The thermodynamic properties of H2O in magnesium and iron cordierite

 The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite. By representing the contribution of H₂O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard enthalpy and entropy of reaction at 298.18° K and 1 bar for, (Mg,Fe)₂Al₄Si₅O₁₈+H₂O ⇄ (Fe,Mg)₂Al₄Si₅O₁₈.H₂O Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH ^° _r =–37141±3520 J and ΔS ^° _r =–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475, –99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of temperature, T(K), and water fugacity, f(bars), is given by n _H2O=1/[1+1/(K ⋅ f _H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H₂O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T. Received: 2 August 1994/Accepted: 7 February 1996     

Feasibility and cost of creating an iron-phosphate coating on pyrrhotite to prevent oxidation

 Acid mine drainage (AMD) occurs when sulfide minerals are exposed to an oxidizing environment. Most of the methods for preventing AMD are either short-term or high cost solutions. Coating with iron phosphate is a new technology for the abatement of AMD. It involves treating the sulfide with a coating solution composed of H₂O₂, KH₂PO₄, and sodium acetate as a buffer agent. The H₂O₂ oxidizes the sulfide surface and produces Fe³⁺ so that iron phosphate precipitates as a coating on the sulfide surface. Experiments performed under laboratory conditions prove that an iron phosphate coating can be established on pyrrhotite surfaces with optimal concentrations of the coating solution in the range of: 0.2M/0.01M H₂O₂, 0.2M KH₂PO₄, and 0.2M sodium acetate NaAc, depending on the experimental scale. Iron phosphate coating may be a long-term solution to the problem of AMD. The method would be easy to implement; the reagent cost, however, is not low enough, although it is lower than the conventional treatment with lime. Received: 30 March 1995 · Accepted: 6 September 1995     

Characterisation of brown coal humic acids and modified humic acids using pyrolysis gcms and other techniques

It has been proposed that Victorian brown coal can be considered as a two-component structure — a lignocellulosic “host”, containing various amounts of weakly bound or entrapped “guest” material together with very small amounts of inorganic and/or mineral matter. The latter predominantly consists of wax esters and/or terpenoid material. In this paper we describe attempts to gain structural information regarding the more complex, “host” component of the coal. Our initial model compound has been humic acid that can be readily obtained from the coal by alkaline extraction. It has been found that “pure” humic acid, free from material associated with the “guest” components of the coal, can be obtained by a highly selective, low-yielding alkaline extraction. This humic acid has been studied by nmr spectroscopy and pyrolysis gas chromatography-mass spectroscopy (py-gc/ms). The products arising from py-gc/ms have been compared with those obtained from similar pyrolysis of whole coals. Alkylation of humic acids using alkyl halides in the presence of base has been successfully carried out and reactivity of the resulting materials compared with those of the parent coal and humic acid.     

Characteristics and origin of sediment-hosted disseminated gold deposits: a review

Sediment-hosted disseminated gold (SHDG) deposits comprise a major portion of the gold production and reserves in the US. Although presently known to be common only in western North America, SHDG deposits are a significant source of world gold production. These deposits are characterized by extremely fine-grained disseminated gold, hosted primarily by arsenian pyrite. Other metals show very little enrichment although in addition to As, anomalous concentrations of elements such as Sb, Hg, Tl and Ba are utilized as exploration tools. The host rocks are dominantly silty carbonates, but ore concentrations are also present in siliceous and silicified rocks as well as intrusive rocks. Alteration consists of decarbonatization, silicification (jasperoid formation) and argillization, which are arranged both spatially and temporally in that order. Argillic alteration is zoned from kaolinite-dominated cores to sericite-dominated margins. The deposits commonly exhibit significant structural (faults) and stratigraphic (composition/permeability) controls. Until the last few years, SHDG deposits were considered as near-surface, epithermal type deposits in origin. Because of their fine-grained nature and the lack of macroscopic features such as veins, it has proven quite difficult to extract geochemical data that are clearly related to their genesis. However, fluid inclusion data indicate pressures corresponding to depths of 2–4 km under lithostatic conditions. Temperatures are constrained by fluid inclusions and phase equilibria to near 225°C. Stable isotope data from alteration minerals and fluid inclusions indicate that the ore fluids were dominated by meteoric waters, some of which had clearly exchanged oxygen with wallrocks during their passage through the crust. Although the data vary, most ore fluids probably had δD values near −150‰ and δ¹⁸O values ranging from −10 to +5‰. Sulfur isotope values reported from SHDG deposits span a wide range, from −30 to +20‰ (sulfides) and 0 to >45‰ (sulfates). Ore-related sulfides (pyrite, realgar) fall at the upper end of the range reported for sulfides. The alteration and mineral assemblage indicate the ore fluids were probably near neutral and gold was likely carried as a bisulfide complex. The depositional mechanism(s) probably included mixing, cooling and oxidation. These mechanisms are consistent with the observed alteration features, i.e. quartz precipitation, calcite dissolution and sericite-kaolinite coexistence. It also explains the presence of both siliceous ores containing native Au and sulfide ores containing Au in pyrite. The extreme variations in sulfur isotopes as seen at Post and fluid inclusion data from Carlin may be indicative of some phase separation (‘boiling’), but such relations have not been documented in other deposits and the importance of phase separation to gold deposition appears minimal.     

Paleogeography and structure of the central Mediterranean: Sicily and its offshore area

The geology of the mainland and offshore of Sicily is illustrated by a few geologic sections and seismic profiles across the late Cenozoic orogenic belt of central and western Sicily and across the Sardinia Channel and Sicily Straits. This belt is the result of several tectonic events. Deformation involved mainly the sedimentary cover of the old African continental margin characterized by a broad basinal domain, flanked along its external (southern) margin by a shallow-water carbonate platform attached to Africa in the Triassic. Compressional deformation started in the more internal basinal rock assemblages overlying a thinned crust. The most important structural characteristic of the early phase of thrusting is the duplex pile forming the bulk of the chain in Central Western Sicily. The structure consists of a basal allochthon, made up of Permian to Middle Triassic layers, an intermediate duplex wedge, composed of competent Mesozoic carbonates, and a roof complex, including Upper Mesozoic-Lower Tertiary less competent rocks. Large-scale clockwise rotation of the thrusts accompanied transpressional movements in the hinterland during the Pliocene. Right oblique reverse faults modified the previous tectonic contacts between the allochthons in the hinterland zones. Contemporaneous south-directed imbrications affected the southern external areas, progressively incorporating foreland and piggyback basins. Development of the Gela Thrust System appears to be linked to the transpressional event; its accretion is also related to contemporaneous underthrusting at deeper levels of Mesozoic carbonate substratum. The older buried thrust sheets were pushed up to the surface breaching the deformed Tertiary cover of the Gela TS. Northwards in the belt post-Messinian normal growth faults opened half graben whose sedimentary fill underwent structural inversion. Alternation of extension and compression tectonics characterizes the Sicilian continental margin in the last million years.     

Overview of in-situ thermomechanical experiments in clay: Concept, results and interpretation

The HADES project (High Activity Disposal Experimental Site) aims at demonstrating the technical feasibility and the long-term safety of geological disposal of reprocessed HLW (High-Level Wastes) radioactive wastes. This disposal could be realised in the Tertiary Boom clay formation below the Mol/Dessel nuclear site. Previous studies in the 80's on the geomechanical behaviour of Boom clay, at host rock temperature (15°C), have demonstrated the mining capabilities of this clay. European partners have collaborated to increase the number of in-situ tests to be developed and operated from the Underground Research Facility (URF). Integrated large-scale experiments have been developed during the last four years, within the framework of the Commission of the European Communities (CEC) research contracts, in order to gain more insight into the thermal influence of heat-emiting wastes on the clay behaviour in the near field.

This paper summarizes the knowledge gained from the in-situ hydro-thermo-mechanical experiments realised from the URF. The objectives and main issues of the tests are described. Information regarding selection of parameters and representativeness of their measurements is discussed. Despite their different aims and designs, three hydro-thermo-mechanical experiments are compared in order to emphasize similarities in the general behaviour of the clay massif submitted to a thermal load. It is observed that its behaviour is qualitatively similar to the reaction upon a constant total stress increase. With the information now available, it is not obvious to draw conclusions with respect to the irreversibility of some phenomena.

Simple in-house analytical codes are able to simulate with a good accuracy the hydro-thermo-mechanical behaviour of clay. Nevertheless, these codes are not sufficient to describe the complexity of the phenomena that are involved in hydro-thermo-mechanical processes as pointed out by field data. In order to validate more complex models, the possibilities to improve the representativeness of the measurements are investigated.     

Benchmarking the thermomechanical behaviour of clays — a progress report — the CEC interclay project

In 1989 a small-scale exercise conducted by the CEC as part of its research programme into the underground storage of radioactive waste highlighted the difficulty of making accurate predictions a priori, of the geomechanical behaviour of clay. Given source data about the Boom clay from the underground research facility at Mol, three organisations independently performed f.e. analyses of two somewhat hypothetical problems. While the results were very broadly in agreement, they were shown to be sensitive to both the type of constitutive model used and the way data was fitted to the model.

Subsequently, the CEC sponsored a more comprehensive benchmark exercise called INTERCLAY II. This involves eleven organisations from various member states of the EC in a project encompassing the three aspects of software development for predictive modelling: verification, validation on the laboratory scale and the treatment of in-situ tests. The paper briefly describes the exercise currently in progress and presents the main results achieved to date concentrating on validation aspects.     

Effects of temperature on strength and compressibility of sand-bentonite buffer

Dense sand-bentonite buffer (γ_d = 1.67Mg/m³) has been proposed in Canada as one of several barriers for isolating nuclear fuel waste. The buffer will be required to function under conditions of high total pressures and elevated temperatures approaching 100°C. Summary results are presented from two test programs: (1) isothermal consolidated undrained triaxial (CIU¯) tests; and (2) isothermal drained constant-p′ (CID) triaxial tests. Specimens were consolidated at effective stresses up to 9.0 MPa and temperatures up to 100°C.

The results indicate parallel hardening lines at systematically lower values of specific volume at elevated temperatures. In shear, increased temperatures produced lower values of maximum deviator stressq_f, and higher pore water pressure changesΔu_f. The net result is curved peak strength envelops in plots ofq_f versusp_f′ that are higher at elevated temperatures, even though the strengths,q_f of individual specimens are lower. The critical state strength envelope is curved inq, p′-planes.

The effect of drained heating on buffer to 100°C is not marked. Compressibilities, stiffnesses, strengths, and pore water pressure generation are all affected, but none of the changes are great.