Origin of the color in cobalt-doped quartz

Synthetic Co-doped quartz was grown hydrothermally in steel autoclaves at the Technological Center of Minas Gerais (CETEC), Brazil. The quartz samples, originally yellow in the as-grown state acquired blue coloration after prolonged heat treatment times at 500°C near the alpha–beta transition temperature. UV–VIS–NIR absorption spectroscopy shows the characteristic spectra of Co³⁺ before heat treatment. After heat treatment, the optical absorption spectrum is dominated by two split-triplet bands the first in the near infrared region centered at about 6,700 cm⁻¹ (1,490 nm) and the second in the visible spectral range at about 16,900 cm⁻¹ (590 nm). Both split-triplet bands are typical for Co²⁺ ions in tetrahedral coordination environments. From the absence of electron paramagnetic resonance (EPR) spectra, we conclude that the Co²⁺ found in the optical absorption spectra of the blue quartz is not due to an isolated structural site in the quartz lattice. Instead, the blue color is associated with electronic transitions of Co²⁺ in small inclusions in which the Co site has tetrahedral symmetry. The non-observation of polarization-depend optical absorption spectra is also in agreement with this model. The results for Co²⁺ in quartz are different from Co-bearing spinel and staurolite and other silicates like orthopyroxene, olivine, and beryls. The formation process of the color center is discussed.     

Origin of red beds in the Ringerike Group (Silurian) of Norway

The Ringerike Group is a meandering fluviatile succession which is about 60% red. Most of the red zones are formed of mudrocks and siltstones and correspond to the fine members of fining-upwards cyclothems. The majority of coarse members are drab coloured.Textural studies of thin and polished sections show that the red colour is caused by finely crystalline hematite as matrix and grain-coatings. This hematite apparently crystallized post-depositionally. Hematite also occurs in other textural sites: within altered phyllosilicates, as detrital grains and as totally pseudomorphed phyllosilicates. This, and the lack of consistency between colour and clay mineralogy, suggests that the red beds have had a long and complex diagenetic history.Iron analyses indicate that the red beds are enriched in Fe³⁺ and total iron (FeO) by about 1%. This is thought to have been derived from the pre-depositional weathering of iron minerals and introduced into the sediments as amorphous iron hydroxide or iron-bearing clays. Crystallization of iron hydroxide under oxidizing conditions and the post-depositional alteration of iron-silicates and oxides is thought to be responsible for the formation of the red beds.     

黔东北地区地热水化学特征及起源

目前在黔东北地区未系统地开展过地热水水文地球化学特征以及地热水来源方面的研究,存在地热水来源、补给区域、径流和排泄等特征不清等问题.在充分了解黔东北地热地质条件的基础上,采集区内15组地热水进行水化学全分析、收集12组地热水氢氧同位素和3组地热水碳同位素数据,得到了该区地热水的水化学特征和同位素特征,分析出地热水的补给来源,估算了地热水的补给高程、补给温度、热储温度、循环深度以及冷水混入比例.结果表明,受地形地貌及地质构造的影响,该区地热水总体由南向北径流,水化学类型主要为HCO₃·SO₄-Ca·Mg、HCO₃-Ca·Mg、HCO₃-Na、SO₄-Ca·Mg及SO₄-Ca型,有益元素主要有F-和H₂SiO₃,沿径流方向地热水呈现pH降低、TDS增加的趋势,水化学类型则由重碳酸盐型水变为硫酸盐型水.同位素分析结果表明,该区地热水补给源为大气降水,补给区为海拔1 500~2 000 m的梵净山地区,地热水年龄为（6 400~11 570）±560 a,补给时的年平均气温为7.0~9.1℃;选用二氧化硅温标及lg（Q/K）-T法估算热储温度为45.0~107.0℃,地热水循环深度为1 000~3 000 m;硅-焓混合模型估算地热水混合前的热储温度极大值为110~200℃,地热水在上升过程中受浅部冷水混合,冷水混入比例为50%~90%.      

鄂尔多斯盆地东北缘保德地区煤层气成因

       鄂尔多斯盆地东北缘保德地区煤层气井组试采取得较好效果,但煤层气成因尚未形成统一认识,影响到对区域煤层 气勘探开发潜力的进一步认识。本文基于各类煤层气样品组分和稳定同位素的分析,对其成因进行了探讨。结果表明,煤 层气组分以甲烷为主,重烃浓度极低;δ13CCH4明显偏轻,部分δ13CCH4和δDCH4分布在热成因与二氧化碳还原型生物成因气过 渡区间;δ13CCO2相对较重,且与δ13CCH4之间存在负相关关系。分析认为,该区煤层气具有混合成因,以热成因气为主,兼 具生物成因气的特征,生物甲烷形成于二氧化碳还原途径,煤层水的化学和动力条件以及煤岩孔渗条件有利于产甲烷菌的 大量繁殖。     

黔东北双河重晶石-萤石矿床流体包裹体组合研究及成因

黔东北双河重晶石-萤石矿床的矿体主要产于下奥陶统碳酸盐岩中,严格受断裂构造控制,为探讨其成因,采用流体包裹体组合(FIA)方法对成矿流体进行了研究.结果显示,流体包裹体类型分为单相水溶液包裹体和富液相两相水溶液包裹体;成矿早期萤石和晚期萤石的均一温度分别为89～131℃和99～122℃,盐度分别为3.83％～20.84...     

Biostratigraphy of oligocene succession in the High Folded Zone,Sulaimani, Kurdistan region,Northeastern Iraq

Oligocene and Lower Miocene sediments from High Folded Zone of Iraqi Zagros have been studied paleontologically at south of Sulaimaniyah, Kurdistan Region, NE Iraq. The identified fauna are consisted of (25) genera and species of benthonic and (16) species of planktonic foraminifera. The fauna comprises relatively abundant foraminiferal assemblages of moderate diversity. Based on the stratigraphic distribution of these species, two biozones have been recognized which are Nummulites—Rotalia and Globoquadrina dehiscens zones. These biozones indicate that the studied sections of Basara and Khewata are of Late Oligocene–Early Miocene age. Based on the microfossils, it has been found that the age of sediments is equivalent to or represents Anah and Serikagni Formations. Some previous studies described Oligocene rocks (Kirkuk Group) as interior sag basin. In the present study, the occurrence of the group inside High Folded Zone and its rich fauna content are used for the discussion of the sag basin versus normal marine water.     

Facies architecture and sequence development of the Euphrates formation in western Iraq

The Euphrates formation is widely exposed in western Iraq near Al-Qaim area. It extends eastward parallel to the Euphrates River on both sides, crossing Anah, Hadetha, and Al-Baghdadi vicinities. Based on the benthic foraminiferal assemblages and microfacies features, 12 different microfacies types have been recognized into two Stratigraphic sections that are lime mudstone, wackestone, bioclastic wackestone, miliolids wackestone, alveolinids wackestone, packstone, bioclastic packstone, peloidal packstone, miliolids packstone, peneroplids packstone, rotaliids packstone, grainstone, peloidal grainstone, oolitic grainstone, and miliolids grainstone. Accordingly, the depositional environments were recognized on the basis of microfacies identification and interpretation ranging from restricted marine, shoal, to open marine environments. The exposed Euphrates succession is represented by four fourth-order cycles (A, B, C, and D); they are mostly asymmetrical showing slightly lateral variation in thickness and symmetry. These cycles represent a succession episode of sea level rises and stillstands. The nature of these cycles reflects variation in the relative sea level resulted from eustatic and tectonic subsidence. Cycle A is underlain by SB1 of Sheik Alas formation in Al-Baghdadi section and by Anah formation in Wadi Hjar section. Cycle D is overlain by SB1 of Fatha formation in Al-Baghdadi section and by conformable contact of Nfayil formation in Wadi Hjar section. According to sequence development, the study area shows low rate of subsidence and the major controlling factor that affects eustatic sea level fluctuation.     

川东北地区中侏罗世新田沟期古气候波动的地球化学响应

四川盆地中侏罗统新田沟组作为侏罗纪大面积红层之下的重要岩石地层单元,见证了盆地由潮湿向干旱转变的整个过程。然而,以往的研究主要集中在古气候类型方面,缺乏系统深入的气候变化研究。本文以川东北地区宣汉七里乡塔坝剖面为对象,系统分析了泥岩的主微量元素地球化学特征,结果表明:化学蚀变指数(CIA)、C值、Sr/Cu、Sr/Ba、Rb/Ti、Cs/Ti、Rb/K等7种古气候替代指标综合显示新田沟期气候存在从炎热潮湿逐渐向干旱转变的趋势,整体以半干旱-半潮湿气候为主。且该时期气候变化过程并不稳定,存在多个短期的湿热与干旱的气候波动。在新田沟组沉积末期,气候条件变得极端干燥。川东北地区中侏罗世的元素地球化学特征记录了区域内重大地质转折期将至时的气候变化,在侏罗纪大面积红层发生之前,气候条件已经开始响应。在中侏罗世早期,四川盆地与欧洲地区并不处于同一个气候带内。     

Origin and Superposition Metallogenic Model of the Sandstone-type Uranium Deposit in the Northeastern Ordos Basin, China

This paper deals with the metallogenic model of the sandstone type uranium deposit in the northeastern Ordos Basin from aspects of uranium source, migration and deposition. A superposition metallogenic model has been established due to complex uranium mineralization processes with superposition of oil-gas reduction and thermal reformation.     

赣东北侏罗纪陆盆的沉积建造及构造旋回

阐述下侏罗统林山组、中侏罗统罗坳组、上侏罗统至下白垩统武夷群的建造类型及建造序列 ,总结了侏罗纪陆盆的发展阶段。本区中生代构造运动划分为 3个亚旋回 :印支亚旋回 (分为Ⅰ幕金子运动 ,Ⅱ幕南象运动 )、早期燕山亚旋回、晚期燕山亚旋回 ;4个构造幕 ,尤以燕山Ⅲ幕最为剧烈 ,使本区地势由东升西降转变为西升东降。     

From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study

The best known cause for colors in insulating minerals is due to transition metal ions as impurities. As an example, Cr³⁺ is responsible for the red color of ruby (α-Al₂O₃:Cr³⁺) and the green color of eskolaite (α-Cr₂O₃). Using X-ray absorption measurements, we connect the colors of the Cr _x Al_2−x O₃ series with the structural and electronic local environment around Cr. UV–VIS electronic parameters, such as the crystal field and the Racah parameter B, are related to those deduced from the analysis of the isotropic and XMCD spectra at the Cr L_2,3-edges in Cr_0.07Al_1.93O₃ and eskolaite. The Cr–O bond lengths are extracted by EXAFS at the Cr K-edge in the whole Cr _x Al_2−x O₃ (0.07≤x< 2) solid solution series. The variation of the mean Cr–O distance between Cr_0.07Al_1.93O₃ and α-Cr₂O₃ is evaluated to be 0.01₅ Å (≈1%). The variation of the crystal field in the Cr _x Al_2−x O₃ series is discussed in relation with the variation of the averaged Cr–O distances.     

Origin of spheroidal chert nodules, Drunka Formation (Lower Eocene), Egypt

Chert nodules of the Drunka Formation (Lower Eocene) are mostly spherical, have diameters from 40 to 120 cm, are quasi-uniformly spaced 2–3 m apart in the plane of bedding, have concentric internal structure and, except for rare small (<6 cm) solid chert nodules, are less than 85% chertified. Nodules formed after moderate alteration of limestone by meteoric water (δ¹⁸O_calcite = –4 to –8‰) at shallow (<100 m) burial depths; more extensive alteration of limestone (δ¹⁸O = –10 to –16‰) by meteoric water followed nodule growth. Chertification was by low-temperature meteoric water (δ¹⁸O_quartz = +18‰ in margins to +22‰ in nuclei) at shallow burial depths. Meteoric water may have invaded the Drunka Formation in association with shelf progradation during the Early Eocene, or during the development of a Middle Eocene unconformity. Replacement of carbonate mud by microcrystalline quartz was the dominant chertification process, but fossils were replaced in part by fine-grained equant megaquartz, quartzine and chalcedony; the last of these occurs in places as beekite. Opal A-secreting marine organisms are the inferred source of silica, but none are preserved. There is no compelling evidence of an opal-CT precursor, so quartz may have formed by direct precipitation. Self-organization processes of enigmatic character established the spacing pattern of the nodules and also the Liesegang-banded internal structure of the chert nodules. Nodules grew chiefly by diffusive supply of silica, although one locality has elongate nodules that grew when there was some porewater advection. Chertification patterns and δ¹⁸O values of both calcite and quartz indicate that nodule growth was complex and variable. Some nodules probably grew from the centre outwards. Many nodules, however, initially grew simultaneously across the entire nodule, but late-stage growth was predominantly at the outer margins or at selective internal sites.     

Origin and differentiation of picritic arc magmas,Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg 80 to 93.4) and clinopyroxene (mg 80 to 92), and include accessory Cr-rich spinel (cr 50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to 20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with 10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo₂ FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has 15 wt% MgO and an mg ^Fe2 value of 81. It requires deep (3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T _p 1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.     

Origin of nodular chert in the carbonate rocks of the Kaladgi group (younger precambrian), Karnataka,India

Lensoid, irregular nodules and laminations of chert appear along the bedding planes and laminations of the Vidyanagar Dolomite Member of the Kaladgi Group, Karnataka, India. The field relations of the chert nodules with the host dolomite and evidence on polished hand-specimens clearly demonstrate that the cherts are secondary after dolomite. This contention finds extensive support from thin-sections which reveal floating relict dolomite fragments on a microcrystalline quartz mat, dolomite rhombs in various stages of progressive replacement, polycrystalline quartz rhombs apparently after dolomite. Chertification has also resulted in an aggrading neomorphic recrystallization of dolomicrite into dolomicrospar which rims the dislodged floating fragments of dolomite.Scanning electron microscopic studies revealed three distinct types of surfaces of chert; the polyhedral surface characteristic of microcrystalline quartz, the spongy surface characteristic of chalcedonic quartz (documented in thin-sections as semi-radiating to radiating aggregates of elongated crystals growing on the rhombic surfaces of dolomite, and the intermediate type showing both. The microcrystalline quartz seems to have originated through multinucleation at equally spaced centres on the dolomite mat, whereas the chalcedonic quartz originated by slow but direct and unhindered precipitation into cavities and residual pore-spaces, following dolomitisation. The silica invasion seems to have been accomplished by alkaline interstitial waters charged with silica.The occurrence and relation of chert-nodules with the host dolomite not only helps in unravelling their own origin, but also aids in drawing the dolomitisation curtain and in turn the post-dolomitisation diagenetic modifications of the host carbonates. It may be added, however, that the Kaladgi Carbonates have undergone mineralogical/textural modifications during diagenesis with superimposition of changes affected later, i.e. during tectonism which folded and cross-folded these sediments.     

The role of fracturing and mineralogical alteration of basement gneiss in the oil exhsudation in the Sousa Basin (Lower Cretaceous), Northeastern Brazil

This work focuses the geological context of an oil sample obtained from an exhsudation in a superficial well, located at Salguinho Farm, Sousa Basin, at the central Borborema Province, Brazil. It is a light oil, with 81,1% of saturated compounds and a predominance of C17 and C23 n-paraffins, in which biomarkers point out to a non-biodegraded mature oil. Although the source rocks of this oil are unknown, the reservoir are metamorphic rocks from the basement, in a structural arch nearside the depocenter area of the basin. This lithology corresponds to ortho-derived gneiss strongly modified by ductile and brittle deformation (fracturing), as well as mineralogical alteration due to retrometamorphic and hydrothermal reactions. Here we emphasize and discuss the importance of such modifications underwent by the gneiss in the development of a propitious porosity and permeability for the further oil accumulation.     

Petrogenesis Significance of Chromian Spinels from Northeastern Jiangxi Province Ophiolite (NEJXO), China: Paleogeodynamic Implications

<正>1 Introduction The northeastern Jiangxi Province ophiolite is located in the SE margin of the Yangtze Block,along the Zhangshudun(Yiyang)-Xingangshan(Dexing),ophiolite belt extending northeast for about 100km(Zhou,1989;Li et al.,1997).The original sequence of the NE Jiangxi ophiolite suite has been structurally disrupted and     

Origin of manganese carbonates in Jurassic red shale, central Japan

Manganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well-preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn_86.7?92.2Ca_2.2?2.9Mg_2.6?6.7Fe_2.6?5.6)CO₃ Carbon and oxygen isotope values, which range from ?7.99 to ?4.78‰ and ?4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction.