Omongwaite, Na2Ca5(SO4)6.3H2O, is a new mineral, found as inclusions in gypsum crystals in recent salt lake deposits at Omongwa pan, Namibia. It is monoclinic, with space group C2, a = 12.08(3) A, b = 6.96(1) Å, c = 6.39(2) Å, B = 90.2(3)°, V= 537(2) Å3, and Z = 1. The six strongest lines in the X-ray powder diffraction pattern [dobs. (Å), (I/I° meas), (hkl)] are: 6.028, (40), (110); 3.484, (29), (310); 3.019, (51), (400); 3.014, (100), (220); 2.824, (34), (1̄12); and 2.820, (65), (112). Electron microprobe analysis, recalculated on the basis of 3H2O per formula unit (p.f.u.), gave 56.16 wt.% SO3, 30.82 wt.% CaO, 5.25 wt.% Na2O, 3.21 wt.% K2O, 6.25 wt.% H2O, totalling 101.69 wt.%. The empirical formula, based on 24 anhydrous oxygens p.f.u., is (Na1.47K0.59)Σ;=2.06Ca4.76S6.07O24.3H2O, yielding Na2Ca5(-SO4)6.3H2O as the end-member formula. Na/K ratios are variable, with an average of ∼2.5. The crystals are elongated, with pseudohexagonal transversal cross-sections and with sphenoidal terminations that are commonly developed at one end. The crystal structure of omongwaite is similar to that of bassanite, CaSO4.0.5H2O. Published studies of the synthetic phase show that it can be described as a bassanite structure in which one out of six Ca2+ ions are replaced by Na+ and a second Na+ ion occupies a position near those sites. The crystals are parallel to the [001] axis of the gypsum crystals in which they occur as inclusions. The mineral formed by topotactic replacement during interaction of gypsum with concentrated solutions. It is preserved where the affected surface became covered by gypsum by rapid growth shortly after the formation of omongwaite. The mineral is named after the locality where it was found.

Steady-state dissolution rates of basaltic glass were measured in mixed-flow reactors at 50ºC at pH 3 and 4 as a function of aqueous sulphate concentration. Dissolution rates in the presence of 0.1 moles/kg SO42- were found to be ~3 times greater than those in corresponding SO42- free solutions. This rate increase is found to be approximately consistent with that calculated using a rate equation previously proposed by Gislason and Oelkers (2003). These results suggest that the addition of sulphate to injected CO2 may facilitate CO2 sequestration in basalts by accelerating basaltic glass dissolution rates thus more rapidly releasing aqueous Ca and Mg to solution.

A new method to measure thermal diffusivity in samples at simultaneous high pressure and high temperature is presented. The sample is placed inside a cylindrical heater and subjected to a heater power that varies sinusoidally with time. The diffusion of the temperature variations into the sample is monitored using radiographic imaging of marker foils. This provides measurements of the phase lag and amplitude variation of the temperature cycle at a range of radii from the sample axis without the need for multiple thermocouples. The technique is tested using a NaCl sample at 4 GPa; the best-fit thermal diffusivity for NaCl at 4 GPa and 673 K is 2.4±0.5x10-6 m2/s.

Coal slag and ash, obtained from burning coal in a textile factory in Duga Resa (Croatia) was discharged directly into the Mrežnica River for 110 y (1884–1994), from where it travelled to the Korana River and further to the Kupa River at Karlovac, a total of ~50km from its source. Of 54 elements determined by inductively coupled plasma mass spectrometry (ICP-MS) in the <2 mm sediment fraction, a number of anomalously high levels were recorded. The geoaccumulation index (Igeo) for the anomalous elements were: Hg (1.88), B (4.05), Na (1.44), Al (2.05), V (1.65), Cr (1.20), Fe (1.18), Ni (2.10), Cu (2.37), Sr (0.97), Zr (3.27), Mo (3.34) and U (4.03). Low-field magnetic susceptibility (MS) was measured for each sample. The Igeo for MS in the anomalous region is 5.85. Correlation analysis showed good correlation (>0.90) of MS with: B (0.96), U (0.95), Zr (0.94), Sr (0.93), Na (0.92), Mo (0.92) and Ni (0.90). Cluster analysis of R-modality indicates that MS is linked to B, Mo, Na and U. Low correlation of MS with Fe (0.36) suggests that Fe in stream sediments is not in a ferromagnetic form. Neither maghemite, nor magnetite phases were identified by X-ray diffraction (XRD) in the sediments. Low-field magnetic susceptibility provides an indicator of contamination of river sediments by transported coal slag and ash, although it cannot be prescribed to a single element.

The mechanisms by which coatings develop on weathered grain surfaces, and their potential impact on rates of fluid-mineral interaction, have been investigated by examining feldspars from a 1.1 ky old soil in the Glen Feshie chronosequence, Scottish highlands. Using the focused ion beam technique, electron-transparent foils for characterization by transmission electron microscopy were cut from selected parts of grain surfaces. Some parts were bare whereas others had accumulations, a few micrometres thick, of weathering products, often mixed with mineral and microbial debris. Feldspar exposed at bare grain surfaces is crystalline throughout and so there is no evidence for the presence of the amorphous ‘leached layers’ that typically form in acid-dissolution experiments and have been described from some natural weathering contexts. The weathering products comprise sub-urn thick crystallites of an Fe-K aluminosilicate, probably smectite, that have grown within an amorphous and probably organic-rich matrix. There is also evidence for crystallization of clays having been mediated by fungal hyphae. Coatings formed within Glen Feshie soils after ∼1.1 ky are insufficiently continuous or impermeable to slow rates of fluid-feldspar reactions, but provide valuable insights into the complex weathering microenvironments on debris and microbe-covered mineral surfaces.

First-principles simulations and high-pressure experiments were used to study the stability of BaCO3 carbonates at high pressures. Witherite, which is orthorhombic and isotypic with CaCO3 aragonite, is stable at ambient conditions. As pressure increases, BaCO3 transforms from witherite to an orthorhombic post-aragonite structure at 8 GPa. The calculated bulk modulus of the post-aragonite structure is 60.7 GPa, which is slightly less than that from experiments. This structure shows an axial anisotropicc ompressibility and the a axis intersects with the c axis at 70 GPa, which implies that the pressure-induced phase transition reported in previous experimental study is misidentified. Although a pyroxene-like structure is stable in Mg- and Ca-carbonates at pressures >100 GPa, our simulations showed that this structure does not appear in BaCO3.

At Niquelândia, Cr extracted from the soil (5,000–9,300 mg.kg-1) is likely the result of the Cr-bearing Fe-oxides compared to the Cr-spinels, showing that low Cr-containing minerals present in the dunite (enstatite, olivine and clay minerals) have been completely dissolved. The chromites, accumulated inside soil profiles, have undergone chemical weathering, leading to a Cr enrichment during soil genesis. Traces of dissolution inside the soil chromites suggest that they can be slowly weathered. In this case chromites could represent a diffuse source of available Cr(III) within the soil profiles.

The effects of CO2 on water-basaltic glass interaction have been simulated at 25ºC. The calculations indicate that addition of CO2 (2—30 bar) to water significantly changes the reaction path. Initially, the pH is buffered between 4 and 6 by CO2 ionization, with dissolution of basaltic glass and the formation of secondary minerals with SiO2, Mg-Fe carbonates and dolomite predominating. Upon the dissolution of additional basaltic glass and mineral fixation of CO2, the pH increases to >8 and (Ca)-Fe-Mg smectites, SiO2, Ca-Na zeolites and calcite become the dominant secondary minerals forming. The overall reaction path depends on the initial water composition, reactive surface area, and the composition of the phyllosilicates and carbonates forming. The key factors are the mobility of Mg2+, Fe2+ and Ca2+ and the competing reactions for these solutes among secondary minerals.

Despite the growing geological evidence that fluid boiling and vapour-liquid separation affect the distribution of metals in magmatic-hydrothermal systems significantly, there are few experimental data on the chemical status and partitioning of metals in the vapour and liquid phases. Here we report on an in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of antimony speciation and partitioning in the system Sb2O3-H2O-NaCl-HCl at 400°C and pressures 270—300 bar corresponding to the vapour-liquid equilibrium. Experiments were performed using a spectroscopic cell which allows simultaneous determination of the total concentration and atomic environment of the absorbing element (Sb) in each phase. Results show that quantitative vapour-brine separation of a supercritical aqueous salt fluid can be achieved by a controlled decompression and monitoring the X-ray absorbance of the fluid phase. Antimony concentrations in equilibrium with Sb2O3 (cubic, senarmontite) in the coexisting vapour and liquid phases and corresponding SbIII vapour-liquid partitioning coefficients are in agreement with recent data obtained using batch-reactor solubility techniques. The XAFS spectra analysis shows that hydroxy-chloride complexes, probably Sb(OH)2Cl0, are dominant both in the vapour and liquid phase in a salt-water system at acidic conditions. This first in situ XAFS study of element fractionation between coexisting volatile and dense phases opens new possibilities for systematic investigations of vapour-brine and fluid-melt immiscibility phenomena, avoiding many experimental artifacts common in less direct techniques.