A computer simulation approach to modelling the structure,thermodynamics and oxygen isotope equilibria of silicates

We use an atomistic model to simulate the structure, lattice dynamics and thermodynamics of silicate minerals. Our approach uses the Born model of a solid, in which the interaction between atoms is described by an interatomic pair potential. We have extended the study of thermodynamics to its very limit by looking at the subtle reaction of oxygen isotope exchange. We have modelled equilibria involving the important metamorphic minerals; albite, diopside, forsterite, pyrope, quartz and wollastonite. The predicted structural and thermodynamic data for these silicates are in very good agreement with the observed values. In addition, we predict not only the correct direction for the phase equilibria for oxygen isotope exchange, but also fractionation factors for the reaction to within a factor of two of the available experimental data. Hence, the potentials used in our approach have shown excellent transferability and have performed very well against the most stringent of tests.     

Modelling the interaction of bentonite with hyperalkaline fluids

Many designs for geological disposal facilities for radioactive and toxic wastes envisage the use of cement together with bentonite clay as engineered barriers. However, there are concerns that the mineralogical composition of the bentonite will not be stable under the hyperalkaline pore fluid conditions (pH > 12) typical of cement and its properties will degrade over long time periods. The possible extent of reaction between bentonite and cement pore fluids was simulated using the reaction-transport model, PRECIP. Key minerals in the bentonite (Na-montmorillonite, analcite, chalcedony, quartz, calcite) were allowed to dissolve and precipitate using kinetic (time-dependent) reaction mechanisms. Simulations were carried out with different model variants investigating the effects of: temperature (25 and 70 °C); cement pore fluid composition; dissolution mechanism of montmorillonite; rates of growth of product minerals; solubilities of product minerals; and aqueous speciation of Si at high pH. Simulations were run for a maximum of 3.2 ka. The results of all simulations showed complex fronts of mineral dissolution and growth, driven by the relative rates of these processes for different minerals. Calcium silicate hydrate (CSH) minerals formed closest to the cement-bentonite boundary, whereas zeolites and sheet silicates formed further away. Some growth of primary bentonite minerals (analcite, chalcedony, calcite and montmorillonite) was observed under certain conditions. Most alteration was associated with the fluid of highest pH, which showed total removal of primary bentonite minerals up to 60 cm from the contact with cement after ∼1 ka. The maximum porosity increase observed was up to 80–90% over a narrow zone 1–2 cm wide, close to the cement pore fluid- bentonite contact. All simulations (except that with alternative aqueous speciation data for Si) showed total filling of porosity a few cms beyond this interface with the cement, which occurred after a maximum of 3.2 ka. Porosity occlusion was principally a function of the growth of CSH minerals such as tobermorite. There was very little difference in the alteration attained using different model variants, suggesting that bentonite alteration was not sensitive to the changes in parameters under the conditions studied, so that transport of pore fluid through the bentonite governed the amount of alteration predicted. Principal remaining uncertainties associated with the modelling relate to assumptions concerning the evolution of surface areas of minerals with time, and the synergy between changing porosity and fluid flow/diffusion.     

REGULARITIES IN THE DISTRIBUTION OF NONFERROUS AND PRECIOUS METALS IN PRINCIPAL ORE MINERALS AND SILICATES OF THE NORIL'SK DEPOSITS

The results of a study of copper, nickel, cobalt, platinum, palladium, gold, and rhodium distribution in basic ore-forming sulfide minerals (chalcopyrite and pyrrhotites) as well as in the silicate portion of the host rocks of the Noril'sk deposit, are given. Interrelations between metals in different sulfide minerals and silicates are characterized. It has been established that palladium, platinum, and gold are concentrated in chalcopyrite while cobalt and rhodium are concentrated in pyrrhotite. The cobalt and precious metal content in sulfides grows with the increase of their nickel content. The relative content, however, of all these elements estimated for 1% of nickel is different in chalcopyrites and pyrrhotites. The former show a high platinum, palladium, and gold content and the latter--cobalt and rhodium. Ratios of these elements in sulfides and silicates are different. The relative platinum content increases in silicates due to a considerably greater content of mineralized platinum. Both nickel and cobalt form silicate compounds the latter to a much greater extent than the former. The relative cobalt content is therefore higher in silicates than in sulfides. In descending order, the element concentrations in sulfides range as follows: copper, nickel, palladium, gold, cobalt, platinum. — Auth. English Summ.     

Interactions between <Emphasis Type="Italic">Bacillus mucilaginosus</Emphasis> and silicate minerals (weathered adamellite and feldspar): Weathering rate,products, and reaction mechanisms

Bacillus mucilaginosus is a common soil bacterium,and usually used as a model bacterium in studying microbe-mineral interactions.Several reaction mechanisms of B.mucilaginosus weathering silicate minerals were proposed.However,the molecule mechanisms and detailed processes were still unclear.In this paper,bacterium-mineral interactions were studied in terms of variations in pH value over the experimental period,variations in mineral composition,weathering rates of silicate minerals and volatile metabolites in the culture medium,etc.,to further explore the bacterium-mineral interaction mechanisms.The results showed that B.mucilaginosus could enhance silicate mineral weathering obviously.The weathering rates were quite different for various kinds of silicate minerals,and the weathering rate of weathered adamellite could reach 150 mg/m2/d.Although B.mucilaginosus produced little acidic substance,pH in the microenvironment of bacterium-mineral complex might be far lower than that of the circumjacent environment;a large amount of acetic acid was found in the metabolites,and was likely to play an important role as a ligand.These results appear to suggest that acidolysis and ligand degradation are the main mechanisms of B.mucilaginosus dissolving silicate minerals,the formation of bacterium-mineral complexes is the necessary condition for the bacteria weathering silicate minerals,and extracelluar polysaccharides played important roles in bacterium-mineral interaction processes by forming bacterium-mineral complexes and maintaining the spe-cial physicochemical properties of microenvironment.     

Experimental study of acidity-consuming processes in mining waste rock: some influences of mineralogy and particle size

Weathering reactions producing and consuming acid in fresh waste rock samples from the Aitik Cu mine in northern Sweden have been investigated. Batch-scale (0.15 kg) acid titrations with waste rock of different particle sizes were operated for 5 months. The pH was adjusted to a nearly constant level, similar to that in effluents from waste rock dumps at the site (pH near 3.5). The reactions were followed by analysing for all major dissolved elements (K, Na, Mg, Ca, Si, Al, SO₄, Cu, Zn, Fe) in aliquots of solution from the reaction vessels. In addition, the solids were physically and chemically characterised in terms of mineralogy, chemical composition, particle size distribution, surface area and porosity. The results show that the alkalinity production is initially dominated by a rapid dissolution of small amounts of calcite and rapidly exchangeable base cations on silicate surfaces. Steady-state dissolution of primary silicate minerals also generates alkalinity. The total alkalinity is nearly balanced by input of acid from the steady-state oxidation of sulphides, such that the pH 3.1–3.4 can be maintained without external input of acid or base. There is a large difference in weathering rates between fine materials and larger waste rock particles (diameters (d) >0.25 mm) for both sulphides and silicates. As a result particles with d smaller than 0.25 mm contribute to approximately 80% of the sulphide and silicate dissolution. Calcite dissolution can initially maintain a neutral pH but with time becomes limited by intra-particle diffusion. Calcite within particles larger than 5–10 mm reacts too slowly to neutralise the acid produced from sulphides.     

Calculated silica activities in carbonatite liquids

Carbonatite magmas precipitate silicates, in addition to the abundant carbonates, oxides, and phosphates. Calculated silica activities for equilibria involving silicates and a silica component in magmatic liquids predict specific assemblages for silicate and oxide phases in carbonatites. These assemblages provide tests of alternative sources (carbonatite magma, coeval silicate magma, or older rock) for silicate minerals in carbonatites. Quartz, feldspars, and orthopyroxene are unlikely to be primary magmatic phases in carbonatites, because the silica activity in carbonatite magmas is too low to stabilize these minerals. Zircon and titanite should be unstable relative to baddeleyite and perovskite, respectively, but they do occur in carbonatites. Liquids dominated by carbonate are strongly nonideal with respect to dissolved silica. Consequently, activity coefficients for a silica component in carbonatite liquids are >>1, so that small mole fractions of SiO₂ translate into silica activities sufficient to stabilize phlogopite, clinopyroxene, amphibole, monticellite, and forsterite, among other silicates. Examination of silicate mineral assemblages in carbonatites in the light of silica activity indicates that many carbonatites are contaminated by solid silicate phases from external sources but these xenocrysts can be discriminated from magmatic minerals.     

地幔内超高压矿物的爆炸

在西藏雅鲁藏布江缝合带的蛇绿岩的豆荚状铬铁矿中,首次发现硅酸盐包裹体具有爆炸结构。在矿石薄片中普遍见到这种高能硅酸盐包体的爆炸结构。在爆炸结构中,铬尖晶石被炸落成不规律的显微碎块,具有尖锐棱角,碎块大小不一,堆积在硅酸盐包体内。此外,许多张性显微裂隙围绕爆炸的硅酸盐包体呈蛛网状分布。推断爆炸前的原生矿物是含水的镁硅酸盐和镁铝硅酸盐矿物。当这种高能级的超高压含水矿物受地幔柱带动超过“水线”时发生爆炸和放水作用,同时产生地震。     

Geochemical modeling of arsenic sulfide oxidation kinetics in a mining environment

Arsenic sulfide (AsS (am), As₂S₃ (am), orpiment, and realgar) oxidation rates increase with increasing pH values. The rates of arsenic sulfide oxidation at higher pH values relative to those at pH∼2 are in the range of 26-4478, 3-17, 8-182, and 4-10 times for As₂S₃ (am), orpiment, AsS (am), and realgar, respectively.Numerical simulations of orpiment and realgar oxidation kinetics were conducted using the geochemical reaction path code EQ3/6 to evaluate the effects of variable DO concentrations and mineral reactivity factors on water chemistry evolution during orpiment and realgar oxidation. The results show that total As concentrations increase by ∼1.14 to 13 times and that pH values decrease by ∼0.6 to 4.2 U over a range of mineral reactivity factors from 1% to 50% after 2000 days (5.5 yr). The As release from orpiment and realgar oxidation exceeds the current U.S. National Drinking Water Standard (0.05 ppm) approximately in 200-300 days at the lowest initial dissolved oxygen concentration (3 ppm) and a reactivity factor of 1%. The results of simulations of orpiment oxidation in the presence of albite and calcite show that calcite can act as an effective buffer to the acid water produced from orpiment oxidation within relatively short periods (days/months), but the release of As continues to increase.Pyrite oxidation rates are faster than orpiment and realgar from pH 2.3 to 8; however, pyrite oxidation rates are slower than As₂S₃ (am) and AsS (am) at pH 8. The activation energies of arsenic sulfide oxidation range from 16 to 124 kJ/mol at pH∼8 and temperature 25 to 40°C, and pyrite activation energies are ∼52 to 88 kJ/mol, depending on pH and temperature range. The magnitude of activation energies for both pyrite and arsenic sulfide solids indicates that the oxidation of these minerals is dominated by surface reactions, except for As₂S₃ (am). Low activation energies of As₂S₃ (am) indicate that diffusion may be rate controlling.Limestone is commonly mixed with sulfide minerals in a mining environment to prevent acid water formation. However, the oxidation rates of arsenic sulfides increase as solution pH rises and result in a greater release of As. Furthermore, the lifetimes of carbonate minerals (i.e., calcite, aragonite, and dolomite) are much shorter than those of arsenic sulfide and silicate minerals. Thus, within a geologic frame time, carbonate minerals may not be present to act as a pH buffer for acid mine waters. Additionally, the presence of silicate minerals such as pyroxenes (wollastonite, jadeite, and spodumene) and Ca-feldspars (labradorite, anorthite, and nepheline) may not be important for buffering acid solutions because these minerals dissolve faster than and have shorter lifetimes than sulfide minerals. However, other silicate minerals such as Na and K-feldspars (albite, sanidine, and microcline), quartz, pyroxenes (augite, enstatite, diopsite, and MnSiO₃) that have much longer lifetimes than arsenic sulfide minerals may be present in a system. The results of our modeling of arsenic sulfide mineral oxidation show that these minerals potentially can release significant concentrations of dissolved As to natural waters, and the factors and mechanisms involved in arsenic sulfide oxidation warrant further study.     

Crystallization conditions of the Wiborg rapakivi batholith, SE Finland: an evaluation of amphibole and biotite mineral chemistry

Summary The wiborgite and dark wiborgite rapakivi granite phases of the Wiborg batholith in southeastern Finland compose about 80% of the total batholith area. A new study of the dominant mafic silicate minerals, in comparison with mafic silicates from more evolved granite phases, hybridized granite and mafic magmatic enclaves provide insights into the overall petrogenesis of the Wiborg batholith. All of the mafic silicate minerals are iron-rich, reflective of the whole rock compositions. Biotite is annitic, calcic amphibole is ferro-edenite to hastingsite, and subsolidus Fe-Mg amphibole is found as accessory grunerite. Temperatures derived from amphibole-plagioclase thermometry suggest crystallization at about ∼ 740 °C. Pressure estimates derived from Al in amphibole barometry range between 2.5 and 5.4 kilobars. This is noticeably higher than the previous estimates of 1 kbar for the Wiborg batholith. Oxygen fugacity estimates from biotite suggest low fO₂ initial values and increase from FMQ to above NNO for late stage granite phases. Received February 29, 2000; revised version accepted December 27, 2000     

硅酸盐中非沸石类孔道结构矿物及其环境属性

对硅酸盐中非沸石类矿物进行的晶体结构特征分析发现,某些链层状硅酸盐矿物(如坡缕石和海泡石)、钛硅酸盐中的硅钛铌钠石、硅钛铌钠矿、钙霞石、方钠石等,以及锰、锆、钒、钇硅酸盐矿物中的水硅钡锰石、堇青石、水钠锆石、水硅钒钙石等矿物中都存在有孔道结构,其孔道直径为0.25~1.0 nm。本文综述了这些孔道结构矿物吸附和交换有害重金属离子、催化降解有机污染物的环境功能和属性。     

含硫化物镁铁质-超镁铁质侵入体中橄榄石Mg、Ni含量的制约:原理、模式及Voisey's Bay侵入体样品研究

镁铁质-超镁铁质岩浆结晶分离早期形成镁铁矿物,镁铁矿物中的Ni和Mg是相容元素。随着结晶分离作用的进行,Ni、Mg在硅酸盐岩浆及后形成的硅酸盐物质中的丰度下降。橄榄石中Ni含量及硅酸盐物质MgO/FeO比值都与母岩浆的相关值相关,据此可推断母岩浆的信息,它们之间可由实验测得的系数相联系。当岩浆饱和硫化物时,在结晶分离过程中硫化物珠滴会与镁铁硅酸盐物质一道析出,同时,与硫化物非饱和岩浆相比,过多的Ni会随之析出。这也反映在Ni、Mg含量比无硫化物分离时有更迅速的降低上。Ni、Mg含量变化值可以在VoiseysBay侵入体的模式曲线上反映出,加拿大Labrador的这一侵入体赋存了一个世界级的Ni-Cu-Co硫化物矿床。过去的作法是将侵入体中橄榄石的Ni、Mg含量与Simkin和Smith得出的各种火成岩中橄榄石的Ni、Mg含量相比较以确定Ni亏损,进而假定橄榄石来自硫化物饱和、有经济价值的岩浆。现在的研究显示这种简单的对比会导致错误。将样品数据与模式曲线对比并反映出侵入体矿物结晶堆积特征是重要的方法。使用这一方法,样品数据能很好地被模式曲线拟合。以在VoiseysBay的研究为例,当硫化物液相与硅酸盐矿物被去除后,硫化物非饱和的分离作用期就会显现出来,随后是硅酸盐结晶作用期。     

Late-stage magmatic to deuteric/metasomatic accessory minerals from the Cerro Boggiani agpaitic complex (Alto Paraguay Alkaline Province)

This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.     

粤北下庄铀矿田中铀酰矿物的成分特征——对核废料处置库中UO2氧化行为的启示

下庄铀矿为一花岗岩型铀矿，矿田地处湿热气候条件下，沥青铀矿普遍产于破碎带中，这种特定的产出环境致使该区沥青铀矿经受了强烈的风化，形成种类繁多、数量丰富的铀酰矿物；而我国高放废物地质处置库拟建在花岗岩体中。因此，下庄铀矿田是开展核废料氧化的天然类比研究的理想地区，并对我国的高放废物地质处置库的安全性评价有重要的指导作用。下庄铀矿田的铀酰矿物组合为铀酰氢氧化物、铀酰硅酸盐和铀酰磷酸盐，包括柱铀矿、黄钙铀矿、calciouranoite、红铀矿、富硅铀酰相、硅钙铀矿、钙铀云母和盈江铀矿等。根据它们的空间分布特征可划分成两个风化系列，即硅酸盐风化系列和磷酸盐风化系列，其共生次序分别为：沥青铀矿→铀酰氢氧化物（氧化物）→富硅铀酰相→硅钙铀矿和沥青铀矿→铀酰氢氧化物（氧化物）→钙铀云母→盈江铀矿。在该矿田中，铀酰氢氧化物是亚稳定相矿物，常常被铀酰硅酸盐或磷酸盐取代，因此，铀酰氢氧化物仅出现在少数样品中，而铀酰硅酸盐和铀酰磷酸盐矿物则非常普遍。矿田中的铀酰矿物在化学成分上以富钙为其显著特征，由于核废料地质处置库近场地下水中的Ca^2＋含量应该明显比下庄矿田地下水中的高，因此，我们预测含Ca的铀酰硅酸盐和铀酰磷酸盐矿物等热力学上的稳定物相很可能是地质处置系统中最主要的铀酰矿物，处置库内放射性核素的迁移主要是由这些矿物控制的。     

Silicate and carbonate mineral weathering in soil profiles developed on Pleistocene glacial drift (Michigan, USA): Mass balances based on soil water geochemistry

Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO₂. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg²⁺/Ca²⁺ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca²⁺ and Mg²⁺ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg²⁺ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry.     

The effect of malonate on the dissolution kinetics of albite,quartz, and microcline as a function of pH at 70°C

The effect of malonate on the dissolution rate of the reservoir minerals: albite, quartz, and microcline has been examined. The effect of pH on rate has been decoupled from the effect of the ligand by controlling pH using dilute buffer solutions at values of pH 4 through pH 10 at 70°C. Any effect of the buffer ligands (acetate and borate) or alkali cation (Na) on rate was accounted for by comparing the measured dissolution rates in buffer solutions with and without malonate. Flowing reactors were used and malonate concentration was varied from 1 to 100 mmolal.As malonate anion concentration is increased, the dissolution rate of these minerals also increases. At moderately acidic to near neutral pH in the solutions containing the highest malonate concentration, malonate most significantly increases the dissolution rate of the silicates over that expected due to simple hydrolysis. At alkaline pH the effect of malonate on the dissolution rate of these silicates is not significant, compared to the dissolution rate due to simple hydrolysis alone.     

Apophyllite (001) surface alteration in aqueous solutions studied by HAFM

Depending on pH and temperature, two different types of surface reactions occur on the apophyllite (001) surface in aqueous HCl-solutions at temperatures from 20 to 130 °C. At low pH, laterally spreading hillocks cover the surface. The hillocks are softer than the pristine surface, chemical analysis shows a depletion in Ca + K, and the spreading velocity of hillocks depends on pH. This indicates a change in chemical bond strength, non-stoichiometric dissolution and a mechanism involving protons. External disturbances such as the AFM scanning tip cause the upper surface layers to peel off revealing that the active sites of hillock formation are between the silicate layers of apophyllite. The observed process can therefore be described by a penetrative ion-replacement reaction which proceeds well below the surface monolayer. By this ion-replacement, the silicate layers eventually become destabilised. The observed reaction, therefore, is equivalent to an incongruent dissolution process. Despite structural similarities, this process is only superficially similar to the ion-exchange occurring in clay minerals or zeolites. In these minerals, the structural backbone is not destabilized. At a more neutral pH and high temperatures, step retreat and etch pit formation can be observed on the apophyllite (001) surface thus indicating a more congruent dissolution mechanism.     

Sorption of silica by clay minerals

Clay minerals were reacted with silica-spiked solutions of unbuffered distilled water; water buffered at pH 5.5, 8 and 10; alkali chloride solutions; natural and artificial sea water to assess the influence of pH, silica and cation activities. The data are plotted as silica produced by dissolution or sorption of silica by clay surface as a function of initial silica concentration at a given pH and solution composition. This allows the determination of the dissolved silica value at which the clay mineral surface neither dissolves nor sorbs silica. The values of the various activities in different solutions are used to infer the phase equilibria between solution, clay mineral and the surface phase produced either by dissolution or sorption. Most intensively investigated were sorption reactions of kaolinite in sea water and other ionic solutions to form silica-rich, cation-rich surface phases in cationic solutions and silica-rich phases in cation-free solutions.Inferred equilibrium constants imply that silicate reconstitution is doubtful as a mechanism for partial control of silica and cation composition of sea water but is reasonable in silica-rich interstitial waters.     

Rock alteration in alkaline cement waters over 15 years and its relevance to the geological disposal of nuclear waste

The interaction of groundwater with cement in a geological disposal facility (GDF) for intermediate level radioactive waste will produce a high pH leachate plume. Such a plume may alter the physical and chemical properties of the GDF host rock. However, the geochemical and mineralogical processes which may occur in such systems over timescales relevant for geological disposal remain unclear. This study has extended the timescale for laboratory experiments and shown that, after 15 years two distinct phases of reaction may occur during alteration of a dolomite-rich rock at high pH. In these experiments the dissolution of primary silicate minerals and the formation of secondary calcium silicate hydrate (C–S–H) phases containing varying amounts of aluminium and potassium (C–(A)–(K)–S–H) during the early stages of reaction (up to 15 months) have been superseded as the systems have evolved. After 15 years significant dedolomitisation (MgCa(CO₃)₂ + 2OH⁻ → Mg(OH)₂ + CaCO₃ + CO₃²⁻_(aq)) has led to the formation of magnesium silicates, such as saponite and talc, containing variable amounts of aluminium and potassium (Mg–(Al)–(K)–silicates), and calcite at the expense of the early-formed C–(A)–(K)–S–H phases. This occured in high pH solutions representative of two different periods of cement leachate evolution with little difference in the alteration processes in either a KOH and NaOH or a Ca(OH)₂ dominated solution but a greater extent of alteration in the higher pH KOH/NaOH leachate. The high pH alteration of the rock over 15 years also increased the rock’s sorption capacity for U(VI). The results of this study provide a detailed insight into the longer term reactions occurring during the interaction of cement leachate and dolomite-rich rock in the geosphere. These processes have the potential to impact on radionuclide transport from a geodisposal facility and are therefore important in underpinning any safety case for geological disposal.     

The effect of crystallinity on dissolution rates and CO2 consumption capacity of silicates

Comparison of measured far-from-equilibrium dissolution rates of natural glasses and silicate minerals at 25 °C and pH 4 reveals the systematic effects of crystallinity and elemental composition on these rates. Rates for both minerals and glasses decrease with increasing Si:O ratio, but glass dissolution rates are faster than corresponding mineral rates. The difference between glass and mineral dissolution rates increases with increasing Si:O ratio; ultra-mafic glasses (Si:O ? 0.28) dissolve at similar rates as correspondingly compositioned minerals, but Si-rich glasses such as rhyolite (Si:O ∼ 0.40) dissolve ?1.6 orders of magnitude faster than corresponding minerals. This behaviour is interpreted to stem from the effect of Si-O polymerisation on silicate dissolution rates. The rate controlling step of dissolution for silicate minerals and glasses for which Si:O > 0.28 is the breaking of Si-O bonds. Owing to rapid quenching, natural glasses will exhibit less polymerisation and less ordering of Si-O bonds than minerals, making them less resistant to dissolution. Dissolution rates summarized in this study are used to determine the Ca release rates of natural rocks at far-from-equilibrium conditions, which in turn are used to estimate their CO₂ consumption capacity. Results indicate that Ca release rates for glasses are faster than those of corresponding rocks. This difference is, however, significantly less than the corresponding difference between glass and mineral bulk dissolution rates. This is due to the presence of Ca in relatively reactive minerals. In both cases, Ca release rates increase by ∼two orders of magnitude from high to low Si:O ratios (e.g., from granite to gabbro or from rhyolitic to basaltic glass), illustrating the important role of Si-poor silicates in the long-term global CO₂ cycle.     

Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ∼ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O = U = O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm⁻¹ for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.