Thermodynamics and crystal chemistry of the hematite–corundum solid solution and the FeAlO3 phase

High-temperature oxide-melt calorimetry and Rietveld refinement of powder X-ray diffraction patterns were used to investigate the energetics and structure of the hematite–corundum solid solution and ternary phase FeAlO₃ (with FeGaO₃ structure). The mixing enthalpies in the solid solution can be described by a polynomial ΔH_mix=WX _hem(1?X _hem) with W=116 ± 10 kJ mol^?1. The excess mixing enthalpies are too positive to reproduce the experimental phase diagram, and excess entropies in the solid solution should be considered. The hematite–corundum solvus can be approximately reproduced by a symmetric, regular-like solution model with ΔG _excess=(W _H ?TW _S )X _hem X _cor, where W _H= 116 ± 10 kJ mol^?1 and W _S =32 ± 4 J mol^?1 K^?1. In this model, short-range order (SRO) of Fe/Al is neglected because SRO probably becomes important only at intermediate compositions close to Fe:Al=1:1 but these compositions cannot be synthesized. The volume of mixing is positive for Al-hematite but almost ideal for Fe-corundum. Moreover, the degree of deviation from Vegard's law for Al-hematite depends on the history of the samples. Introduction of Al into the hematite structure causes varying distortion of the hexagonal network of oxygen ions while the position of the metal ions remains intact. Distortion of the hexagonal network of oxygen ions attains a minimum at the composition (Fe_0.95Al_0.05)₂O₃. The enthalpy of formation of FeAlO₃ from oxides at 298 K is 27.9 ± 1.8 kJ mol^?1. Its estimated standard entropy (including configurational entropy due to disorder of Fe/Al) is 98.9 J mol^?1 K^?1, giving the standard free energy of formation at 298 K from oxides and elements as +19.1 ± 1.8 and ?1144.2 ± 2.0 kJ mol^?1, respectively. The heat capacity of FeAlO₃ is approximated as C _p (T in K)= 175.8 ? 0.002472T ? (1.958 × 10⁶)/T ²? 917.3/T ^0.5+(7.546 × 10^?6) T ² between 298 and 1550 K, based on differential scanning calorimetric measurements. No ferrous iron was detected in FeAlO₃ by Mössbauer spectroscopy. The ternary phase is entropy stabilized and is predicted to be stable above about 1730 ± 70 K, in good agreement with the experiment. Static lattice calculations show that the LiNbO₃-, FeGaO₃-, FeTiO₃-, and disordered corundum-like FeAlO₃ structures are less stable (in the order in which they are listed) than a mechanical mixture of corundum and hematite. At high temperatures, the FeGaO₃-like structure is favored by its entropy, and its stability field appears on the phase diagram.     

The chemistry of pyrite formation in aqueous solution and its relation to the depositional environment

Experimental investigations on pyrite synthesis indicate that before pyrite can be produced by a reaction involving ferrous iron, the disulphide ion must be formed; in experiments described the ion was obtained by the action of H₂S in aqueous solution on elemental sulphur. Conditions under which the experiments were conducted indicate that pyrite will not form above pH 6.0. The reaction to produce pyrite is fastest when oxygen is excluded and elemental sulphur is produced from the oxidation of H₂S by ferric iron. A reaction between FeS and elemental sulphur will yield pyrite at a much slower rate, although the same basic reaction is involved. An attempt has been made to relate the occurrence of pyrite in different sedimentary environments to this basic chemistry.

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Zusammenfassung Wie Versuche zeigen, ist die Voraussetzung der Pyrit-Bildung das Vorliegen von S ₂ ^2– -Ionen, die dann mit Fe^II reagieren. Die S ₂ ^2– -Ionen wurden durch Einwirken einer verdünnten H₂S-Lösung auf elementaren Schwefel erhalten. Pyrite entstehen in diesen Experimenten somit nur unterhalb pH 6. Pyrit erhält man am schnellsten, wenn Sauerstoff abwesend ist und der H₂S durch Fe^III oxidiert wird. Die Umsetzung von FeS mit elementarem Schwefel liefert Pyrit wesentlich langsamer, wenn auch die zugrunde liegenden Reaktionen sich entsprechen. Es wird versucht, sedimentäre Pyrit-Vorkommen entsprechend diesen Reaktionsabläufen zu deuten.

     

柴达木盆地早始新世大暖期古土壤中赤铁矿和针铁矿的定量分析及其气候意义

赤铁矿和针铁矿是古土壤中常见的两种风化产物,定量研究赤铁矿和针铁矿有利于了解成壤时期古气候特征。文章利用可见光波段的漫反射光谱法对早始新世大暖期期间柴达木盆地大红沟剖面路乐河组古土壤中的铁氧化物矿物进行了定量分析。测试结果清晰显示了赤铁矿和针铁矿的存在,其中赤铁矿含量约在0.09~17.16 g/kg之间,针铁矿的含量大致为0.13~24.6 g/kg;针铁矿（Gt）相对赤铁矿（Hm）平均含量较高（Gt/Hm变化范围0.04~34.34,平均值约为2.18）,且数据存在较大波动。古土壤在早始新世大暖期时经历了3个阶段的古气候变化,早期古降雨量偏高,针铁矿/赤铁矿平均值为3.06,数据波动明显,指示相对温暖湿润且干湿波动大的气候条件;中期时古降雨量稍低,针铁矿/赤铁矿平均值降低至1.75,指示相对干旱的气候条件;晚期时古降雨量增加,针铁矿/赤铁矿平均值升高至1.80,指示相对温暖湿润的气候条件。其中早期的路乐河组古土壤对应了全球早始新世气候大暖期的峰值,路乐河组古土壤整体的古气候特征是对全球早始新世气候大暖期的区域响应。

     

Tourmalines from the siderite-quartz-sulphide hydrothermal veins,Gemeric unit,western Carpathians,Slovakia: crystal chemistry and evolution

Mineralogy and Petrology - Tourmaline is an important gangue mineral in a large number of Cretaceous siderite-quartz-sulphide hydrothermal veins in the Gemeric Unit, Slovak Ore Mountains, Slovakia,...     

Garnet crystal plasticity in the continental crust,new example from south Madagascar

Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ～300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (10¹⁰ m^?3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.     

Fluorapatite surface composition in aqueous solution deduced from potentiometric, electrokinetic, and solubility measurements, and spectroscopic observations

The surface chemistry of fluorapatite in aqueous solution was investigated using electrokinetic techniques, potentiometric titrations, solubility measurements, and attenuated total reflection infrared spectroscopy. All methods indicate the formation of Ca/F depleted, P enriched altered layer via exchange reactions between H⁺ and Ca²⁺, and OH⁻ and F⁻ at the fluorapatite (FAP) surface. Observations suggest that this leached layer has a di-calcium phosphate (CaHPO₄) composition and that it controls the apparent solubility of FAP. Electrokinetic measurements yield an iso-electric point value of 1 ± 0.5 consistent with a negatively charged FAP surface at pH > 1. In contrast, surface titrations give an apparent pH of point of zero charge of ∼7.7, consistent with a positively charged surface at pH < 7.7. These differences are shown to stem from proton consumption by both proton exchange and dissolution reactions at the FAP surface. After taking account for these effects, FAP surface charge is shown to be negative to at least pH 4 by surface titration analysis.     

Temporal variations in the chemistry of circum-neutral drainage from the 10-Level portal,Myra Mine,Vancouver Island,British Columbia

The Myra mine, now inactive, produced Zn and Cu concentrates from a Zn-rich, Kuroko-type, volcanogenic massive sulfide deposit located in the mountainous interior of Vancouver Island. The climate at the site is classified as “Marine West Coast”, with annual precipitation exceeding 2200 mm. Water from a losing stream on the mountainside above the mine follows preferential, fracture-controlled pathways to the upper workings before draining through the 10-Level portal. With a view toward mine decommissioning, portal discharge rate was monitored continuously over a 17-month period during which 46 water samples were collected. Effluent chemistry, dominated by Ca, HCO₃ and SO₄, shows moderate to high total base metal concentrations and near-neutral pH. Carbonatization, mainly of mafic rocks in the hangingwall, provides significant acid neutralizing potential. Metal concentrations vary seasonally, with smaller spikes associated with summer storm events, and a main peak associated with flushing of the workings during the first heavy autumn rains. Aqueous speciation modeling suggests that Fe and Al concentrations are controlled by the solubilities of hydrous ferric oxides and microcrystalline gibbsite, respectively. Concentrations of Zn, Cu and Cd appear controlled by sorption rather than by the solubilities of mineral phases. A comparison of precipitate concentrations observed in portal effluent with predictions from mass balance (inverse) modeling results suggests that less than 5% of the precipitated Fe and Al hydroxides are transported from the mine. However, amounts of sorbed Cu, Zn and Cd measured in the effluent are only slightly lower than modeled values. This suggests that the small fraction of (probably finer) Fe precipitates in portal effluent sorbs most of the Zn, Cu and Cd predicted by modeling. Based on mass balance calculations, metal loadings are explained by the oxidation of 3830 kg of pyrite, 600 kg of sphalerite and 190 kg of chalcopyrite, annually. Circum-neutral drainage conditions are maintained by the reaction of almost 19,800 kg of calcite, annually.     

Nitrogen-bearing,aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides

Minerals in eclogites from different localities in the Western Gneiss Region of the Norwegian Caledonides (age 425 Ma) contain a variety of fluid inclusions. The earliest inclusions recognized are contained in undeformed quartz grains, protected by garnet, and consist of H₂O+N₂ (with ). The reconstructed P-V-T-X properties of these fluid inclusions are compatible with peak or early-retrograde metamorphic conditions. Matrix minerals (quartz, garnet, apatite, plagioclase) contain a complex pattern of mostly truly secondary inclusions, dominated by CO₂ and N₂. The textural patterns and P-V-T-X properties of these inclusions are incompatible with the high pressures of the eclogite-forming metamorphic event, but suggest that they were formed during uplift, by a combination of remobilization of preexisting inclusions and influx of external fluids. The fluid introduced at a late stage was dominated by CO₂, and did not contain N₂. The present data agree with theoretical predictions of eclogite fluids from mineral equilibria, and highlight the differences between granulite (CO₂) and eclogite (H₂O+N₂) fluid regimes. The provenance of the nitrogen in the eclogite fluid inclusions represents an important, but unsolved question in the petrology of high-pressure metamorphic rocks.Contribution no. 68 to the Norwegian programme of the International Lithosphere Project     

铜陵叶山铁矿赤铁矿微尺度矿物学研究及地质意义

铜陵隆起区内广泛分布着铜-铁-金多金属矿床,也存在众多小型铁矿,其中叶山铁矿是典型代表之一。矿体主要产于石炭系黄龙组内,具有层控特征,铁矿化以镜铁矿/赤铁矿为主。为了深入探讨该铁矿的形成机制及与沉积环境的关系,本文对该矿中不同产状的赤铁矿进行了详细野外地质观察和显微结构分析,并将赤铁矿划分为3类(Hem 1、Hem 2、Hem 3)。激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)分析表明Hem 1具有高Si、Al、Ti的特征,微量元素V、Cr、Co、Ni、Cu、Zn、Sr、Ba、Sn、Pb含量相对较高,Co/Ni比值小于1。Hem 1酸不溶物的扫描电镜(SEM)观察显示石英粒径为2～600μm,表面有风成的断面、孔洞、长条状"V"型凹痕等微结构和形貌,同时粘附有纳米硅质小球,表明石英为潮间带环境下的碎屑。以上结果表明该区黄龙组地层存在沉积赤铁矿型铁质建造,发生重结晶作用。Hem 2和Hem 3在主量元素上以低Ti含量,Si、Al、Mn含量变化大为特征;化学组成及元素相关性图解表明Hem 2、Hem 3并非单一成因,可分为两类:高W,低Mn、Si、Co/Ni比值(1. 5)为沉积-热液叠加改造成因(Hem 2_(SH)、Hem 3_(SH));低W,高Mn、Si、Co/Ni比值( 1. 5)为热液成因(Hem 2_H、Hem 3_H)。因此,微量元素含量上,Hem 2_(SH)与其共生磁铁矿差异大,Hem 2_H与其共生磁铁矿差异小; Hem 2_(SH)/(Hem 3_(SH))相对Hem 2_H/(Hem 3_H)显示亏损(W元素除外); Hem 2_(SH)、Hem 3_(SH)、Hem 2_H和Hem 3_H各自之间的波动变化,反映了热液改造过程元素的继承性和热液组分演化过程及其对前驱体的叠加影响,但元素相关性图解排除了Hem 1作为Hem 2_(SH)、Hem 3_(SH)前驱体的可能。综上分析结果表明:叶山铁矿可能属于沉积-热液叠加改造型矿床,黄龙组地层存在以菱铁矿为主的沉积铁建造并提供了该矿床的主要成矿物质,受到燕山期岩浆活动驱动的热液流体叠加改造。赤铁矿的多成因特征不仅为认识叶山铁矿的富集过程提供了重要依据,同时也为进一步了解铜陵地区热液改造型铁矿与黄龙组同生沉积环境之间的关系提供了重要启示。     

A critical evaluation of the thermodynamic data for boron ions,ion pairs,complexes, and polyanions in aqueous solution at 298.15 K and 1 bar

Energy related industrial development, municipal waste treatment processes and some natural sources, contribute to B (being present) in the environment in elevated concentrations. Boron forms numerous aqueous ion pairs and complexes, some of which would not normally exist in significant concentrations under typical baseline environmental conditions. A complete understanding of the aqueous chemistry of B depends on an accurate evaluation of the thermodynamic data for these species.Equilibrium constants for 29 inorganic complexes, ion pairs, and polyanions of B have been critically evaluated. Apparent equilibrium constants have been extrapolated to zero ionic strength, and the selected thermodynamic values are tabulated. Gibbs free energies of formation for the aqueous species have been computed; these are presented with entropy and enthalpy values where available     

Competing effects of crystal chemistry and silicate melt composition on trace element behavior in magmatic systems: insights from crystal/silicate melt partitioning of the REE,HFSE, Sn,In, Ga,Ba, Pt and Rh

We present new partition coefficients for the REE, HFSE, Sn, In, Ga, Ba, Pt and Rh between clinopyroxene, olivine and basaltic melt as a function of crystal chemistry and melt composition at temperatures of 1190–1300 °C and 1-bar pressure. Two components, namely \(\mathrm {Al_2O_3}\) and \(\mathrm {Na_2O}\), were chosen to be investigated since they are known to affect the structure of silicate melts and especially clinopyroxene crystal chemistry. The amount of \(^{[4]}\mathrm{Al}\) in clinopyroxene will result in an increase of \(D_i^\mathrm{{cpx/melt}}\) even after applying a correction factor to account for the effect of melt polymerization. Moreover, the positive correlation between \(^{[4]}\mathrm{Al}\) and \(D_i^\mathrm{{cpx/melt}}\) is not restricted to the REE, but also applies for Sn, Ga, In, and Ba. The addition of up to 2.6 wt% \(\mathrm {Na_2O}\) to the silicate melt universally increases the \(D_i^\mathrm{{cpx/melt}}\) without any concomitant change in crystal chemistry or a significant effect in melt polymerization. This compositional effect is likely due to the ability of Na to break REE–Al complexes in the melt. Our results emphasize the importance of considering all variables that affect the behavior of trace elements in magmatic systems before applying the lattice strain model and derive meaningful results for the changes in the parameters of the crystallographic sites.     

Composition of brines in halite‐hosted fluid inclusions in the Upper Ordovician,Canning Basin,Western Australia: new data on seawater chemistry

Analyses of primary and early diagenetic fluid inclusions in the halite from the Late Ordovician Mallowa Salt, Canning Basin, Western Australia indicate a Ca‐rich composition and high concentration of parent brines in the basin which were close to sylvite and carnallite precipitation. The salt‐bearing series in the sampled interval was overheated up to 62 °C. The recorded differences in gas compositions result from the input of several gas sources including dispersed organic matter in the salt series and hydrocarbon deposits in the underlying rocks. The high concentration of the brines in fluid inclusions does not allow quantitative reconstruction of the chemical composition of Late Ordovician parent seawater. Using the information from Early Cambrian and Late Silurian basins as a proxy, however, the new data indicate that Late Ordovician seawater was undoubtedly Ca‐rich and, in comparison with modern seawater, had a similar K content, considerably lower Mg content (c. 30%), approximately three times the Ca content and one‐third the SO₄ content.     

Temporal variation in discharge chemistry and portal flow from the 8-Level adit, Lynx Mine, Myra Falls Operations, Vancouver Island, British Columbia

The Lynx mine, currently inactive, has produced copper and zinc concentrates from massive sulfide deposits on a lease within the rainy, mountainous interior of Vancouver Island. Tailings, used to back-fill a mined-out stope, are being leached by percolating groundwater and the resulting acidic, metal-laden drainage is discharging from the portal of the 8-Level adit. Temporal variations in the flow rate, specific conductance and temperature of the discharge were monitored continuously over a 2-year period while effluent chemistry was sampled weekly. Conductivity was relatively constant throughout most of the year but peaked with the first autumn storm events as accumulated soluble sulfide oxidation products were flushed from the workings. Concentrations of sulfate and most metals were closely correlated with conductivity as were low pH values as stored acidity was released along with dissolved species. Variations in pH controlled the speciation and partitioning of metals between dissolved and particulate phases.     

Effect of contact time,pH, and ionic strength on Cd(II) adsorption from aqueous solution onto bentonite from Gaomiaozi,China

Copper smelting and toxic emissions in Sarcheshmeh Copper Complex have resulted in soil pollution especially in the vicinity of the smelting plant. Calculated geoaccumulation index, contamination factor (C _f), and contamination degree (C _deg) indicate surface soil enrichment in potentially toxic metals (As, Cu, Pb, Zn, Mo, and Cd). The results also indicate that most contaminated areas are located in the prevailing wind directions (N and NE). However, continuous copper smelting can result in extensive pollution in the study area. This is especially alarming for adjacent townships. Since, the sampled sites are also used as grazing land, the soils are likely to become phytotoxic and provide a potential pathway for the toxic elements to enter the food chain. C _f based on distance and direction give more reasonable results; that is, the decrease of contamination degree with distance. This is in agreement with I _geo and also statistical analysis, which show a decreasing trend of metal loadings of soil with distance from the smelter. Statistical analysis reaffirms the polluting role of the smelting plant.     

贵州三都巴言鲕状赤铁矿构造热液成因的推断及意义

贵州三都县巴言一带石炭系下统祥摆组及断裂带中可见鲕状赤铁矿。区域成矿条件对比研究发现,祥摆组无沉积赤铁矿的条件;钻孔显示赤铁矿呈脉状赋存于祥摆组至上司组断裂带中,而祥摆组正常层位无赤铁矿;沿断裂带走向追索,赤铁矿化蚀变明显,远离断裂带,祥摆组岩性为黑色炭质泥岩夹石英砂岩,岩相条件不适宜赤铁矿沉积;矿体仅分布于断裂带附近地层及破碎带中,不符合沉积矿床的一般特征。排除通过风化作用充填的可能,结合矿体的展布特征及围岩蚀变特征,推断巴言赤铁矿为构造热液成因。对巴言构造热液成因的赤铁矿"鲕粒"特征研究发现,其与正常沉积的赤铁矿"鲕粒"具有相似的特征,这将为"鲕粒"成因的深入研究提供新的线索。     

The effects of temperature,pressure, and oxygen on copper and iron sulphides synthesised in aqueous solution

Experiments with both iron sulphide and copper sulphide precipitates in aqueous solution show that oxygen is significant in determining the mineral phases resulting after heating up to 180 °C. Tetragonal FeS converts to FeS₂ and Fe₃O₄ in the presence of oxygen, while in the absence of oxygen under the same conditions it converts to hexagonal FeS. Suspensions of covellite and of chalcocite under the same conditions convert to digenite in the presence of oxygen, but remain unchanged when oxygen is absent. Experimental evidence is advanced to support structure determinations that CuS contains disulphide ions.

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Zusammenfassung Experimente mit in wässrigen Lösungen ausgefällten Eisen- und Kupfersulfiden dokumentieren die außerordentliche Bedeutung des Sauerstoffes auf die Bildungs- und Verhaltensweise von Mineralphasen, beobachtet nach dem Aufheizen auf 180 °C. So bildet sich aus ursprünglich tetragonalem FeS letzten Endes Pyrit und Magnetit, wenn Sauerstoff zugegen war, während bei dessen Abwesenheit, aber sonst gleichen Versuchsbedingungen, die Umwandlung in hexagonales FeS erfolgte. Suspensionen von Covellin und Chalkosin wurden, unter analogen Versuchsbedingungen und in Gegenwart von Sauerstoff, in Digenit überführt, während sonst, wenn keine Sauerstoffeinwirkung stattfand, die Mineralphasen keinerlei Veränderungen erkennen ließen. Mittels solcher experimenteller Methoden lassen sich bei der Strukturbestimmung des CuS offenbar Merkmale des Vorhandenseins von Disulfidionen erkennen.

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Published by permission of the Director, Bureau of Mineral Resources, Canberra A. C. T., Australia.

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Temporary Address: Mineralog.-Petrograph. Inst. Univ. Heidelberg, Berliner Str. 19, 69 Heidelberg, Germany     

Laminaset geometries in fine grained shallow marine sequences: core data from the Rannoch Formation (North Sea) and outcrop data from the Kennilworth Member (Utah, USA) and the Bencliff Grit (Dorset, UK)

Sandbodies from storm-dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones. A characteristic of such lamination is the presence of low angle cross lamination. In order to model correctly the effects of such lamination on a waterflood of an oil-bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements. This challenge has been greatly complicated by the lack of outcrop of the formation of interest. The Middle Jurassic Rannoch Formation of the North Sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly. In this study, the laminaset geometry data that can be obtained from core (e.g. apparent set thicknesses) were collected for the Rannoch Formation. These data were compared with similar data from potential outcrop analogues in (1) the Cretaceous Kennilworth Member of the Blackhawk Formation in Utah, USA and (2) the Upper Jurassic Bencliff Grit from the Dorset coast, UK. A quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues. The Rannoch Formation core is characterized by numerous low angle truncations. We have measured these features in two wells (means of 7.2° and 12.1°). Mean apparent set thicknesses were 0.24 and 0.19 m. In the outcrop sections studied, truncation angles ranged from 9.6° to 13.4° and mean set thicknesses from 0.24 to 0.34 m. Mean bounding surface dips of 5.8° and 8.6°, and mean laminaset lengths of 2.3 and 4.1 m were also measured. directly in the field and by using photomosaics. On the basis of this comparison, the Kennilworth Member in Utah was found to be the most suitable and the geometries (i.e. aspect ratios) measured there were used to generate an appropriate geometry of Rannoch laminaset geometries for use in engineering studies: laminaset length, 2.0 m; laminaset thickness, 0.2 m.     

Calculation of the structures, stabilities, and vibrational spectra of arsenites, thioarsenites and thioarsenates in aqueous solution

Structures, stabilities and vibrational spectra have been calculated using molecular quantum mechanical methods for As(OH)₃, AsO(OH)₃, As(SH)₃, AsS(SH)₃ and their conjugate bases and for several species with partial substitution of S for O. Properties for the neutral gas-phase molecules are calculated with state-of-the-art methods which yield AsL distances within 0. 01 Å and AsL stretching frequencies within 10 cm⁻¹ of experiment. Similar accuracy is obtained for neutral molecules in solution using a polarizable continuum model (PCM). For monoanions such as and frequencies can be calculated to within 20 cm⁻¹ of experiment using the polarizable continuum model. Multiply charged anions remain a challenge for accurate frequency calculations, but we have obtained results within the PCM model which at least semiquantitatively reproduce the available data. This allows us to assign the controversial features D, E and F in the Raman data of (Wood S. A., Tait C. D. and Janecky D. R. (2002) A Raman spectroscopic study of arsenite and thioarsenite species in aqueous solution at 25 °C. Geochem. Trans. 3, 31-39).To help in the assignment of the arsenic sulfide spectra we have also calculated energetics for the oxidation of As(III) to As(V) compounds by polysulfides, disproportionation of As(III) compounds and for the dissociation of the oxo- and thio-acids. We have determined that As(III) oxyacids can be transformed to thioacids which can in turn be oxidized to As(V) sulfides by polysulfides and that the pK_a1s of the acids involved can be ordered as follows: AsS(SH)₃ < As(SH)₃ < AsO(OH)₃ < As(OH)₃ in order of increasing pK_a1. We have also established from the calculated energies that the most stable form of the As(III) oxyacid in acidic aqueous solution is indeed As(OH)₃, consistent with previous assignments.