Stability of biological and inorganic hemimorphite: Implications for hemimorphite precipitation in non-sulfide Zn deposits

Hemimorphite, Zn₄Si₂O₇(OH)₂·H₂O, one of the most common minerals in non-sulfide Zn deposits, together with smithsonite and hydrozincite, is one of the most abundant minerals in the “calamine” deposits in SW Sardinia. In spite of their importance for the development of ore genesis models, the stability properties of hemimorphite are poorly known. This paper presents solubility experiments on two different types of hemimorphite: a “geological” hemimorphite from a supergene non-sulfide Zn deposit, of supposed abiotic origin, and a hemimorphite precipitated by bacterial activity. Both specimens were characterized, before and after the experiment, by Synchrotron Radiation X-ray powder diffraction, Scanning Electron Microscopy, and X-ray Absorption Spectroscopy. The calculated solubility product constants (logK_s) are similar for both “geological” and biogenic hemimorphite (30.3 ± 0.4 and 30.5 ± 0.1, respectively). During the solubility experiment, biological hemimorphite undergoes an amorphous to crystalline phase transition, and the distinctive features (mineralized bacterial sheaths and organic filaments), that allowed us to demonstrate its biological origin, are no longer recognizable by Scanning Electron Microscopy.The results of this study may be useful for developing ore genesis models, including evolution in a supergene environment, and in general for performing geochemical speciation and equilibrium calculations. Moreover, our findings open the way for a new interpretation of hemimorphite-forming processes, and suggest the possibility that bacteria may have played a role in the formation of this mineral in ore deposits.     

Dielectric loss in ore-forming chromium spinel at temperatures from 20 to 800 °C

Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95–98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe₂O₃)^? : (FeO/Fe₂O₃)^??, where (FeO/Fe₂O₃)^? and (FeO/Fe₂O₃)^?? are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe₂O₃ ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr₂O₃) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr₂O₃), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 ? 11.52N + 1.20N², with an error of σ = 0.19 (N = T · 10^?2, T is temperature in °C).     

Cobalt sorption–desorption behavior of calcareous soils from some Iranian soils

Little research has been done to study the role of soil parameters in cobalt (Co) retention, release and the processes involved in calcareous soils of arid and semi-arid regions. We studied the Co sorption and desorption capacity of various calcareous soils using batch technique. The sorption and desorption behavior of Co varied greatly among the studied soils. The sorbed fraction ranged from 92.3% to 97.2% and from 51.0% to 71.8%, when 5 and 200 mg Co l⁻¹, was added to the soil samples, respectively. Cobalt sorption curves were well fitted with Langmuir, Freundlich, and linear equations. The values of the distribution coefficients obtained from linear equation ranged from 9.5 l kg⁻¹ to 23.4 l kg⁻¹. Desorption experiments resulted in a Co recovery ranged from 3.6% to 11.4%, indicating a low desorption of Co from soils. The results of the geochemical modeling indicated that under low Co addition, the solutions were undersaturated with respect to Co(OH)_2(am), Co(OH)_2(c), Co₃(PO₄)_2(s), CoCl_2(s), CoHPO_4(s), CoCl₂·6H₂O_(s), and CoO_(s), whereas under higher Co addition, the solutions were undersaturated with respect to Co(OH)_2(am), CoCl_2(s), CoCl₂·6H₂O_(s), CoO_(s), CoHPO_4(s), and saturated with respect to Co₃(PO₄)_2(s), and CoCO_3(s). The hysteresis indices indicated that desorption of freshly sorbed Co with 0.01 M CaCl₂ was hysteretic in all soils and low mobility and leaching potential of freshly sorbed Co can be expected from these calcareous soils. Statistical correlations revealed that Co sorption and desorption onto the soils were influenced by the presence of CaCO₃ in soils. These findings suggested that calcareous soils are able to retain strongly Co in which the movement of Co in the soil profile would be negligible. Thus, little risk of groundwater contamination can be expected with Co in these calcareous soils.     

Thermodynamic properties of tooeleite,Fe63+(As3+O3)4(SO4)(OH)4·4H2O

Tooeleite, nominally Fe₆³⁺(As³⁺O₃)₄(SO₄)(OH)₄·4H₂O, is a relatively uncommon mineral of some acid-mine drainage systems. Yet, if it does occur, it does so in large quantities, indicating that some specific conditions favor the formation of this mineral in the system Fe-As-S-O-H. In this contribution, we report the thermodynamic properties of synthetic tooeleite. The sample was characterized by powder X-ray diffraction, scanning electron microscopy, extended X-ray absorption fine-structure spectroscopy, and Mössbauer spectroscopy. These methods confirmed that the sample is pure, devoid of amorphous impurities of iron oxides, and that the oxidation state of arsenic is 3+. Using acid-solution calorimetry, the enthalpy of formation of this mineral from the elements at the standard conditions was determined as −6196.6 ± 8.6 kJ mol⁻¹. The entropy of tooeleite, calculated from low-temperature heat capacity data measured by relaxation calorimetry, is 899.0 ± 10.8 J mol⁻¹ K⁻¹. The calculated standard Gibbs free energy of formation is −5396.3 ± 9.3 kJ mol⁻¹. The log K_sp value, calculated for the reaction Fe₆(AsO₃)₄(SO₄)(OH)₄·4H₂O + 16H⁺ = 6Fe³⁺ + 4H₃AsO₃ + SO₄²⁻ + 8H₂O, is −17.25 ± 1.80. Tooeleite has stability field only at very high activities of aqueous sulfate and arsenate. As such, it does not appear to be a good candidate for arsenic immobilization at polluted sites. An inspection of speciation diagrams shows that the predominance field of Fe³⁺ and As³⁺ overlap only at strongly basic conditions. The formation of tooeleite, therefore, requires strictly selective oxidation of Fe²⁺ to Fe³⁺ and, at the same time, firm conservation of the trivalent oxidation state of arsenic. Such conditions can be realized only by biological systems (microorganisms) which can selectively oxidize one redox-active element but leave the other ones untouched. Hence, tooeleite is the first example of an “obligatory” biomineral under the conditions prevailing at or near the Earth's surface because its formation under these conditions necessitates the action of microorganisms.     

Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with andradite (Ca₃Fe ₂ ⁺³ Si₃O₁₂) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO₃)+eskolaite (Cr₂O₃) at 1,385 ± 10 ° C. The incorporation of Ca₃Fe ₂ ⁺³ Si₃O₁₂ component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnet_ss (Ca₃(Cr,Fe⁺³ ₂)Si₃O₁₂)+pseudowollastonite (CaSiO₃)+hemeskolaite_ss(Cr,Fe⁺³O₃). Pure andradite decomposes to pseudowollastonite (CaSiO₃)+hematite (Fe₂O₃) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO₃-Cr₂O₃-Fe₂O₃ ternary system. This minimum is located at about 64.5 wt.-% CaSiO₃, 0.5 wt.-% Cr₂O₃, and 35.0 wt.-% Fe₂O₃. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr⁺³ by Fe⁺³ increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).     

Solubility of Fe2(OH)3Cl (pure-iron end-member of hibbingite) in NaCl and Na2SO4 brines

Pure-iron end-member hibbingite, Fe₂(OH)₃Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe₂(OH)₃Cl(s), the pure-iron end-member of hibbingite (Fe^II, Mg)₂(OH)₃Cl(s), and Fe(OH)₂(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe₂(OH)₃Cl(s) + 3H⁺ ? 3 H₂O + 2 Fe²⁺ + Cl^?,the solubility constant of Fe₂(OH)₃Cl(s) at infinite dilution and 25 °C has been found to be log₁₀ K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)₂(s) + 2H⁺ ? 2 H₂O + Fe²⁺,the solubility constant of Fe(OH)₂ at infinite dilution and 25 °C has been found to be log₁₀ K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe₂(OH)₃Cl(s) and Fe(OH)₂(s), the Na⁺–Fe²⁺ pair Pitzer interaction parameter θ_{Na⁺/Fe²⁺} has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)₂(s) to Fe₂(OH)₃Cl(s). Additionally, when Fe₂(OH)₃Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.     

Mineralogy and geochemistry of laterites from the Morro dos Seis Lagos Nb (Ti,REE) deposit (Amazonas,Brazil)

The Morro dos Seis Lagos niobium deposit (2897.9 Mt at 2.81 wt% Nb₂O₅) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100 m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10 m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23 wt% Nb₂O₅), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe³⁺ + (Nb, Ta) for 2Ti] substitution and both contain up to 2 wt% WO₃. The laterites have an average Nb₂O₅ content of 2.91 wt% and average TiO₂ 5.00 wt% in the upper parts of the sequence. Average CeO₂ concentration increases with increasing depth, from 0.12 wt% in the pisolitic type to 3.50 wt% in the brown laterite. HREE concentration is very low.     

Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study

Pristine diorite drill cores, obtained from the Äspö Hard Rock Laboratory (HRL, Sweden), were used to study the retention properties of fresh, anoxic crystalline rock material towards the redox-sensitive uranium. Batch sorption experiments and spectroscopic methods were applied for this study. The impact of various parameters, such as solid-to-liquid ratio (2–200 g/L), grain size (0.063–0.2 mm, 0.5–1 mm, 1–2 mm), temperature (room temperature and 10 °C), contact time (5–108 days), initial U(VI) concentration (3 × 10⁻⁹ to 6 × 10⁻⁵ M), and background electrolyte (synthetic Äspö groundwater and 0.1 M NaClO₄) on the U(VI) sorption onto anoxic diorite was studied under anoxic conditions (N₂). Comparatively, U(VI) sorption onto oxidized diorite material was studied under ambient atmosphere (pCO₂ = 10^−3.5 atm). Conventional distribution coefficients, K_d, and surface area normalized distribution coefficients, K_a, were determined. The K_d value for the U(VI) sorption onto anoxic diorite in synthetic Äspö groundwater under anoxic conditions by investigating the sorption isotherm amounts to 3.8 ± 0.6 L/kg which corresponds to K_a = 0.0030 ± 0.0005 cm (grain size 1–2 mm). This indicates a weak U sorption onto diorite which can be attributed to the occurrence of the neutral complex Ca₂UO₂(CO₃)₃(aq) in solution. This complex was verified as predominating U species in synthetic Äspö groundwater by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Compared to U sorption at room temperature under anoxic conditions, U sorption is further reduced at decreased temperature (10 °C) and under ambient atmosphere. The U species in aqueous solution as well as sorbed on diorite were studied by in situ time-resolved attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. A predominant sorbing species containing a UO₂(CO₃)₃⁴⁻ moiety was identified. The extent of U sorption onto diorite was found to depend more on the low sorption affinity of the Ca₂UO₂(CO₃)₃(aq) complex than on reduction processes of uranium.     

An Apatite II permeable reactive barrier to remediate groundwater containing Zn,Pb and Cd

Phosphate-induced metal stabilization involving the reactive medium Apatite II™ [Ca_10−xNa_x(PO₄)_6−x(CO₃)_x(OH)₂], where x < 1, was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater containing elevated concentrations of Zn, Pb, Cd, Cu, SO₄ and NO₃. The groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d’Alene River, Idaho. Microbially mediated SO₄ reduction and the subsequent precipitation of sphalerite [ZnS] is the primary mechanism occurring for immobilization of Zn and Cd. Precipitation of pyromorphite [Pb₁₀(PO₄)₆(OH,Cl)₂] is the most likely mechanism for immobilization of Pb. Precipitation is occurring directly on the original Apatite II. The emplaced PRB has been operating successfully since January of 2001, and has reduced the concentrations of Cd and Pb to below detection (2 μg L⁻¹), has reduced Zn to near background in this region (about 100 μg L⁻¹), and has reduced SO₄ by between 100 and 200 mg L⁻¹ and NO₃ to below detection (50 μg L⁻¹). The PRB, filled with 90 tonnes of Apatite II, has removed about 4550 kg of Zn, 91 kg of Pb and 45 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier, wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).     

Geochemistry of ash leachates during the 1994–1996 activity of Popocatépetl volcano

Increasing fumarolic activity at Popocatépetl volcano has been observed since 1992. On 21 December 1994, a series of eruptions at Popocatépetl volcano produced ash emissions that reached the city of Puebla located to the east of the volcano. Eruptive activity declined sharply from June 1995 until 5 March 1996 when ash emissions and fumarole flux increased to levels similar to those of December, 1994. Intermittent ash production has continued to 1997. Ash was sampled at more than 80 different locations around the volcano during the various eruptions. Gas produced during an eruption may be scavenged by the ash and leaching of the ash with water allows determination of the concentration of ions adsorbed from the volcanic gases. The leachates obtained from eruptions from December 1994 until 28 November 1996, were analyzed by potentiometry with selective electrodes for Cl⁻ and F⁻ and by ion chromatography for SO₄²⁻. Minor cations (Co²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Sb²⁺, Ti⁴⁺, Cd²⁺, Tl³⁺) were determined in some samples by ICP-MS. The highest concentrations of Cl⁻ and SO₄²⁻ were obtained for the 21 December 1994 ash at the start of the eruptions with 19 550 ppm SO₄²⁻ and 1028 ppm Cl⁻ and for the emission which occurred on 5 March 1996, with 21 775 ppm SO₄²⁻ and 1250 ppm Cl⁻. At both times a concentration decrease was observed, but with particular trends in each case. The composition of the ash leachates suggests that the two Popocatépetl eruptions in 1994 and 1996 began with phreatic and magmatic components. The increase in F⁻ and the decrease in the Cl/F ratio may indicate a heating up of the volcanic system at the beginning of March, 1996, one week before the outpouring of lava in the bottom of the crater on 20 March 1996. The concentration trends for SO₄²⁻, Cl⁻ and F⁻ suggest that during the 1996 activity, the system attained higher temperatures than in 1994–1995.     

An Apatite II permeable reactive barrier to remediate groundwater containing Zn,Pb and Cd

Phosphate-induced metal stabilization involving the reactive medium Apatite II™ [Ca_10−xNa_x(PO₄)_6−x(CO₃)_x(OH)₂], where x < 1, was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater containing elevated concentrations of Zn, Pb, Cd, Cu, SO₄ and NO₃. The groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d’Alene River, Idaho. Microbially mediated SO₄ reduction and the subsequent precipitation of sphalerite [ZnS] is the primary mechanism occurring for immobilization of Zn and Cd. Precipitation of pyromorphite [Pb₁₀(PO₄)₆(OH,Cl)₂] is the most likely mechanism for immobilization of Pb. Precipitation is occurring directly on the original Apatite II. The emplaced PRB has been operating successfully since January of 2001, and has reduced the concentrations of Cd and Pb to below detection (2 μg L⁻¹), has reduced Zn to near background in this region (about 100 μg L⁻¹), and has reduced SO₄ by between 100 and 200 mg L⁻¹ and NO₃ to below detection (50 μg L⁻¹). The PRB, filled with 90 tonnes of Apatite II, has removed about 4550 kg of Zn, 91 kg of Pb and 45 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier, wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).     

Mineralogy and genetic features of the Cu-As-Ni-Sb-Pb mineralization from the Mlakva polymetallic deposit (Serbia) — New occurrence of (Ni-Sb)-bearing Cu-arsenides

(Ni-Sb)-bearing Cu-arsenides are rare minerals within the Mlakva and Kram mining sectors (Boranja ore field) one of the less-known Serbian Cu deposits. (Ni-Sb)-bearing Cu-arsenides were collected from the Mlakva skarn-replacement Cu(Ag,Bi)-FeS polymetallic deposit. The identified phases include β-domeykite, Ni-bearing koutekite and (Ni-Sb)-bearing α-domeykite. (Ni-Sb)-bearing Cu-arsenides are associated with nickeline, arsenical breithauptite, chalcocite, native Ag, native Pb and litharge. Pyrrhotite, pyrite, chalcopyrite, cubanite, bismuthinite, molybdenite, sphalerite, galena, Pb(Cu)-Bi sulfosalts and native Bi, as well as minor magnetite, scheelite and powellite are associated with the sulfide paragenesis. The electron microprobe analyses of the (Ni-Sb)-bearing Cu-arsenides yielded the following average formulae: (Cu_2.73,Ni_0.17,Fe_0.03,Ag_0.01)_∑ 2.94(As_0.98,Sb_0.05,S_0.02)_∑ 1.06–β-domeykite (simplified formula (Cu_2.7,Ni_0.2)_∑ 2.9As_1.1); (Cu_3.40,Ni_1.40,Fe_0.11)_∑ 4.91(As_1.94,Sb_0.13,S_0.02)_∑ 2.08–Ni-bearing koutekite (simplified formula (Cu_3.4Ni_1.5)_∑ 4.9As_2.1); and Cu_1.97(Ni_0.98,Fe_0.03)_∑ 1.01(As_0.81,Sb_0.22)_∑ 1.03–(Ni–Sb)-bearing α-domeykite (simplified formula Cu₂NiAs). The Rietveld refinement yielded the following unit-cell parameters for β-domeykite and Ni–bearing koutekite: a = 7.1331(4); c = 7.3042(5) Å; V = 321.86(2) ų, and a = 5.922(4); b = 11.447(9); c = 5.480(4) Å; V = 371.48(5) ų, respectively. Ore geology, paragenetic assemblages and genesis of the Mlakva deposit are discussed in detail and the Cu-As-Ni-Sb-Pb mineralization has been compared with similar well-known global deposits.     

Paleo-depths reconstruction of the last 550,000 years based on the transfer function on recent and Quaternary benthic foraminifers of the East Corsica margin

In this study, the model H(i) = 109.6103 + C₁ × F1(i) + C₂ × F₂(i) + … + C₃₃ × F₃₃(i) obtained from depth modelling based on 33 recent benthic foraminifer species distribution, has been applied to the fossil benthic foraminifers from the borehole GDEC-4-2 drilled at a water depth of 491 m, in the East-Corsica basin, covering the last 550,000 years. The obtained variations of the paleo-depths show a medium correlation with the oscillations of the relative sea level and also with the fluctuations of the oxygen isotopic ratio (δ¹⁸O G. bulloides and δ¹⁸O C. pachyderma–C. wuellerstorfi). This newly developed transfer function is accompanied by an error margin of ± 86 m, suggesting that this model will probably be more suitable for a time scale of the order of a million years where sea level variations are recorded with larger amplitudes. Without considering these problems related to amplitudes, it also turns out that the “eustatic” signal of the microfauna is accompanied by a “trophic” signal, which should not to be neglected, especially at a millennial scale time resolution. Thus, the application of this method would require taking into account the bottom trophic effects strongly controlling the distribution of benthic foraminifer assemblages.     

Hydrosulfide/sulfide complexes of zinc to 250 °C and the thermodynamic properties of sphalerite

The solubility of synthetic ZnS_(cr) was measured at 25–250 °C and P = 150 bars as a function of pH in aqueous sulfide solutions (~ 0.015–0.15 m of total reduced sulfur). The solubility determinations were performed using a Ti flow-through hydrothermal reactor. The solubility of ZnS_(cr) was found to increase slowly with temperature over the whole pH range from 2 to ~ 10. The values of the Zn–S–HS complex stability constant, β, were determined for Zn(HS)₂⁰_(aq), Zn(HS)₃^?, Zn(HS)₄^2?, and ZnS(HS)^?. Based on the experimental values the Ryzhenko–Bryzgalin electrostatic model parameters for these stability constants were calculated, and the ZnS_(cr) solubility and the speciation of Zn in sulfide-containing hydrothermal solutions were evaluated. The most pronounced solubility increase, about 3 log units at m(S_total) = 0.1 for the temperatures from 25 to 250 °C, was found in acidic solutions (pH ~ 3 to 4) in the Zn(HS)₂⁰_(aq) predominance field. In weakly alkaline solutions, where Zn(HS)₃^? and Zn(HS)₄^2? are the dominant Zn–S–HS complexes, the ZnS_(cr) solubility increases by 1 log unit at the same conditions. It was found that ZnS(HS)^? and especially Zn(HS)₄^2? become less important in high temperature solutions. At 25 °C and m(S_total) = 0.1, these species dominate Zn speciation at pH > 7. At 100 °C and m(S_total) = 0.1, the maximum fraction of Zn(HS)₄^2? is only 20% of the total Zn concentration (i.e. at pH_t ~ 7.5), whereas at 350 °C and 3 <pH_t <10, the fraction of Zn(HS)₄^2? and ZnS(HS)^? is less than 0.05% and 2.5% respectively, of the total Zn concentration and Zn(HS)₂⁰ and Zn(HS)₃^? predominate. The measured equilibrium formation constants were combined with the literature data on the stability of Zn–Cl complexes in order to evaluate the concentration and speciation of Zn in chloride solutions. It was found that at acidic pH, and in more saline fluids having total chloride > 0.05 m, Zn–Cl complexes are responsible for hydrothermal Zn transport with no significant contribution of Zn–S–HS complexes. The hydrosulfide/sulfide complexes will play a more important role in lower salinity (< 0.05 m chloride) hydrothermal solutions which are characteristic of many epithermal ore depositing environments. The value of Δ_fG° (β-ZnS_(cr)) = ? 198.6 ± 0.2 kJ/mol at 25 °C was determined via solubility measurements of natural low-iron Santander (Spain) sphalerite.     

Cyanobacterial mineralisation of posnjakite (Cu4(SO4)(OH)6·H2O) in Cu-rich acid mine drainage at Yanqul,northern Oman

This is the first detailed account of the copper sulfate posnjakite (Cu₄(SO₄)(OH)₆·H₂O) coating cm-long filaments of a microbial consortium of four cyanobacteria and Herminiimonas arsenicoxydans. It was first observed on immersed plant leaves and stalks in a quarry sump of the abandoned Yanqul gold mine in the northern region of Oman; rock surfaces in the immediate vicinity show no immediate evidence of posnjakite. However, a thin unstructured layer without filaments but also containing the brightly coloured turquoise posnjakite covers ferruginous muds in the sump. Although copper is a potent bactericide, the microbes seem to survive even at the extreme heavy metal concentrations that commonly develop in the sump during the dry season (Cu²⁺ ≈ 2300 ppm; Zn²⁺ = 750 ppm; Fe²⁺ ≈ 120 ppm; Ni²⁺ = 37 ppm; Cr_total = 2.5 ppm; Cl⁻ = 8250 ppm; and SO₄²⁻ = 12,250 ppm; pH ∼2.6), thus leading to the precipitation of posnjakite over a large range of physicochemical conditions. Upon exposure to the prevailing arid climate, dehydration and carbonation quickly replace posnjakite with brochantite (Cu₄(SO₄)(OH)₆) and malachite (Cu₂(CO₃)(OH)₂). To characterise and understand the geochemical conditions in which posnjakite precipitates from undersaturated fluids (according to our thermodynamic modelling of the dominant elements), waters from rainy and dry periods were analysed together with various precipitates and compared with the observed field occurrences. The findings imply that posnjakite should not form in the examined environment through purely inorganic mechanisms and its origin must, therefore, be linked to the encountered microbial activities.     

Hydrothermal alteration and ore-forming fluids associated with gold-tellurium mineralization in the Dongping gold deposit,China

The Dongping gold deposit hosted in syenites is one of the largest hydrothermal gold deposits in China and composed of ore veins in the upper parts and altered zones in the lower parts of the ore bodies. Pervasive potassic alteration and silicification overprint the wall rocks of the ore deposit. The alteration minerals include orthoclase, microcline, perthite, quartz, sericite, epidote, calcite, hematite and pyrite, with the quartz, pyrite and hematite assemblages closely associated with gold mineralization. The phases of hydrothermal alteration include: (i) potassic alteration, (ii) potassic alteration - silicification, (iii) silicification - epidotization - hematitization, (iv) silicification - sericitization - pyritization and (v) carbonation. Mass-balance calculations in potassic altered and silicified rocks reveal the gain of K₂O, Na₂O, SiO₂, HFSEs and transition elements (TEs) and the loss of REEs. Most major elements were affected by intense mineral reactions, and the REE patterns of the ore are consistent with those of the syenites. Gold, silver and tellurium show positive correlation and close association with silicification. Fluid inclusion homogenization temperatures in quartz veins range from 154 °C to 382 °C (peak at 275 °C–325 °C), with salinities of 4–9 wt.% NaCl equiv. At temperatures of 325 °C the fluid is estimated to have pH = 3.70–5.86, log fO₂ = − 32.4 to − 28.1, with Au and Te transported as Au (HS)₂⁻ and Te₂^2 − complexes. The ore forming fluids evolved from high pH and fO₂ at moderate temperatures into moderate-low pH, low fO₂ and low temperature conditions. The fineness of the precipitated native gold and the contents of the oxide minerals (e.g., magnetite and hematite) decreased, followed by precipitation of Au- and Ag-bearing tellurides. The hydrothermal system was derived from an alkaline magma and the deposit is defined as an alkaline rock-hosted hydrothermal gold deposit.     

Evolution of groundwater chemistry in and around Vaniyambadi Industrial Area: Differentiating the natural and anthropogenic sources of contamination

Groundwaters in the crystalline aquifers are the major source of drinking water in Vaniyambadi area of Vellore district. Geochemical methods in collaboration with statistical methods were applied in this industrial area to understand the natural and anthropogenic influences on groundwater quality. To accomplish this objective, groundwater samples were collected and analyzed for physicochemical parameters and the results showed a dominance in the order of Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ for anions and cations, respectively. In contrast to this anion dominance were changed to Cl⁻ > HCO₃⁻ > SO₄²⁻ > NO₃⁻ in samples collected near the tannery industries. Groundwater quality evaluation using TDS and TH suggested that 57% of the total samples are hard-brackish type, indicating its unsuitability for drinking purpose. Generally the water type is Na⁺Cl⁻ to Ca²⁺Mg²⁺HCO₃⁻ type with an intermediate Ca²⁺Mg²⁺Cl⁻, suggesting the mixing of fresh groundwater with tannery effluent and cation exchange. Factor analysis and bivariate plots of major ions suggests that both natural and anthropogenic inputs are equally influencing the groundwater quality. Further investigations proved that silicate weathering is the dominant geogenic source of groundwater solute content, whereas tannery effluent is the anthropogenic source. Saline water mixing index (SWMI) and Cl⁻ vs NO₃⁻ bivariate plot were employed to differentiate the tannery contamination from the other anthropogenic inputs such as agricultural fertilizers, municipal sewages, etc. This analysis shows that samples 2, 4, 8 and 9 (located within the tannery cluster) have a SWMI value greater than 1, representing the groundwater–tannery effluent mixing. This study infers that groundwater in the Vaniyambadi area is under serious threat from both natural and anthropogenic contamination. However, the controlling discharge of untreated tannery effluents must be regulated to reduce the further deterioration of this vital resource in this part of the country.     

Importance of hydrogeological conditions during formation of the karstic bauxite deposits,Central Guizhou Province,Southwest China: A case study at Lindai deposit

Karstic bauxite deposits are widespread in Central Guizhou Province, SW China, and high-grade ores are frequently sandwiched with overlying coal and underlying iron-rich layers and form a special “coal–bauxite–iron” structure. The Lindai deposit, which is one of the most representative karstic bauxite deposits in Central Guizhou Province, was selected as a case study. Based on textural features and iron abundances, bauxite ores in the Lindai deposit are divided into three types of ores, i.e., clastic, compact, and high-iron. The bauxite ores primarily comprise diaspore, boehmite, kaolinite, illite, and hematite with minor quartz, smectite, pyrite, zircon, rutile, anatase, and feldspar. The Al₂O₃ (53–76.8 wt.%) is the main chemical contents of the bauxite ore samples in the Lindai district, followed by SiO₂, Fe₂O₃, TiO₂, CaO, MgO, S, and P etc. Our geological data on the Lindai deposit indicated that the ore-bearing rock series and its underlying stratum have similar rare earth elements distribution pattern and similar Y/Ho, Zr/Hf, and Eu/Eu¹ values; additionally, all ore-bearing rock samples are rich in MgO (range from 0.16 wt.% to 0.68 wt.%), and the plots of the dolomites and laterites lie almost on or close to the weathering line fit by the Al-bearing rocks in Zr vs. Hf and Nb vs. Ta diagrams; suggesting that the underlying Middle Cambrian Shilengshui Formation dolomite is the parent rock of bauxite resources in the Lindai district.Simulated weathering experiments on the modern laterite from the Shilengshui Formation dolomite in the Lindai bauxite deposit show that hydrogeological conditions are important for karstic bauxite formation: Si is most likely to migrate, its migration rate is several magnitudes higher than those of Al and Fe under natural conditions; the reducing inorganic acid condition is the most conducive to Al enrichment and Si removal; Fe does not migrate easily in groundwater, Al enrichment and Fe removal can occur only in acidic and reducing conditions with the presence of organic matter.The geological and experimental studies show that “coal–bauxite–iron” structure in Lindai deposit is formed under certain hydrogeological conditions, i.e., since lateritic bauxite or Al-rich laterite deposited upon the semi-closed karst depressions, Si can be continuously removed out under neutral/acidic groundwater conditions; the coal/carbonaceous rock overlying the bauxitic materials were easily oxidized to produce acidic (H₂S, H₂SO₄, etc.) and reductant groundwater with organic materials that percolated downward, resulting in enrichment of Al in underlying bauxite; it also reduced Fe³⁺ to its easily migrating form Fe²⁺, moving downward to near the basal carbonate culminated in precipitating of ferruginous (FeS₂, FeCO₃, etc.) strata of the “coal–bauxite–iron” structure. Thus, the bauxitic materials experienced Al enrichment and Si and Fe removal under above certain hydrogeological conditions forming the high-quality bauxite.     

Miscibility in the CaSO4·2H2O–CaSeO4·2H2O system: Implications for the crystallisation and dehydration behaviour

SeO₄²⁻ ions can substitute for sulphate in the gypsum structure. In this work crystals of different Ca(SO₄,SeO₄)·2H₂O solid solutions were precipitated by mixing a CaCl₂ solution with solutions containing different ratios of Na₂SO₄ and Na₂SeO₄. The compositions of the precipitates were analysed by EDS and the cell parameters were determined by X-ray powder diffraction. Moreover, a comparative study on dehydration behaviour of selenate rich and sulfate rich Ca(SO₄,SeO₄)·2H₂O solid solutions was carried out by thermogravimetry.The experimental results show that the Ca(SO₄,SeO₄)·2H₂O solid solution presents a symmetric miscibility gap for compositions ranging from X_CaSO4·2H2O = 0.23 to X_CaSO4·2H2O = 0.77. By considering a regular solution model a Guggenheim parameter a₀ = 2.238 was calculated. The solid phase activity coefficients obtained with this parameter were used to calculate a Lippmann diagram for the system Ca(SO₄,SeO₄)·2H₂O–H₂O.     

Distance least squares modelling of the cubic sodalite structure and of the thermal expansion of Na8(Al6Si6O24)I2

The Distance Least Squares (DLS) structure modelling technique is used to determine the room-temperature structures of the sodalites Li₈(Al₆Si₆O₂₄)Cl₂, Na₈(Al₆Si₆O₂₄)Cl₂, K₈(Al₆Si₆O₂₄)Cl₂, Na₈(Al₆Si₆O₂₄)Br₂, and Na₈(Al₆Si₆O₂₄)I₂. The technique is also used to calculate the thermal expansion behaviour of Na₈(Al₆Si₆O₂₄)I₂ assuming that the discontinuity in its thermal expansion curve occurred either when the ideal fully-expanded state was achieved (case 1) or when the x-coordinate of the sodium atom became 0.25 (case 2). The results are given as plots of bond lengths and bond angles as a function of temperature. Case 2 was preferred and analysis of the results implied that the driving force for the untwisting of the partially-collapsed sodalite framework was in the framework bonds with the cavity ion bonds resisting the untwisting. Best estimates indicate that the expansion of the Na-O and Na-I bonds are 9% and 27.4% respectively, between room temperature and 810° C, and there is an apparent shortening of the framework bond distances of about 1.5%.