E. Caballero

Chemical, mineralogical and isotope behavior, and phase transformation during the precipitation of calcium carbonate minerals from intermediate ionic solution at 25°C

A set of time-series experiments was performed in which CaCO3 was precipitated (25°C, 1 atm) from aliquots of an NaHCO3 (0.330 M)/CaCl2 (0.0023 M) solution in a closed system to evaluate the effect of phase transformation on calcium carbonate precipitation kinetics and isotope behavior. Monohydrocalcite precipitated initially at a solution saturation state (Ωmhc) slightly greater than one, whereas calcite crystallized later at a significant higher saturation state (Ωcl ≅ 14.4). The precipitation of calcite promoted the dissolution of monohydrocalcite at a rate that exceeded calcite precipitation, producing anomalous behavior in the chemical and isotope composition of the system. The carbon isotope fractionation factors (103 lnα) for monohydrocalcite-HCO3(aq)- and monohydrocalcite-CO2(g) were 0.36 ± 0.01‰ and 8.35 ± 0.01‰, respectively. The oxygen isotope fractionation factor for monohydrocalcite-H2O was 27.8 ± 0.1‰. The carbon isotope fractionation factors for calcite-HCO3 (aq)− and calcite-CO2(g) were 0.94 ± 0.06‰ and 8.93 ± 0.06‰, respectively, whereas the oxygen isotope fractionation factor for calcite-H2O(l) was 28.0 ± 0.2‰. A carbon isotope fractionation factor of 0.58 ± 0.07‰ was determined for the mineral pair calcite-monohydrocalcite, but no fractionation was observed for oxygen isotopes over time steps when both minerals co-precipitated. Fractionation factors for calcite were independent of precipitation rate over the range in rates of 103.96 to 105.63 μmol/m2h. These results extend the upper limit of characterization for the relationship between precipitation rate and isotope partitioning of carbon between calcite, HCO3 (aq)− and CO2(g), and quantitatively document for the first time the independence between precipitation rate and oxygen isotope partitioning in the calcite-H2O(l) system.

Kinetics of montmorillonite dissolution in granitic solutions

Abstract Experiments measuring smectite dissolution rates in granitic solutions were carried out in a semi-batch reactor at 20, 40, and 60°C. The pH conditions of the solutions range from 7.6 to 8.5. Solid samples were confined within a dialysis membrane and introduced in the solution. The solution was renewed every 7 days and the dissolution reaction was investigated by the variation of Si concentration in the solutions. The average rates at pH∼8 were 10−14.13, 10−13.70, and 10−13.46 mol m−2 s−1, at 20, 40, and 60°C, respectively, and the activation energy for the dissolution reaction at pH ∼8 was 30.5±1.3 kJ mol−1. Comparison of the present results with other studies reveals that the montmorillonite dissolution rate depends strongly on the pH of the solution, with a minimum value at pH 8–8.5. At room temperature, the dissolution rate was found to be linearly dependent on proton (acidic conditions) or hydroxyl (basic conditions) activity in solution: Rate =10 −11.39 a H + 0.34 pH Rate =10 −12.31 a OH − 0.34 pH >8.5 The comprehension of the dissolution mechanism can be improved by using surface complexation theory. Correlation between speciation of surface sites and kinetic results indicated that at room temperature the dissolution rate was directly proportional to the surface concentration of >AlOH2+ and >AlO− surface complexes, under acidic or alkaline conditions, respectively. Rate =10 −8.0 {> AlOH 2 + } pH Rate =10 −8.2 {> AlO − } pH >8.5 A multiple variable model is proposed to take into account simultaneously the effect of pH on dissolution rates and on activation energy. The rates estimated using the model are in good agreement with experimental dissolution rates.

A stable isotope study of cave seepage waters

An isotope study (δ18O, δ2-H) of 62 samples of waters from both the interior of the Nerja cave (Malaga, Southern Spain) and the exterior sinkholes and springs at the site was carried out. Rainfall water in the study area was also studied. The mean value of seepage waters can be considered as the mean isotopic value of the precipitation waters of the area. The origin and aquifers of these different types of waters has also been studied, obtaining a clear meteoric origin both for the seepage waters and the waters from the wells and springs, but the aquifer of the latter is different, showing more negative isotopic values. This suggests that they could have mixed with waters from other aquifers which are either deeper or originate at other altitudes.

Bentonites from Cabo de Gata, Almería, Spain: a mineralogical and geochemical overview

Abstract The Neogene volcanic region of Cabo de Gata, Almería, SE Spain, is dotted with many outcrops of bentonite, some of them of significant economic interest. The bentonites have their origin in the hydrothermal alteration of pyroclastic rocks (15-7 Ma). The deposits are usually associated with fractures. The major mineral is a dioctahedral Fe- and Mg-smectite (89-75%) and this is accompanied by minor amounts of feldspars, quartz, amphiboles, pyroxenes, biotite, zeolites, disordered tridymite, calcite, etc. This paper describes the geological background, the general characteristics of the bentonites and major aspects of their formation, e.g. type of low-temperature hydrothermal solutions, mass balance, chemical evolutions of the smectites and geochemistry of trace elements. Finally, the characteristics of three of the most important deposits are described.

Experimental alteration of volcanic tuff; smectite formation and effect on 18 O isotope composition

Three samples of volcanic tuff were hydrothermally altered at ∼82°C in a soxhlet apparatus for periods from 745 to 2706 h. The samples correspond to partially altered specimens of volcanic tuff with 6 wt. % (T3a) and 9 wt. % (T3b) smectite and to the calcination product of the latter (T3c). The calcination treatment melted the smectite in the sample. Untreated samples and alteration products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetry (TG), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, and oxygen isotope analysis. DTA-TG of the <2-µm size fractions showed that there was a small increase of smectite relative to glass after alteration for samples T3a and T3b, with the amount of smectite increasing exponentially with time. No smectite formed in sample T3c. These results suggest that smectite acts as a nucleation site for the precipitation of new smectite. The amount of glass in the <2-µm size fraction increased, although slightly decreased relative to smectite. SEM-EDX analysis showed smectite with the approximate structural formula of Na0.22K0.08Mg0.12Ca0.03VI(Al1.47Fe0.05Mg0.48)IV(Si3.97Al0.03)O10(OH)2.Oxygen isotope composition of the <2-µm size fraction became enriched in 18O by alteration, the >2-µm size fraction of T3b did not vary, and that of T3c was depleted in 18O. Our results are consistent with three processes during alteration: 1) oxygen isotope exchange between volcanic glass and water, 2) neoformation of smectite, and 3) hydration and consequent hydroxylation of the calcined glass.

A record of Pleistocene climate from a stalactite, Nerja Cave, southern Spain

A study of stable isotopes (N 18 O, N 13 C) of a uranium-series dated aragonitic stalactite from Nerja Cave (Malaga, southern Spain) was carried out in order to determine the conditions of deposition in isotopic equilibrium and non- equilibrium. We obtained a record of climate change from 190 000 to 160 000 years ago. A series of carbon (N 13 C) and oxygen (N 18 O) isotopes analyses have been carried out in parallel to the stalactite growth axis. Sampling was done in each growth layer. A curve of the secular N 18 O aragonite variations for the stalactite suggests cooler climate conditions

The formation of bentonite : mass balance effects

Abstract The alteration of volcanic material into bentonite produces an important loss of mass originated by the hydrolytic action of a large amount of hydrothermal solutions. The latter solutions also provide essential elements for the formation of these materials. Mass balance gains and losses in 52 samples from several bentonite deposits show that the greatest element losses correspond to Si, and to a lesser extent to Na and K. There are some gains in Fe, Ca, Mg and H 2 O. On the other hand, there is a good correlation ( r =0.87) between the amount of matter lost and the amount of smectite formed. In addition, the relationship between matter loss and the chemical composition of the neoformed smectite has been studied, and it has been observed that the chemical composition of smectite varies with increasing matter loss. The initial smectite contains less A1, and more Fe and Mg in comparison to the final smectite.

Early-stage smectites from pyroclastic rocks of Almería (Spain)

Abstract In the volcanic region of Cabo de Gata (Almeria, Spain) bentonite deposits were formed from pyroclastic rocks. There is evidence that the smectite composition changed from Fe-rich to Al-rich members as the degree of hydrothermal alteration increased. Because true initial stages of alteration do not occur in this region, artificial hydrolysis of rocks is needed to obtain early-stage smectites. Three typical volcanic rocks were hydrolyzed in Soxhlet extractors at 82°C for 1457 hr. with 243–302 l of percolating water. In these conditions, 11–12% of smectite was formed. Mineralogical and chemical data of synthetic smectites reveal that they are Fe-bearing beidellites. In fact, the mean value of octahedral Fe of synthetic smectite is 0.85 whereas for the natural one is 0.21. The tetrahedral charge is close to 0.6 and 0.2, respectively. The differences between the synthetic and the natural processes of formation are strictly kinetic and do not seem to affect either the mechanism of the hydrolytic reaction or the composition of neoformed smectites.

An optimized thermal extraction system for preparation of water from fluid inclusions in speleothems

The fluid inclusions present in speleothems (secondary mineral deposits formed in caves) are a relict sample of the parent seepage water from which the speleothem was deposited and determination of their composition can solve the palaeotemperature equation for the precipitation of speleothem carbonate. We have extracted fluid inclusions using a newly-designed thermal vacuum extraction method in stalagmites and stalactites from Nerja Cave, Southern Spain. Optimal conditions were found to involve heating samples crushed to 0.8-2mm for 3 hours at 300-400oC. Waters extracted from modern aragonitic speleothem samples produced results on the Meteoric Water Line defined by modern infiltration and other waters associated with the cave and hence demonstrate an absence of fractionation effects. This successfully demonstrates the utility of the method.

Natural acid sulphate alteration in bentonites (Cabo de Gata, Almeria, SE Spain)

Abstract This study is located on the bentonite outcrop of ‘El Toril’ (Cabo de Gata, Almeria, Spain), where acid solutions have resulted in an alteration front that has caused physical, chemical and mineralogical changes in the bentonite material. The main objective is to assess these changes and to draw a model of evolution of this front in the outcrop. The most important chemical and mineralogical changes observed consist mainly of the loss of mass in the bentonite, the formation of kaolinite, the precipitation of jarosite and alunite from leachates and important changes in the rare earth element distribution pattern. No sulphurs have been identified in the pyroclastic rock at the top of the outcrop, but the presence of dissolution marks and the high concentrations of As, Pb and V in the rock overlying the bentonite point towards the existence, in principle, of these sulphurs and suggest that meteoric alteration is mainly responsible for the genesis of the outcrop.