Origin of a late Neoproterozoic (605 ± 13 Ma) intrusive carbonate–albitite complex in Southern Sinai, Egypt

New geochemical, isotopic, and geochronological data and interpretations are presented for late Neoproterozoic intrusive carbonates and related rocks of southern Sinai, Egypt (northernmost Arabian–Nubian Shield). The Tarr carbonates are coarsely crystalline and related to explosive emplacement of hypabyssal and volcanic albitite at 605 ± 13 Ma. The carbonates associated with the albitites are divisible into two types: primary dolomitite and secondary breunneritite (Fe-rich magnesite). The dolomitite was clearly intrusive but differs from classic igneous carbonatites, containing much lower abundances of incompatible elements, such as REE, U, Th, Rb, Nb, Y, P, Sr, Zr, Ba, and total alkalies. The breunneritite is a secondary replacement of dolomitite, probably marking the roots of a vigorous hydrothermal system. Albitites show pristine abundances of major and trace elements and were not subjected to a major metamorphic overprint. They are relatively more fractionated, alkaline and related to within-plate A-type magmas, were emplaced in an extensional or non-compressive tectonic regime in the cupola of high-level A-type granite. Tarr albitites may represent residual magma remaining after near-total crystallization of an A-type granite pluton at depth, forcibly emplaced into the roof above the cooling pluton. The intrusive dolomitite exsolved from highly differentiated albitite melt, in the apical regions of a still-buried alkaline “A-type” granite pluton that was rich in CO₂; these volatiles migrated upwards and towards the cooler margins of the magma body. Late NNE-SSW extension allowed a shallow-level cupola to form, into which albitite melts and carbonate fluids migrated, culminating in explosive emplacement of albitite breccia and intrusive carbonate. Isotopic compositions of Tarr dolomitite and albitite indicate these are consanguineous and ultimately of mantle origin. Magmatic volatiles fenitized the wall rock, while submarine hydrothermal activity transformed some of the dolomitite into breunneritite. Recognition of Tarr-type should encourage similar hypabyssal complex intrusions to be sought for in association with A-type granitic plutons elsewhere.     

Immiscible silicate-carbonate liquids as evidenced from ocellar diabase dykes,southeast Sinai

Diabase dykes containing spherical patches of carbonate intrude the Tarr albitite complex of SE Sinai. The morphology of these dykes indicates a highly gas-charged magma. Petrographic evidence points toward equilibrium during cooling of immiscible carbonate ocelli and silicate matrix. Dolomite is the main component of these ocelli, which are geochemically and isotopically similar to carbonatite. However, the low total REE content, and the presence of considerable marble in the country rock, suggest a process of stoping and melting of carbonate, followed by the “in situ” development of silicate-carbonate immiscibility.     

A new type U-Th-REE-Nb mineralization related to albitite: A case study from the Chachaxiangka deposit in the northeastern Qaidam Basin of China

The U-Th-REE-Nb (Ta)-polymetallic mineralization is generally related to either the silica-undersaturated syenites, the silica-oversaturated alkaline/peralkaline granites or igneous carbonatites. In this study, the authors report a new mineralization type, which is related to the magmatic-hydrothermal albitite (with mineral assemblage predominated by albite with volume content > 90%), as exemplified by the Chachaxiangka deposit in Qinghai Province of China. The Chachaxiangka deposit is the first albitite-related U-Th-REE-Nb deposit recognized in China and the mineralization can be divided into 3 types: the vein-type, the disseminated veinlet type and breccia type, of which the former 2 are predominant. Three mineralization stages can be identified according to the detailed mineralogical analyses, including the magmatic stage, main hydrothermal mineralization stage and post-ore stage. By comprehensive analyses of the mineralogical, major and trace element compositions, the authors suggest that the albitite vein is magmatic-hydrothermal in origin and both the magmatic evolution and overprint of the hydrothermal fluids play important roles in the formation of the albitite and related polymetallic mineralization. Phase separation between the silicate melt and carbonate/phosphate melt might take place in the magmatic stage, yet the immiscibility between the silicate melt and chloride-dominated fluids is the most important mechanism for the REE mineralization and also causes the Nb-Th re-mobilization and enrichment. The red color of the albitite aplite vein is an eye-catching prospecting mark in the field and more mineralization can be expected at depth and in the surrounding areas. The discovery of the new albitite type U-Th-REE-Nb mineralization give rise to new ideas during future U-Th-REE-Nb exploration, not only in the Qaidam-Altun belt, but also other areas across China.     

南秦岭富钠长石岩石的岩石学和地球化学研究

钠长石岩是秦岭造山带中的一类特色岩石,主要分布于南秦岭凤-太、山-柞和镇-旬盆地的泥盆纪和志留纪地层中,与秦岭众多大中型铅锌矿、金矿成矿关系密切。钠长石岩呈层状、似层状产于碎屑岩或碳酸盐岩岩系中,与区域地层整合产出,渐变过渡。具有条带状、块状、角砾状构造;XRF分析数据显示钠长石岩化学成分以富Na2O、Al2O3、SiO2等常量元素及Pb、Zn、Au、Ag、Cu等微量元素为特征;富含元素Ba和Fe-Mn-(Co+Cu+Ni)×10分布图投影表明钠长石岩具有热水沉积的特征。而TiO2与Al2O3关系图解又反映出钠长石岩中有陆源碎屑物混入的特点,其Ti、Al可能来自水成沉积物中的粘土;ICP-MS分析所得稀土元素数据显示,钠长石岩与地层稀土元素在稀土总量、轻重稀土量和配分模式均十分相似,其较高∑REE就是成岩过程中水成沉积物混入造成的。富钠长石岩石在形成时间上发生在沉积和早期成岩阶段,与正常沉积岩同时形成。兼具有正常沉积岩石和热水沉积岩石的特点,是一种有别于二者的混合型非典型热水沉积岩石。研究认为钠长石岩是由一种富钠和金属元素的热水通过对地层元素的萃取并与海底未固结沉积物混合而成的混合热液经沉积成岩作用形成的。     

陕西双王钠长石岩特征及金矿床形成期次分析

双王含金钠长石岩带主要为层状 ,产于泥盆系星红铺组粉砂质板岩中。钠长石岩带主要由层状钠长石岩及钠长石岩角砾岩组成。角砾岩角砾主要为层状钠长石岩碎裂形成。层状钠长石岩及钠长石岩角砾韵律纹层构造及条带状构造发育 ,矿物组合和热水沉积钠长石岩相同。钠长石岩主元素及微量元素特征表明钠长石岩具热水沉积的一般特征。角砾岩角砾棱角明显 ,具可拼接破裂构造或典型角砾状构造 ;钠长石岩角砾和早期胶结物矿物组成基本相同。胶结充填物中晶洞构造发育 ,显示同生液压致裂角砾的一般特征。据钠长石岩沉积特征构造及角砾特征 ,认为钠长石岩及金矿床主要是热水沉积 -隐爆作用形成的 ,可分为 4个形成阶段 ,金矿矿化主要和第 3、4阶段有关。     

Spectral delineation of albite zone using ASTER data in Khetri Copper Belt

The Khetri Copper Belt is well known because of its economically viable Cu deposits. In this region, there is a linear narrow zone following a major crustal fracture. Along this zone, due to hydrothermal activity and subsequent alteration, a new rock, albitite has formed. The study presents an account of the spectral analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) dataset for albitite mapping in the Khetri Copper Belt. Spectral angle mapper algorithm is applied on the ASTER data taking the ground sample reflectance spectra (VNIR–SWIR) and image derived spectra (TIR) as reference spectra. The albitite-classified areas are analyzed spatially and statistically to determine the extent of albitite zone in the study area. This research is to contribute to the better understanding of the spatial extent of the albitite zone which will assist in determining the potential for the mineralization in the area. The remote sensing approach of utilizing multichannel data offers the capability to resolve the key geological features of this geologically dynamic area.     

X-ray crystallography of uraninites associated with the albitite belt of western India: Evidence for the high-temperature origin of uranium and associated mineralisation

X-ray diffraction (XRD) studies on the radioactive ore samples from various parts of Rajasthan and Haryana have revealed the presence of several uranium and other atomic mineral occurrences in the albitite belt of western India. The primary uranium minerals (PUMs) are uraninite and brannerite, whereas, the secondary uranium minerals (SUMs) show considerable speciations: phosphate, silicate, hydrous oxide hydrate, and vanadate. Multiple oxides (MOs) are davidite, fergusonite, aeschynite-(Y), microlite, samarskite, euxenite, betafite, and columbite-tantalite. The thorium minerals are huttonite, thorite, uranoan-thorite, thorianite, thorutite, and brabantite. The yttrium and REE-bearing minerals are xenotime, britholite, allanite, chevkinite, tritomite, and monazite. It is noted that the measured unit cell dimension (a₀) of the investigated uraninites ranges from 5.4110 Å to 5.4646 Å. The highest unit cell dimension (5.4646 Å) represents a composition (or oxidation grade) of UO_2.05, whereas, the lowest one (5.4110 Å) corresponds to a composition of UO_2.54. Furthermore, it is also apparent that, with increase in oxidation grade, there is a concomitant decrease in unit cell dimension. As most of the values of a_o of uraninites from the albitite belt are high (> 5.45 Å), it may be inferred that the overall temperature of formation of uraninites of the albitite belt was higher (ca. 400°C). However, the low values of a₀ in certain localities could be due to the prevalence of relatively low and fluctuating temperature regimes locally (ca. 400°–100° C). Numerous occurrences of refractory, multiple oxides, and REE minerals, in association with uranium mineralisation, also support a high-temperature origin for the investigated uraninites. Binary data plots of unit cell dimension (a₀) versus oxidation grade/composition (UO_2+x) of uraninites (n = 36) suggest that the gross uranium mineralisation in the albitite belt of western India is mainly linked to regional metamorphism, anatexis, granitic intrusion, metasomatism, and contact metamorphosed granite-pegmatite aureoles and granite-related vein type with hydrothermal overprints, including redistribution of intrinsic sedimentary uranium and its concentration along suitable structural locales. These interpretations are consistent with the known gross geologic features of the albitite belt. Furthermore, the presence of marialite (calcian) in many places in the albitite belt also supports such a contention, as this mineral is known to be restricted to metamorphic and metasomatic environments. The speciation of secondary uranium minerals could be due to the higher oxidation of U⁴⁺ to U⁶⁺ in surface to near-surface conditions and its (U⁶⁺) remobilisation as uranyl ions. The combination of moving uranyl ions with available cations and anions en route caused re-precipitation of U as diversified assemblages of low-temperature uranyl minerals under suitable physicochemical conditions.     

大沟谷钠长石岩及金矿床形成分析

对富金钠长石岩的岩石化学、微量元素及稀土元素分析显示,大沟谷钠长石岩与区域上震旦系乐昌峡群片岩及附近花岗岩没有成因联系.富金钠长石岩主要是热水沉积形成的.钠长石岩与金矿有紧密的内在联系,钠长石岩型金矿床是热水沉积+改造作用形成的.     

Zircon Geochronology and Trace Element Geochemistry from the Xiaozhen Copper Deposit, North Daba Mountain: Constraints on Albitites Petrogenesis

Albitite often accompanies with various metal and gem mineral deposits and a large number of occurrences have been reported globally, including the South Qinling orogen, China. The Xiaozhen copper deposit is a typical deposit in the North Daba Mountain area of the South Qinling orogen whose distribution is controlled by albitite veins and fractures. As there are few studies on the petrogenesis of albitite in Xiaozhen copper deposit, this paper focuses on the petrogenesis of albitite and its mineralization age. Detailed fieldwork and mineral microscopic observations initially suggest that albitite from the Xiaozhen copper deposit is igneous in origin. Further zircon trace element geochemistry studies indicate that these zircons have high Th/U ratios(0.5), low La content, high(Sm/La)N and Ce/Ce*values, and a strong negative Eu anomaly, which are commonly seen in magmatic zircons. The chondrite–normalized rare earth element(REE) patterns are consistent with magmatic zircons from throughout the world, and they fall within or near the field of magmatic zircons on discriminant diagrams. The calculated average apparent Ti–in–zircon temperature for young zircons is 780°C, consistent with magmatic zircon crystallization temperatures. Therefore, zircon geochemistry indicates that the albitite origin is magmatic. SIMS U–Pb dating on nine magmatic zircons yielded a concordia age of 154.8±2.2 Ma, which represents the formation of albitite and the metallogenic age. More importantly, it is consistent with the ages of Yanshanian magmatism and metallogenesis in the South Qinling orogen, so formation of the Xiaozhen copper deposit may be a closely related geological event.     

青海查查香卡铀多金属矿床钠长岩脉地质特征及其对矿床成因的约束

查查香卡铀多金属矿床是青海省探明的第一个中型硬岩型U-Th-Nb-LREE矿床.为进一步明确矿床成因,文章在详细描述矿床地质特征的基础上,使用LA-ICP-MS对钠长岩脉中锆石进行U-Pb定年,同时对钠长岩脉和不同矿化强度斜长角闪片岩的岩石学、矿物学和地球化学研究.测试和研究结果表明,该矿床是与钠长岩脉有关的岩浆型铀多金属矿床,其成岩成矿年龄为(401.7±6.3)Ma,矿石的微量和稀土元素特征与钠长岩脉更为相似.文章认为钠长岩脉是在柴达木盆地北缘早古生代造山作用结束后的板内环境中,由深部富集地幔低程度部分熔融形成.矿区西部具有钠长岩脉型铀多金属矿的找矿潜力,而东部则有可能发现岩体型矿体,具有更大的找矿潜力.     

四川会理小青山地区钠长(石)岩成因初探

笔者从岩石学、岩石化学、稀土元素地球化学特征等方面对小青山地区的钠长岩进行了初步探讨。该区的钠长岩主要受构造控制，分布于破碎带及其两侧，具有从中心到围岩的对称分带性特征。钠长岩中存在大量的交代残余结构。通过岩石化学成分和稀土元素地球化学特征，尤其是通过Q型群分析表明：该区的钠长岩化学成分复杂，且与干枚岩／板岩有一定的相关性。笔者认为该区的钠长岩为热液交代的产物，其被交代的原岩主要为古元古界的一套富含Si和Al的浅变质岩系。区内的铜(金)矿床与钠长岩的形成作用密切相关。     

Geochemistry of Pan-African volcanic arc sequences in southeastern Sinai Peninsula and plate tectonic implications

Major and trace elements of lavas, dykes and plutonic rocks of the late Proterozoic orogenic sequence in SE Sinai, the Kid Group, have been analysed. The dykes and lavas of the southermost sequences, the Tarr Complex and Heib Formation, are calc-alkaline, whereas the lavas of the Malhak Formation and the Sharira Gabbro (to the north of the Heib Formation) show both calc-alkaline and tholeiitic trends. The trace element characteristics of the Tarr Complex and the Heib and Malhak Formations, despite between-sequence variations, are all comparable with ensialic island arc magmatism, whereas the Sharira Gabbro shows some MORB characteristics in addition to the island arc imprints. The Sharira Gabbro and the lavas of the Malhak Formation possibly formed in a developing back-arc basin behind a continental-marginal ensialic island arc (the Tarr Complex and Heib Formation). During the Pan-African orogeny, the constituent units of the Kid Group were mutually juxtaposed along major ductile shear zones of thrust-fault character. This plate-convergence regime involved initial magmatic arc development following northward subduction, and subsequent collision between the arc complex and the Proterozoic continental margin.     

广东大沟谷钠长石岩地球化学特征及成因研究

粤西大沟谷一带钢长石岩主要以层状、透镜状及脉状产于震旦系乐昌峡群变质片岩中,具块状及条带状构造,是大沟谷金矿的主要赋矿岩石。钠长石岩富Na、St、Al、Mg、Mn、K,在微量元素方面富An、As、Sn、B、Cu、Ni、Co、Ph、Zn,稀土元素的含量较低,配分模式为平缓右倾曲线。矿物组合及铁白云石碳、氧同位素组成特征与秦岭地区热水沉积钠长石岩的基本相同。据大沟谷钢长石岩的地质地球化学特征认为钠长石岩是热水沉积的产物。     

Origin of trondhjemite and albitite at the expense of A-type granite,Aravalli orogen,India:Evidence from new metasomatic replacement fronts

The A-type Harsora-Dadikar granites in the Alwar complex of northern Aravalli orogen,NW India provide evidence for subsolidus-requilibration of feldspars.They record three new discrete stages of albitisation,producing trondhjemite and albitite sequentially at the expense of original granite.Stage-Ⅰ metasomatism deanorthised the magmatic oligoclase and transformed the grey least-albitised granite to pinkish grey microcline-oligoclase granite.Stage-II converted the latter to trondhjemite by replacement of microcline to oligoclase.Stage-Ⅲ metasomatism led to the formation of albitite/albite granite from trondhjemite,where the metasomatically formed oligoclase was replaced by albite.This stage of metasomatism resulted in nearly complete disappearance of amphibole and biotite,producing a monomineralic rock(albitite),which is consistent with Korzhinskii theory of infiltration metasomatism.The reaction fronts delineating the Stage-Ⅱ and Stage-Ⅲ are sharp and easily discernible by their prominent color differences in Harsora on the outcrop scale.Chemically,the mineral transformations during three stages are manifested by the differential gains/losses in Na,K,Ca,Rb,Ba,Sr,Fe and Mg.The formation of albite,Cl-rich marialitic scapolite and Cl-rich amphibole in the albitised granites are suggestive of Naand Cl-brines as the metasomatising fluids.The fluid-rock interactions,which can significantly transform the pristine mineralogy of granitoids,should be carefully considered to avoid any misinterpretations about their petrological history.     

Geology,geochemistry and genesis of the Makou magnetite-apatite deposit in the Luzong volcanic basin,Middle-Lower Yangtze River Valley Metallogenic Belt,Eastern China

The Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB) is located on the northern margin of the Yangtze Plate (Eastern China). Ore deposits in the belt are mainly clustered in seven ore districts, and are closely associated with Mesozoic intermediate-felsic magmatic rock. Among the seven ore districts, the Luzong and Ningwu districts host large-scale iron resources in volcanic basins. The Makou magnetite-apatite deposit in the southern Luzong Basin was previously interpreted to be related to a quartz syenite porphyry. In this study, we conducted field geological studies and determined the age and geochemistry of the Makou intrusive rocks. Petrography and electron probe micro analysis (EPMA) indicated that the Makou ore-hosting rocks have intense albite alteration. The wallrock alteration is spatially restricted, and comprises albite alteration (Stage I), magnetite mineralization (Stage II), quartz-sulfide alteration (Stage III) and carbonate alteration (Stage IV) stages. Fluid inclusions in syn-mineralization apatite homogenized at 252.2–322.6 °C, which slightly lower than is typical for magnetite-apatite deposits in the region. Field study revealed that the quartz syenite porphyry at Makou disrupted the orebodies along clear-cut intrusive contacts, and that the quartz syenite porphyry does not contain iron mineralization, suggesting it has no direct genetic relationship with the iron mineralization. The ore-hosting albitite and ore-forming biotite diorite have LA-ICP-MS zircon U-Pb ages of 129.6 ± 1.2 Ma and 131.2 ± 3.3 Ma, respectively, and the iron mineralization was dated by mass spectrometer phlogopite ⁴⁰Ar-³⁹Ar at 130.76 ± 0.77 Ma. We propose that the Makou magnetite-apatite deposit is genetically related to the biotite diorite, rather than to the quartz syenite porphyry in the mine pit. The biotite diorite closely resembles intrusions related to magnetite-apatite deposits elsewhere in the region.     

赣东北高压变质岩的岩石类型、矿物组成与变质过程

赣东北高压变质岩包括含硬玉霓辉石钠长角闪片岩、含硬玉霓辉石石英钠长石岩、含霓辉石角闪石英钠长石岩、含霓辉石钠长角闪片岩、蓝透闪石石英钠长石岩、镁钠闪石石英钠长石岩等岩石类型，主要组成脏矿物为硬玉、霓辉石、镁钠闪石、蓝透闪石、镁角闪石、阳起石、石英、钠长石、金红石和榍石。研究表明，高压变质峰期后经历了近等温降压退变质过程。     

甘肃龙首山成矿带新水井铀(钍)矿床元素迁移规律及成矿作用过程研究

新水井铀(钍)矿床位于甘肃省龙首山成矿带,是碱交代型铀矿床的典型代表,其矿体完全产于钠交代蚀变花岗岩中,成矿过程可划分为钠交代蚀变、铀钍矿化和成矿后3个主要阶段。文章对该矿床花岗岩原岩、蚀变岩及矿石开展了系统主微量元素分析,采用Grant等浓度线法探讨了钠交代蚀变和铀钍矿化阶段的元素迁移规律,结果表明:钠交代蚀变阶段为富含Na、Ca、过渡族元素(Sc、V、Cr、Co、Ni)、U、Th及CO2、H2O等挥发分的复杂流体,钠交代过程中原岩中的大离子亲石元素(Rb、Ba)和部分轻稀土元素(LREE)不同程度带出;而铀钍成矿阶段成矿流体则富集重稀土元素(HREE)、U、Th、PO43-等成分,CO2等挥发分大量逸出。结合前人研究,认为新水井矿床成矿流体可能来自地幔流体和大气降水热液的混合;等挥发分CO2的逸出是新水井矿床最重要的矿质沉淀机制,导致了铀钍矿物和磷酸盐矿物(磷灰石)的共沉淀,而磷灰石的沉淀又促进了以磷酸盐形式搬运的Th元素的沉淀。     

Geology and geochemistry of the Changba SEDEX Pb-Zn deposit,Qinling orogenic belt,China

The Changba Pb-Zn SEDEX deposit occurs in the Middle Devonian sequence of the Anjiaca Formation of the Western Qinling Hercynian Orogen in the Gansu Province, China. The Changba-II orebody is hosted in biotite quartz schist and is the largest of 143 stratiform orebodies that are hosted either in biotite quartz schist or marble. The Changba-II comprises two types of mineralization: a bedded facies and an underlying breccia lens. The bedded section exhibits three sulfide sub-facies zoned from bottom to top: 1) banded sphalerite intercalated with quartz albitite; 2) interbedded massive pyrite and sphalerite ore; and 3) banded sphalerite ore intercalated with banded baritite. Major metallic minerals are sphalerite, pyrite, galena, with minor arsenopyrite, pyrrhotite, boulangerite, and rare chalcopyrite. The bedded sulfides are underlain by a lens of brecciated and albitized biotite-quartz schists cemented by sulfides and tourmaline.Massive and bedded sulfide ³⁴S values range from 8.1 to 29.3, whereas barite ³⁴S values range from 20.8 to 31.5. Disseminated pyrite in footwall schists has ³⁴S values ranging from 8.1 to 10.6, and increase to values ranging from 11.1 to 14.7 in the hangingwall. The lower ³⁴S values for massive and bedded sulfides are interpreted to be derived from progressive bacterial sulfate reduction (BSR) of Devonian seawater in a sulfate-restricted sub-basin. The higher ³⁴S values for massive and bedded sulfides could be a product of quantitative BSR but this is incompatible with barite being more abundant above the bedded sulfides. Instead, it is more likely that thermochemical sulfate reduction of seawater sulfate or of evaporite was the source of heavy hydrothermal sulfur. Heavy hydrothermal sulfur was injected into a sulfate-restricted sub-basin where it mixed with low ³⁴S BSR sulfide to form the massive and bedded sulfides. The REE patterns of sulfide layers and associated quartz albitite and baritite are similar to those of the host biotite quartz schists, suggesting that the hydrothermal fluids leached REE from the underlying rocks. Pb isotope ratios in galena form an array between the Upper Crust and the Mantle reservoir curves, which indicates that the lead is derived from upper crustal rocks comprising mafic igneous units. The Sr⁸⁷/Sr⁸⁶ ratio of 0.7101 for carbonate within the sulfide layers also suggests that Sr is derived from the mixing of Sr leached from upper crustal rocks with Middle Devonian seawater Sr. A Rb-Sr isochron age of 389.4 ± 6.4 Ma for sulfide layers and the interbedded hydrothermal sediments is consistent with the age of host Mid-Devonian strata. Ar³⁹/Ar⁴⁰ plateau age at 352.8 ± 3.5 Ma and Ar³⁹-Ar⁴⁰ isochron age of 346.6 ± 6.4 Ma for albite in the quartz albitite intercalated with sulfide layers indicate either albite formation after the sulfides or thermal resetting of the Rb-Sr system at about 350 Ma, the age of collision between the North China and Yangtze cratons.Editorial handling: E. Frimmel     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

ATR-FTIR spectroscopic characterization of coexisting carbonate surface complexes on hematite

The speciation of carbonate adsorbed to hematite in air-equilibrated aqueous solutions has been studied using ATR-FTIR spectroscopy. Samples were measured over a range of pH conditions, at 0.1 M NaCl and at low ionic strength, and in H₂O and D₂O solutions to permit a multispecies analysis of the data. Second-derivative analyses and fits to the spectra indicate the presence of two major and two minor surface-bound carbonate species. The two major complexes coexist at near-neutral pH and low ionic strength. One of these two complexes is relatively sensitive to ionic strength, being displaced at 0.1 M NaCl, whereas the other is not. Comparison of experimental to DFT/MO-calculated frequencies suggest these two major species to be (a) a monodentate binuclear inner-sphere carbonate surface complex, and (b) a fully or partially solvated carbonate (CO₃²⁻) species that is symmetry broken and appears to reside in the structured vicinal water layers at the hematite-water interface, retained by hydrogen bonding and/or other forces. Minor carbonate complexes include diffuse layer CO₃²⁻ and an unidentified inner-sphere species. Both of the dominant species observed here are likely to be significant controls of the surface charge and sorptive properties of Fe-oxides.