysis amount to 0.6–1.6% (0.9% average) for kamacite, 0.7–1.5% (1.1% average) for taenite, and 0–1.5% (0.4% average) for monocline pyrrhotite. (5) No magnetite was found in the M(T) curve during the first heating of the samples. Hence, the content of magnetite is much below 0.1. 相似文献
18.
《Chemie der Erde / Geochemistry》2019,79(4):125528
Primitive CO3.00–3.1 chondrites contain ∼2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H2 and reacts with SiO2 to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr2O3 in a few R4-6 chondrites. Magnetite grains with high Cr2O3 behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ∼4 vol.% magnetite with substantial Cr2O3 (up to ∼15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7. 相似文献
19.
Ahmed El Goresy 《Contributions to Mineralogy and Petrology》1968,17(4):331-346
Numerous magnetic spherules and grains collected from the Greenland ice and suspected of being of cosmic origin were studied microscopically and with the microprobe. Seven types of spherules and grams were recognized.Several magnetite spherules contain metallic cores. The metallic cores of three spherules are composed of nearly pure Fe with traces of Ni. The metallic nuclei of two other spherules contain appreciable amounts of Ni; the nucleus of one of these is composed of a Ni-rich NiFe alloy (96.9% Ni), and that of the other contains 3% Ni. This latter spherule is probably of cosmic origin, perhaps formed in the fusion crust of an iron meteorite. Its magnetic shell contains no detectable Ni.The majority of the spherules consist of magnetite, which is more or less transected by martite lamellae [parallel to {111} planes of the magnetite]. One composite grain of titanomagnetite, ilmenite, hematite, and pyroxene was also found. This grain is of terrestrial origin, probably derived from the metamorphic crystalline complex of Greenland. The mineralogy and chemistry of the observed magnetite spherules and grains are discussed in detail.This work was begun at Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, where it was supported in part by Grant GA-855 from the National Science Foundation, and completed at Max-Planck-Institut für Kernphysik, Heidelberg. 相似文献
20.
Polished sections of 5 enstatite chondrites have been irradiated with 30 MeV 4He ions to produce the alpha-radioactive nuclei 211At and 210Po from 209Bi and 208Pb, respectively. The distribution of alpha activity can be mapped, using cellulose nitrate as an alpha track detector, to give the corresponding Bi or Pb distributions in the meteorite. No strong localization of Bi or 208Pb was found; relatively uniform track distributions were observed. In particular, metal or sulfide grains are not enriched in Bi or Pb (relative to bulk), which is in agreement with the predictions of nebular condensation calculations. While the track distributions appear uniform, the results of detailed, track-by-track mappings of the Bi detectors indicate that the Bi is not totally randomly distributed; the statistical fluctuations in the observed track density are different for the cases where the Bi is totally randomly distributed and where the Bi is localized in point sources. Assuming that the Bi in a given sample is localized in identical point sources which are uniformly distributed throughout the sample, the observed relative population densities of clusters (‘stars’) of small numbers of tracks (2–5) corresponds to Bi being localized, with ~90% in grains with about 10?16g-Bi (~3 × 105Biatoms), and with ~10% in 4 × 10?14 g-Bi sources. If these are elemental Bi, as predicted theoretically, they are ~ 102 Å and 103 Å in size, respectively. 相似文献