Mineralogy and origin of stichtite in chromite-bearing serpentinites

Stichtite, a rare (14 known localities worldwide) hydrated carbonate-hydroxide of Mg and Cr with ideal formula Mg₆Cr₂ (OH)₁₆ CO₃ · 4H₂O, occurs exclusively in Cr-rich serpentinites of ophiolites or greenstone belts. Physical properties (hardness = 1.5–2, specific gravity = 2.16–2.2, perfect basal [0001] cleavage, grain size commonly < 100 μm) resemble talc, but the mineral has an attractive purple to lilac color; chemical analyses demonstrate it to be a non-silicate. Stichtite generally occurs as irregular to rounded masses (< 1 cm – 30 cm across) and as veinlets (< 1 mm – > 2 cm wide) within serpentinite. Macroscopic and microscopic textures, such as crosscutting veinlets and stringers, demonstrate that stichtite formation invariably post-dated serpentinization. In some specimens stichtite surrounds relict grains of Cr-rich spinel; in others stichtite has completely replaced euhedral or subhedral chromites. Chemical analyses of stichtites reveal substantial substitution of Al and Fe³⁺ for Cr in specimens from many localities, reflecting a possible compositional continuum between stichtite and rhombohedral polymorphs hydrotalcite (Mg₆Al₂ (OH)₁₆ CO₃ · 4H₂O) and pyroaurite (Mg₆Fe₂ (OH)₁₆ CO₃ · 4H₂O). We report the first electron microprobe analyses of stichtites from seven localities, and summarize all available published chemical data. Stichtites very likely inherited part of their trivalent cation chemistry from precursor Cr-rich spinels, but stichtite growth apparently post-dated characteristic “ferritchromit” alteration, as demonstrated by the depletion of Al and enrichment in Fe³⁺ in stichtite relative to primary chromite core compositions. Stichtite appears to form by reaction between serpentine and altered chromite, during addition of substantial fluid, either as separate H₂O and CO₂ phases, or as a mixed volatile phase. Such reactions must involve removal of substantial SiO₂, possibly by transport and remote deposition of silica by throughgoing aqueous and carbonic fluid. Received: 4 April 1996 / Accepted: 16 September 1996     

Electrokinetic remediation of metal contaminated glacial tills

This paper presents the results of an experimental investigation which studied the feasibility of using the electrokinetic process to remediate contaminated clays of glacial origin, otherwise known as glacial tills. An overview of the electrokinetic phenomena, as well as previously performed laboratory and field investigations, is first presented. The methodology of the electrokinetic experiments which were conducted to investigate the removal of metals from a glacial till is then described. A total of 16 experiments were conducted using glacial till samples obtained from a project site near Chicago. Sodium and calcium were used as the surrogate cationic metallic contaminants. These experiments demonstrated that ion transport during the electrokinetic process occurs due to both electro-osmosis and electromigration, but that due to electromigration is significantly higher than that due to electro-osmosis. Unlike other clays such as kaolinite, the glacial till used for this investigation possessed high buffering capacity because of its high carbonate content which prevented the acid front migration from the anode to the cathode during the electrokinetic process. The ion removal efficiency of the electrokinetic process was found to increase when: (1) the voltage gradient applied to the soil was increased, (2) the initial concentration of the contaminants was increased, and (3) the duration of the treatment process was increased. The ion removal efficiency was also greater for smaller ions which possess less ionic charge and when the ions existed independently in the soil as compared to when they coexisted. This investigation suggests that the electrokinetic process has significant potential for remediating glacial tills contaminated with metals. However, the properties of Na and Ca are not representative of contaminants, such as heavy metals, so further investigations are needed.     

The economic mineral potential of the mid-Proterozoic Waterberg Group, northwestern Kaapvaal craton, South Africa

The 1900–1700 Ma Waterberg Group in the main Waterberg fault-bounded basin consists of dominantly coarse siliciclastic red beds with minor volcanic rocks. The sedimentary rocks were deposited mainly by alluvial fans, fluvial braidplains and transgressive shallow marine environments, with lesser lacustrine and aeolian settings. Uplifted, largely granitic source areas were located along the Thabazimbi-Murchison lineament (TML) fault system in the south, and along the Palala shear zone in the northeast. Palaeoplacer titanomagnetite-ilmenite-zircon heavy mineral deposits, best developed in the Cleremont Formation in the centre of the basin, reflect initial fluvial reworking and subsequent littoral marine concentration. Coarse alluvial cassiterite placer deposits are found in the Gatkop area in the southwest of the basin, and appear to have been derived from stanniferous Bushveld Complex lithologies south of the TML. Hydrothermal zinc and U-Cu mineralisation in the Alma lithologies in the same area appears to be related to the TML fault system. Small manganese deposits and anomalous tungsten values occur in the south of the basin, where they are again closely spatially associated with the TML. Copper-barium mineralisation is found associated with dolerite dykes, and in stratigraphically controlled, inferred syngenetic settings. The most interesting of these apparently syngenetic occurrences is found within green coloured reduced mudrocks and inferred volcanic rocks, at an unconformity developed within the overall red bed sequence of the Waterberg Group, adjacent to the TML in the southwest of the basin. The most important potential mineralisation in the main Waterberg basin thus encompasses shoreline placer Ti and the possibility of substantial sediment-hosted copper deposits. Received: 31 May 1996 / Accepted: 17 February 1997     

Diffusion of tetravalent cations in zircon

Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients in m²sec⁻¹): log D _Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol⁻¹ /2.303 RT) log D _U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol⁻¹ /2.303 RT) log D _Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol⁻¹ /2.303 RT) The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited cores. Received: 12 July 1996 / Accepted: 2 December 1996     

Effect of tunnel depth on modulus of deformation of rock mass

Summary Deformability of rock mass significantly influences its behaviour and is, therefore, an important consideration for the design of underground openings. The modulus of deformation of rock mass is, however, normally obtained from expensive and time-consuming uniaxial jacking tests, whose results often have a large scatter. An empirical correlation has, therefore, been proposed for a quick and inexpensive preliminary estimation of the modulus of deformation of rock mass on the basis of field instrumentation carried out in tunnels in India.     

Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA

Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year⁻¹, whereas mean evaporation from the brine-covered areas was 872 mm year⁻¹. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 10⁶ m³ of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 10⁸ kg (7.5 × 10⁵ t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area.     

The separation of the influence of nutrients and climate on the varve time-series of baldeggersee, Switzerland

The varve data-set from a freeze-core taken in the deepest part of Baldeggersee was subjected to different multivariate statistical analyses in order to estimate the amount of variance in the varve thickness measures explicable by past climate and by the trophic state of the lake. A comparison of two different time-periods (1902 to1992 versus 1920 to 1980) revealed that the lake restoration programme since 1982 has had a significant impact on the formation of the seasonal layers. Results of the partitioning of the variance in the varve thickness measures showed that about two thirds of the variance are unexplained by a climate and trophic state model and that trophic state explains 6%, whereas climate accounts for about 28% of the variance before the effect of lake restoration had a strong impact on the varves. Among the climate parameters the amount of annual precipitation is a strong predictor for explaining the thickness of both dark layer and total couplet thickness, whereas summer precipitation is important for the thickness of the light layer.