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1.
Nuwaifa Formation is a part of sequence stratigraphy that belongs to the Jurassic system exposed in the western desert of Iraq. The Jurassic system consists of Ubaid, Hussainiyat, Amij, Muhaiwir, and Najmah formations. Each formation is composed of basal clastic unit overlain by upper carbonate unit. Nuwaifa karst bauxite was developed in fossil karsts within the Ubaid Formation in areas where maximum intersection of fractures and faults exist. This bauxitization process affected the upper surface of the Ubaid limestone formation, which directly underlies the Nuwaifa bauxite Formation. Nuwaifa Formation represents karst-filling deposit that consists of a mixture of allochthonous (sandstone, claystone, and mudstone) and autochthonous lithofacies (bauxite kaolinite, kaolinitic bauxite, iron-rich bauxite, and flint clay). Most bauxite bodies occur within the autochthonous lithofacies and are lenticular in shape with maximum thickness ranges from few meters to 35 m and in some place up to 100 m. Petrographically, the bauxite deposit exhibits collomorphic-fluidal, pisolitic, oolitic, nodular, brecciated, and skeletal textures indicative of authigenic origin. Mineralogy boehmite and gibbsite are the only bauxite minerals; the former is dominant in the upper parts of the bauxite profiles, whereas the latter is dominant throughout the lower and middle part of the bauxite. Kaolinite, hematite, goethite, calcite, and anatase occur to a lesser extent. The study bauxites are mainly composed of Al2O3 (33–69.6 wt.%), SiO2 (8.4–42 wt.%), Fe2O3 (0.5–15.9 wt.%), and TiO2 (0.7–6.1 wt.%) with LOI ranging from 13.5 to 19.1 wt.%. Geochemical investigations indicate that the immobile elements like Al2O3, TiO2, Cr, Zr, and Ni were obviously enriched, while SiO2, Fe2O3, CaO, MgO, Zn, Co, Ba, Mn, Cu, and Sr were depleted during bauxitization process. The results of this study strongly suggest that the bauxite deposits of the Nuwaifa Formation are derived from the kaolinite of the Lower Hussainiyat Formation.  相似文献   

2.
Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al2O3 contents as low as 2.3?wt.%, FeO* contents up to 17?wt.% and SiO2 contents ~65?wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5?wt.% Al2O3, 13?wt.% FeO* and 66?±?2?wt.% SiO2. The dominant fractionating assemblage is alkali feldspar?+?fayalite?+?hedenbergite?+?oxides?±?quartz. Feldspar – melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.  相似文献   

3.
The Neoarchean Bundelkhand greenstone sequences at Mauranipur and Babina areas within the Bundelkhand Gneissic Complex preserve a variety of magmatic rocks such as komatiitic basalts, basalts,felsic volcanic rocks and high-Mg andesites belonging to the Baragaon, Raspahari and Koti Formations.The intrusive and extrusive komatiitic basalts are characterized by low SiO_2(39-53 wt.%), high MgO(18-25 wt.%).moderately high Fe_2O_3(7.1-11.6 wt.%), Al_2O_3(4.5-12.0 wt.%), and TiO_2(0.4-1.23 wt.%)with super to subchondritic(Gd/Yb)N ratios indicating garnet control on the melts. The intrusive komatiitic suite of Ti-enriched and Al-depleted type possesses predominant negative Eu and positive Nb, Ti and Y anomalies. The chemical composition of basalts classifies them into three types with varying SiO_2, TiO_2, MgO, Fe_2O_3, Al_2O_3 and CaO. At similar SiO_2 content of type Ⅰ and Ⅲ basalts, the type II basalts show slightly high Al_2O_3 and Fe_2O_3 contents. Significant negative anomalies of Nb, Zr, Hf and Ti, slightly enriched LREE with relatively flat HREE and low ∑REE contents are observed in type Ⅰ and Ⅱ basalts. TypeⅢ basalts show high Zr/Nb ratios(9.8-10.4), TiO_2(1.97-2.04 wt.%), but possess strikingly flat Zr, Hf, Y and Yb and are uncontaminated. Andesites from Agar and Koti have high SiO_2(55-64 wt.%), moderate TiO_2(0.4-0.7 wt.%), slightly low Al_2O_3(7-11.9 wt.%), medium to high MgO(3-8 wt.%) and CaO contents(10-17 wt.%). Anomalously high Cr, Co and Ni contents are observed in the Koti rhyolites. Tholeiitic to calc alkaline affinity of mafic-felsic volcanic rocks and basalt-andesite dacite-rhyolite differentiation indicate a mature arc and thickened crust during the advanced stage of the evolution of Neoarchean Bundelkhand greenstone belt in a convergent tectonic setting where the melts were derived from partial melting of thick basaltic crust metamorphosed to amphibolite-eclogite facies. The trace element systematics suggest the presence of arc-back arc association with varying magnitudes of crust-mantle interaction. La/Sm, La/Ta,Nb/Th, high MgO contents(20 wt.%), CaO/Al_2O_3 and(Gd/Yb)_N 1 along with the positive Nb anomalies of the komatiite basalts reflect a mantle plume source for their origin contaminated by subductionmetasomatized mantle lithosphere. The overall geochemical signatures of the ultramafic-mafic and felsic volcanic rocks endorse the Neoarchean plume-arc accretion tectonics in the Bundelkhand greenstone belt.  相似文献   

4.
Aluminium tracer diffusivities were measured in polycrystalline mullite. The artificial aluminium isotope 26Al was used as tracer isotope. An advanced preparation technique for the 26Al2O3 tracer source allowed to apply secondary ion mass spectrometry (SIMS) in order to analyse 26Al depth distributions in the polycrystalline material. Pre-exponential factors and activation enthalpies were determined for compositions of 78 wt.% Al2O3, 22 wt.% SiO2 (high-alumina material) and of 72 wt.% Al2O3, 28 wt.% SiO2 (low-alumina material), respectively. A strong dependence of the 26Al grain boundary diffusivity on the composition is observed. The results are discussed in comparison to previous data on grain boundary diffusivities of oxygen in mullite samples from the same batch. Dedicated to Prof. Hermann Schmalzried on the occasion of his 75th birthday.  相似文献   

5.
Porcelain wares have been produced following the directions contained in the Heylyn and Frye patent of 1744, using Cherokee clay and a lime‐alkali glass frit. The wares were fired to the bisque (˜ 950°C), glazed using a clay‐glass mixture, and then fired to a “heat‐work” level of Orton cone 9–90° deflection at 150°C per hour (1279°C). Modal mineralogy comprises Caplagioclase and two glass phases, one relict frit and the other a melt phase. The bulk chemistry of the body comprises 64.3 wt % SiO2, 21.7 wt % Al2O3, and 5.6 wt % CaO. Molecular ratios are SiO2:Al2O3 5.0 and SiO2:CaO 10.7. It is concluded that the patent, whose significance has been questioned over many years, was a practical working recipe, that close comparison may be made with porcelains of the “A”‐marked group, and that the patent represents a remarkable landmark in English ceramic history. © 2004 Wiley Periodicals, Inc.  相似文献   

6.
The body of hydroxylellestadite metasomatic rock penetrated by a borehole drilled at the Gumeshevsk deposit at depths of 530–534 m includes a thin interval of younger lower temperature tobermorite-plombierite metasomatic rock with subordinate amounts of Ca-Si gel, tacherenite, cubic lime, and thaumasite. Hydroxylellestadite has never before been found in calc skarns. The hydroxylellestadite metasomatic rock is cut by gypsum and fukalite veinlets, and the tobermorite-plombierite metasomatic rock is intersected by thaumasite veinlets. The pristine rock of the metasomatics was marble, and the metasomatic rock replaced andradite-bearing wollastonite skarn (with wollastonite replaced by foshagite). The ore minerals (chalcopyrite, valleriite, sphalerite, and others) were formed after the hydroxylellestadite metasomatite but most probably before the tobermorite-plombierite metasomatic rock and the veinlets of calcic minerals. The metasomatic rock was produced at significant variations in the oxygen, sulfur, and carbon dioxide fugacities. The composition of the hydroxylellestadite is, according to its microprobe analysis, as follows (wt %): SiO2 17.10, TiO2 0.01, Al2O3 0.02, FeO 0.20, MnO 0.00, MgO 0.04, CaO 55.40, Na2O 0.14, K2O 0.09, P2O5 0.12, CO2 1.90 (chemical analysis), SO3 21.60, F 0.16, Cl 0.14, total 96.92. The plombierite (SiO2 43.8–44.1 wt %, CaO 30.5–31.1 wt %) in the metasomatic rock notably differs from rare plombierite (SiO2 48.18 wt %, CaO 39.19 wt %) contained in the veinlets of thaumasite (SiO2 12.70 wt %, CaO 30.69 wt %, SO3 17.78 wt %).  相似文献   

7.
FeO*‐Al2O3‐TiO2‐rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono‐volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron‐titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe‐Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al‐rich fine‐gained massive type. The first two types consist of dark globule or band enriched in Fe‐Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*‐Al2O3‐TiO2‐rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*‐Al2O3‐TiO2‐rich rocks are characterized by low concentrations of SiO2 (25–54.2 wt%), Na2O + K2O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al2O3 (20–42%), TiO2 (3–9.2%), and P2O5 (0.26–1.30%). TiO2 is positively correlated with Al2O3 and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al2O3)?(CaO + Na2O + K2O)?(FeO*+ MnO + MgO) and (SiO2)?(FeO*)?(Al2O3), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high‐temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*‐Al2O3‐TiO2‐rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.  相似文献   

8.
Neyriz ophiolite in Abadeh Tashk area appears as four major separated massifs in an area with 125 km2, south of Iran. Peridotites including harzburgite, dunite, and lesser low-Cpx lherzolite are the major constituents of the ophiolite with very minor mafic rocks. Usual gabbros of ophiolite complexes are virtually absent from the study area. Mineral modality associated with bulk rock and mineral chemistry of the peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease in modal pyroxenes and in Al2O3, CaO, SiO2, Sc, Ta, V, and Ga values of the studied rocks by approaching chromite deposits. The Neyriz peridotites vary from low-Cpx lherzolite (MgO, 41.97–43.1 wt.%; Al2O3, 0.8–1.3 wt.%) with low content of Cr# spinel (36.7–37.6) and Fo olivine (90.79–91.5) to harzburgite (MgO, 44.31–45.25 wt.%;Al2O3, 0.29–0.45 wt.%; Cr# spinel, 58.2–73.45; Fo olivine, 91.23–91.56), and then to dunite (MgO, 45.9–49.2 wt.%; Al2O3, 0.18–0.48 wt.%) with higher content of Cr# spinel (74.34–79.36) and Fo olivine (91.75–94.68). Compared to modern oceanic settings, mineral and rock composition of low-Cpx lherzolite plot within the field of mid-ocean-ridge environment, whereas those of harzburgite and dunite fall in the field of fore-arc peridotites. As a result of the studies on minerals and whole rock chemistry along with rock interrelationships, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt from which the high-Cr–Mg, low-Ti chromitites were formed within mantle wedge above the supra-subduction zone in a fore-arc setting.  相似文献   

9.
A geological survey carried out in the Yaoundé (Cameroon) region has revealed the presence of homogeneous clayey laterite in the upper part of a laterite cover on interfluves, thickest on hills (780–800 m altitude) where ferricrete is absent, and clayey heterogeneous hydromorphic material in valleys. We present in this paper the physical, mineralogical and geochemical properties of these occurrences and discuss their potential as raw material for pottery, manufacture of bricks and tiles. These clayey raw materials are mostly made up of fine particles (ranging from 55 to 60% clay + silt in the clayey laterite, more than 70% clay + silt in the clayey hydromorphic material). Their chemical composition is characterized by silica (<60% SiO2), alumina (<35% Al2O3) and iron (ranging from 3 to 14% Fe2O3). Their main clay minerals are disorganized and poorly crystallized kaolinites. The average limits of liquidity (44.56% versus 91.58%) and limits of plasticity (22.4 versus 45.93) revealed that clayey hydromorphic material has the greatest plasticity. The studied raw materials are suitable for making pottery as well as the manufacture of bricks and tiles. However, the high iron content in the clayey laterite (between 11 and 12% Fe2O3) prevents their efficient use in the manufacture of ceramics.  相似文献   

10.
Twelve samples of Nigerian laterites were obtained from Ilorin, a rapidly growing urban center, and capital of Kwara State, Nigeria. Three varieties of laterites (clay, gravel and crust) were identified and subjected to mineralogical, chemical and geotechnical analyses which included: identification of clay and non-clay minerals by X-ray diffraction (XRD) techniques; chemical composition by X-ray fluorescence spectrometer analysis; pH of soil in water; moisture contents and specific gravity determinations, grain size analysis; compaction test by Harvard Compaction Apparatus and unconfined compressive strength determination.The laterite soil samples are composed of kaolinite and illite clay minerals with some quartz and feldspar. They were found to be rich in SiO2 (45%) Fe2O3, (16%) and Al203 (10%).These soils yielded maximum strength when compacted on the dry side of their optimum moisture content (OMC).The soils are not expected to perform very well as concrete aggregates since they contain high amounts of SiO2 and Fe2O3. These oxides are known to have deleterious effects on construction materials, particularly concrete aggregates.  相似文献   

11.
The study area forms part of an emerging iron ore province of southern Cameroon. Geochemistry analyses reveal that the siliceous itabirite has a very simple chemical composition, with Fe2O3 and SiO2 representing more than 96 wt.% of the average composition; suggesting chemical precipitates of silica and iron. Low Al2O3 and TiO2 concentrations and a weak positive correlation between them point to a minor detrital component in the precipitated marine sediments. The Si/Al ratio (average 52.7) indicates the hydrothermal origin of the studied itabirite. The Al–Si discrimination diagram supports this interpretation through the plot of all data in the hydrothermal field. The studied samples have low iron content (about 39.32% Fe), high gangue content (40.97% SiO2 and 1.3 % Al2O3) and low concentration of the deleterious elements (0.16 % P and < 0.01% S). The main gangue mineral is silica which can be efficiently removed from iron ores during preparation of raw materials for the blast furnace process. According to commercial standards for crude iron ores, it may be concluded that the Zambi iron ores are a low‐grade magnetic ore that can be profitably exploited for the production of iron for steel production.  相似文献   

12.
Late Cretaceous shales of the Fika Formation in the Chad (Bornu) Basin, northeastern Nigeria, were analysed to define paleoenvironment and source of the organic matter, and their relation to tectonic setting. The organic carbon and sulphur contents of Fika shale samples are in the range of 0.51–2.13 and 0.31–1.65 wt.%, respectively, pointing that these shales were deposited in suboxic-anoxic marine conditions. The biomarker and chemical compositions provide evidence for a major contribution of aquatic algae and microorganisms with minor terrigenous organic matter input. Moderate salinity stratification and relatively anoxic-suboxic bottom water conditions are also likely in the Fika shales. Therefore, stratified water column with moderate salinity and relatively anoxic-suboxic bottom water conditions have contributed to organic matter (OM) preservation in the Fika shale layer. Fika shale samples are rich in SiO2 (54.80 wt.%), followed by Al2O3 (23.75 wt.%) and Fe2O3 (10.19 wt.%). Compared with average shale, the analysed shale samples are obviously enriched in Al2O3 (23.75 wt.%), TiO2 (1.34 wt.%), and P2O5 (0.30 wt.%), indicating that these sediments are rich in clay minerals and represent a good possibility for enhanced organic matter production and enrichment.Plots of Fika shale on bivariate discriminant function diagram suggest an active continental margin setting for the provenance. The inferred tectonic setting for the late Cretaceous shales of the Fika Formation of the Chad (Bornu) Basin is in agreement with the tectonic evolutionary history of the west and central Africa during the Cretaceous period.  相似文献   

13.
ABSTRACT

The Boein–Miandasht Complex (BMC) is a part of the Sanandaj–Sirjan metamorphic basement and is cut by gabbroic to granitoid bodies. These intrusive bodies comprise gabbro, gabbro–diorite associated with fine-grained, in part porphyritic leucocratic granitoids. Zircon U–Pb dating of representative gabbro–diorite samples yielded ages of 166.4 ± 1.8 Ma and 163.5 ± 6.3 Ma (Callovian, the latest stage of the Middle Jurassic). Mineral chemistry of the gabbro–diorites shows a homogeneous composition of the main minerals, main augite to diopside clinopyroxene and plagioclase (~An17–59). Moreover, low AlZ/TiO2 ratios of the clinopyroxene grains suggest that the rocks were generated in a within-plate tectonic regime. The SiO2 contents of the gabbro-diorite rocks are between 46.36 and 55.61 wt. %, Al2O3 ranges from 7.57 to 17.98 wt. %. The TiO2 contents vary from 1.18 to 3.65 wt. %, Fe2O3 from 7.41 to 12.95 wt. %, the MgO ranges between 3.49 and 15.75 wt. %, Na2O from 0.65 to 5.08 wt. % and K2O from 0.48 to 1.08 wt. %. These rocks mostly plot in the alkali-gabbro field. Compared to chondrite are characterized by enrichment of LREEs over HREE, enrichment of LIL elements (e.g. Rb, Sr and Ba) and obvious positive anomalies of Nb and Ti. Based on the chemical composition, and mineral composition, this complex was generated in an extensional tectonic regime by partial melting of the hot asthenospheric mantle which is not more consistent with previous models which have suggested for SaSZ evolution in before.  相似文献   

14.
The Morro dos Seis Lagos niobium deposit (2897.9 Mt at 2.81 wt% Nb2O5) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100 m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10 m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23 wt% Nb2O5), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe3+ + (Nb, Ta) for 2Ti] substitution and both contain up to 2 wt% WO3. The laterites have an average Nb2O5 content of 2.91 wt% and average TiO2 5.00 wt% in the upper parts of the sequence. Average CeO2 concentration increases with increasing depth, from 0.12 wt% in the pisolitic type to 3.50 wt% in the brown laterite. HREE concentration is very low.  相似文献   

15.
Iron–nickel-laterite deposits in the Balkan Peninsula and Turkey, located in the Mirdita–Sub-Pelagonian and Pelagonian geotectonic zones, extending into the Anatolides zone are a major source of nickel. Repeated marine transgression and regression, and the multistage development of allochthonous laterite deposits by re-working and re-deposition in a shallow sea environment are demonstrated by the alternation of Fe–Ni-laterite layers within marine sequences.Geochemical study of these Fe–Ni laterite deposits shows that arsenic contents are generally low, ranging from less than 2 to a few tens of ppm. However, in the Aghios Ioannis deposit, Lokris, Central Greece As varies significantly and attains values up to 0.26 wt.% As and in the Gordes deposit of W. Turkey, the As content ranges from 0.004 to 1.07 wt.% As (average 0.34), reaching values up to 1.94 in the hematite zone. Investigation of the mineral chemistry (SEM-EDS) shows that goethite is the main host of As, ranging between 0.5 and 1.2 wt.% As2O3 in the Aghios Ioannis deposit, and between 1.2 to 6.9 wt.% As2O3 in the Gordes deposit, whereas, in co-existing calcite As was not detectable. Goethite occurs in fine-grained porous and concretionary, concentric textures. As values are higher in concretionary goethite. Positive correlation (r > 0.74) between As and Al2O3, TiO2 and ∑ REE contents in the laterite deposits of Greece, coupled with the As-enrichment only in certain laterite deposits points to post depositional As-enrichment.Assuming that high pH facilitates the adsorption of As by goethite, due to its high surface area and low values of the activation energy of adsorption (literature data) As-adsorption by goethite is considered to play an important role in its retention. Elevated As-contents in goethite (Fe-oxides) in Fe–Ni-laterites of Greece and Turkey, due to its absorption capacity, are considered to be of particular significance in the remediation of aquifer and soil contamination rather than being a source of environmental risk.  相似文献   

16.
Bauxite deposits in the Usambara Mountains of north eastern Tanzania occur as remnants of residual deposits on two geomorphologically related plateaus of Mabughai-Mlomboza and Kidundai at Magamba in Lushoto, Usambara Mountains. The parent rocks for the deposits are mainly granulites and feldspathic gneisses of Neoproterozoic Mozambique belt. The plateaus represent a preserved Late Cretaceous–Lower Tertiary old land surface (African surface). Other parts of the Usambara Mountains and the neighbouring Pare Mountains are covered mostly by red–brown lateritic soils and impure reddish-brown kaolinitic clays. The bauxite deposits contain mainly Al2O3 (40–69 wt.%), Fe2O3 (3–10 wt.%), SiO2 (0.16–7 wt.%) and other elements occur in quantities not substantial to affect the quality or processing of the bauxite, and are attributed to the presence of relic minerals. Gibbsite makes up to 98 vol.% of the bauxite ore in special cases. Gibbsite is accompanied by goethite in the ore. Boehmite occurs in small amounts and is usually accompanied by hematite. Impurities include goethite, hematite, kaolinite, and minor relic quartz and microcline. Kaolinite is the sole clay mineral encountered in the bauxite ore, suggesting mature soil profiles and a development of the bauxite deposits on a well-drained peneplanation. Ore reserve estimates from the drilling data and surface geological mapping of the deposits yielded bauxite reserves of about 37 million tonnes.  相似文献   

17.
王海芝  程捷 《第四纪研究》2008,28(6):1090-1097
周口店地区的古环境变化研究多数研究集中在中更新世时期,而缺乏对早更新世时期环境变化的研究。这主要是由于缺少保存完好的早更新世沉积记录造成的。随着对20世纪80年代在周口地区发现的东洞剖面,发现这是一个保存完好的早更新世剖面,为研究早更新世时期的古环境变化特征提供了良好的研究材料。为了重建早更新世时期的古环境变化特征,利用XRF对东洞洞穴沉积物的主要元素（SiO2,Al2O3,Fe2O3和CaO）的化学组成进行了高分辨率分析,同时对沉积物中的FeO含量进行了测试。结果显示东洞剖面沉积物的主要化学组成为SiO2,占41.6%～58.9%,其次是Al2O3和Fe2O3,其含量的变化范围分别为13.69%～29.63%和5.00%～9.81%。Al2O3和Fe2O3在剖面上与SiO2含量成明显的镜像变化关系,显示出Al2O3和Fe2O3对沉积物中SiO2含量的稀释作用。另外,Fe2O3与Al2O3在剖面上具有很好的相关性,表明Fe2O3主要富集在富铝的矿物中。从元素含量在剖面的上分布看,东洞剖面的化学组成发生3次大的波动,主要表现为SiO2和FeO含量增高,而Fe2O3与Al2O3含量的减少。这3次波动分别出现在剖面的15.3～14.6m,11.0～9.9m和8.40～7.84m深度处。在3次化学组成的波动出现的同时,指示沉积物风化程度和温度变化的Si/Al（SiO2/Al2O3）和FeO/Fe2O3比值也发生了明显变化,比值增高,指示了3次大的干冷事件。另外,在剖面上部（10.00～7.84m,即第2次事件以后）SiO2/Al2O3和FeO/Fe2O3比值变高且波动频繁,表明自第2次干冷事件后沉积区的环境变得不稳定,逐渐向冷干气候转变。东洞剖面的地球化学记录（SiO2/Al2O3和FeO/Fe2O3）与泾川黄土剖面的粒度曲线具有较好的对比性,支持了东洞剖面记录的环境信息与黄土沉积记录的环境变化具有一致性。通过与泾川粒度曲线的对比发现,东洞剖面记录的3次干冷事件在时段上分别对应于黄土-古土壤序列中的L26,L15和L13。  相似文献   

18.
核桃箐铁铜矿位于滇中武定断陷盆地内,与迤纳厂铁铜矿同属于滇中地区的IOCG型矿产。野外地质调查发现其成矿地质背景和矿床特征虽与迤纳厂铁铜矿有一定相似性,但富集的成矿元素类型和蚀变特征等方面却与迤纳厂不同。核桃箐矿区主要的赋矿地层为落雪组(Pt_2l),成矿元素只大量富集Fe元素,局部富集Cu元素,没有富集Au元素。矿石类型以致密块状磁铁矿石为主,围岩蚀变类型较少但具有一定的分带性,与矿化关系最为密切的蚀变是硅化。岩相学和地球化学研究发现,从围岩到蚀变岩,核桃箐矿区内主要的迁入元素是SiO_2、Al_2O_3、MnO和Na_2O,主要的迁出元素是CaO、MgO、H_2O和CO_2,表明核桃箐矿区内的硅化与去碳酸盐化是同时进行的,而钠化则与其同步或者稍晚于硅化。岩矿地球化学研究表明,核桃箐含矿围岩的n(SiO_2)/n(Al_2O_3) 3. 6,n(Al)/n(Al+Fe+Mn) 0. 5,n(Al_2O_3)/n(Al_2O_3+Fe_2O_3)=0. 12～0. 36,反映其形成于拉张环境、成岩时海水深度较浅的成岩特征。  相似文献   

19.
The heat capacity and vibrational entropy of a calcium aluminate and three peraluminous calcium aluminosilicate glasses have been determined from 2 to 300 K by heat-pulse relaxation calorimetry. Together with previous adiabatic data for six other glasses in the system CaO-Al2O3-SiO2, these results have been used to determine partial molar heat capacities and entropies for five species namely, SiO2, CaO and three different sorts of Al2O3 in which Al is 4-, 5- and 6-fold coordinated by oxygen. Given the determining role of oxygen coordination on low-temperature heat capacity, the composition independent entropies found for SiO2 and CaO indicate that short-range order around Si and Ca is not sensitive to aluminum speciation up to the highest fraction of 25% observed for VAl by NMR spectroscopy. Because of the higher room-temperature vibrational entropy of IVAl2O3 (72.8 J/mol K) compared to VAl2O3 (48.5 J/mol K), temperature-induced changes from IVAl to VAl give rise to a small negative contribution of the order of 1 J/mol K to the partial molar configurational heat capacity of Al2O3 in melts. Near 0 K, pure SiO2 glass distinguishes itself by the importance of the calorimetric boson peak. On a g atom basis, the maximum of this peak varies with the composition of calcium aluminosilicate glasses by a factor of about 2. It does not show smooth variations, however, either as a function of SiO2 content, at constant CaO/Al2O3 ratio, or as a function of Al2O3 content, at constant SiO2 content.  相似文献   

20.
The raw-material base of the Russian aluminum industry is considered. The raw materials include common (bauxites, nepheline syenites) and uncommon (nepheline ores, synnyrites, anorthosites, power-and-heating plant ashes, kaolines) types of ores. With regard to many criteria (reserves and quality of ores, technology of their processing, etc.), the problem of alumina deficit can be solved by mining sillimanite group minerals Al2SiO5 (wt.%: Al2O3 = 62.9, SiO2 = 37.1), namely, andalusite, sillimanite, and kyanite. Their proved reserves converted to the final product (aluminum) exceed 400 mln tons. This will be enough for more than a hundred years provided that aluminum is produced in the present-day output (4 mln tons in 2008). Almost all deposits can be explored by strip mining, with application of the gravitation, flotation, and electromagnetic separation methods for ore dressing. The alumina content in concentrates reaches 60–62 wt.%. Only high-grade bauxites and the above concentration methods can ensure such a high yield of Al2O3. Sillimanite group minerals can be processed together with nepheline ores by sintering or be used for the direct electrothermal production of silumin and aluminum, excluding the alumina production stage. The latter method is the most promising in Russia.  相似文献   

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