The stability of MG-chlorite

The equilibrium Chlorite = Cordierite + Forsterite + Spinel + H₂0 has been experimentally determined with a chlorite of composition Mg₄.₇₅Al₂.₅Si₂.₇₅O₁₀(OH)₈. Brackets have been obtained between 605 and 640° C at 0.5 kb, 644 and 670° C at 1.0 kb and 690 and 704° C at 2.0 kb. These data are not notably displaced from Chernosky's (1974) data for the same equilibrium with a chlorite of clinochlore composition (Mg₅Al₂Si₃O₁₀(OH)₈), however they are more constraining. A thermodynamic analysis of the above data and data on related equilibria included ideal solution models describing compositional variability in cordierite, orthopyroxene and chlorite. The H₂0 content of cordierite was described using a model based on that of Newton and Wood (1979). The hydrous end-member has two moles of H₂0 and the volumes of the end-members are different; this allows the full range of data to be described with one function. Al-content of orthopyroxene was calculated with Gasparik and Newton's (1984) model. Solid solution in chlorite was modelled by choosing the end-members, Mg₆Si₄O₁₀(OH)₈ and Mg₄Al₄Si₂O₁₀(OH)₈, and using ideal configurational entropy to describe the free energy of mixing. Disordering phenomena in cordierite and spinel were accounted for by adding small entropies of disorder to the third law entropies. Linear programming was used to calculate consistent thermochemical properties for all phases considered. Experimental results indicate that the upper thermal stability of Mg-chlorite is affected by only a few degrees for the composition used here. The thermochemical properties derived allow more complete modeling of systems that include chlorite.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

Thermodynamic properties of copper chloride complexes and copper transport in magmatic-hydrothermal solutions

The behaviour of copper in hydrothermal waters and brines is poorly known at the pressure-temperature-salinity conditions typical of magmatic-hydrothermal systems, severely limiting our understanding of how much copper can be transported and deposited i

Copper accumulation, distribution and fractionation in vineyard soils of Victoria, Australia

The use of copper-based fungicides by Victorian vinegrowers has increased the total copper concentration in some vineyard soils to 250 mg/kg compared to background levels of approximately 10 mg/kg. The Australian and New Zealand guidelines for the assessment of contaminated sites recommend that total copper concentrations in soil exceeding 60 mg/kg require environmental investigation, and this level is exceeded in 8 of the 14 vineyards investigated. The focus of this paper is to measure the accumulation, distribution and fractionation of copper in contaminated and uncontaminated soils as a step towards understanding copper existence in soils and its potential for availability to flora and fauna. Physical and chemical properties and total copper concentrations were measured in soil samples from four selected wine-producing regions of Victoria, including vineyard soils and adjacent background soils. Copper fractionation in soils with high total copper concentrations was measured using a selective extraction procedure. Copper in surface soils (0-1 cm) and throughout soil profiles (maximum depth of 50 cm) was separated in the following fractions: water soluble (WS), exchangeable (E), sorbed (S), easily reducible Mn (ERMn), bound to carbonates (CA), organically bound (OM), bound to Fe and Al oxides (FeOx) and residual (RES). The results show that the source (anthropogenic or natural) of copper in soils and the content of organic matter has influenced the copper distribution among various soil fractions, and in turn its potential mobility and risk to biota. In vineyard soils, potentially available copper (defined as fractions WS+E+S) constitutes more than 60 wt.% of total copper in the upper part of soil profiles and the percentage decreases with increasing depth. However, copper associated with the less mobile fractions (FeOx+RES) is less than 10 wt.% in the top part of soil profiles and increases with increasing depth. Copper in uncontaminated soils exists mainly in less mobile fractions (FeOx+RES; 70-90 wt.%), whereas potentially available copper constitutes approximately 10 wt.% of total copper content throughout soil profiles. The study shows that the conversion between copper fractions is slow, indicating that copper can stay active in soils for long periods of time, greater than tens of years, and may result in leaching and transport to deeper soil layers

Bacillus cereus, gold and associated elements in soil and other regolith samples from Tomakin Park Gold Mine in southeastern New South Wales, Australia

Efficient exploration for new Au deposits is increasingly important as existing deposits become depleted. This is particularly relevant in Australia, where exploration can be difficult because of a thick regolith cover. New and effective methods of exploration need to be developed, and possibilities lie in geomicrobiological methods. For instance, Bacillus cereus, a common soil bacterium, has been shown to act as a biogeochemical indicator for concealed mineralisations, including vein-type Au deposits. We report the results of the first Australian case study of the association of B. cereus and Au at the Tomakin Park Gold Mine in southeastern New South Wales. Soil samples from the Ah horizon were analysed for B. cereus spores and 56 major and trace elements. The results show enrichment of Au, As, B. cereus spores and, to a lesser extent, Sb, Bi and Pb over the top of the Au deposit. Gold concentrations over the mineralisation range from 100 ppb to 1.1 ppm compared to a background of 2 ppb and As concentrations are enriched to 100 ppm from a background of 5 ppm. B. cereus spore counts were up to 10 times higher in soils with elevated concentrations of Au. Factor analysis indicates four main associations: TiO2 +lanthanides+actinides; CaO+MgO+Cs+Be+Ba(+Ga+Pb+Rb); B. cereus+Au+As+Sb+Bi(+Pb); Fe2O3+MnO+Co+Ni+Cu+Mo. Selective sequential leaching was used to study the fractionation of Au and As in soils, other regolith materials and Au-bearing vein quartz to infer their mobility and bioavailability. In unweathered quartz vein material, the majority of the Au was extracted only in the strongest, final step, with aqua regia. However, in soils from the Ah horizon, 50% of the Au was present in the water-, ammonium acetate- and sodium pyrophosphate- and hydroxylamine hydrochloride-extractable fractions. In contrast, As displays little change in fractionation with an increasing degree of weathering, and is predominantly associated with the operationally defined Mn- and Fe-oxides and oxyhydroxides. These results indicate that: (i) Au is mobilised during the weathering of the host rock; (ii) Au is bioavailable in these soils; and (iii) the increase in B. cereus spores is likely to be linked to elevated concentrations of bioavailable Au in these soils. The results indicate also that an effective biogeochemical exploration technique may be developed, where B. cereus spore counts are measured in the field and used as a pre-screening method to target areas useful for further sampling and complete geochemical analysis

A spectrophotometric study of aqueous copper(I)-chloride complexes in LiCl solutions between 100°C and 250°C.

Copper transport and deposition in highly saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride and hydrosulphide. However, our understanding of the behavior of copper(I) chloride complexes at elevated temperatures and in highly saline brines is limited by the conditions of existing experimental studies where the maximum chloride concentration is 2 m. This paper presents the results of a study of copper(I) chloride complexes at much higher chloride concentrations, 1.5 m to 9.1 m, using a UV spectrophotometric method. The UV spectra of copper(I)-bearing LiCl solutions were measured at temperatures between 100 °C and 250 °C at vapor-saturated pressures and quantitative interpretation of the spectra shows that CuCl2- , CuCl32-, and CuCl43-, were present in the experimental solutions. The fitted logarithms of formation constants (log K ) for CuCl2- are in good agreement with the previous results of solubility experiments reported by Xiao et al. (1998) and Liu et al. (2001) The log K values for CuCl32- also agree with those of Liu et al. (2001) and theoretical estimates of Sverjensky et al. (1997). This study presents the first experimentally determined formation constants for CuCl43-, at temperatures greater than 25 °C, and indicates that this complex predominates at chloride concentrations greater than 5 m. Based on the new log K values generated from this study, the calculated chalcopyrite solubility in NaCl solutions indicates that in addition to cooling, fluid mixing (dilution of saline fluids) may be an important factor controlling the deposition of copper minerals from hydrothermal solutions. 2002 Elsevier Science Ltd

A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25-28 degrees C

The mobility and transport of gold in low-temperature waters and brines is affected by the aqueous speciation of gold, which is sensitive in particular to pH, oxidation and halide concentrations. In this study, we use UV-Vis spectrophotometry to identif