C. Jiménez de Cisneros

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.

Palaeogeographic significance of black pebbles (Lower Cretaceous, Prebetic, southern Spain)

Abstract The presence of black pebbles in carbonate stratigraphic sections constitutes a criterion for identifying emergence episodes, which are significant to palaeographic reconstructions and to location of sedimentary cycle boundaries. Three genetic models can be inferred from the stratigraphic, petrologic and geochemical study of black pebbles that occur in sediments of the Lower Cretaceous of the Prebetic (southern Spain). In the first model, black pebbles are the product of redeposition in tidal and lacustrine environments from layers rich in organic matter (carbonaceous layers). In the second model, blackening takes place as a result of very mature organic matter, mainly generated from terrestrial plants (possibly burnt ones), selectively staining some of the intraclasts of the storm layers in intertidal environmnts. In the third model, applicable only to pedogenic and lacustrine environments, black pebbles, to a large extent, originate from a redeposition of pedogenic carbonate materials affected by natural forest fires, where blackening was caused by a sudden and rapid increase in temperature. The third hypothesis is supported by data obtained from an experiment in which samples underwent progressive heating to monitor the blackening process and accompanying change in isotopic composition. In all cases, the effects of storms and/or currents and waves caused erosion and redeposition.

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

Calcretes from a palaeosinkhole in jurassic palaeokarst (subbetic, southern Spain)

Abstract Calcretes have been recognized in the bottom of a palaeokarstic sinkhole etched into Early Jurassic carbonates from the Subbetic (Southern Spain). These are 3 to 10 cm thick and they show an internal structure in which three units can be distinguished. The lower unit is composed of alternate irregular bands of unfossiliferous micrite and sparry calcite. In the micrite, several features point out the activity of endolithic microorganisms as fungi and bacteria. The middle unit, which has a laminated structure with palisade calcite spar and some vadose silt is interpreted as a speleothem and, finally, the upper unit shows calcrete pisoids and rhizocretions. The isotope ratio analysis gives results in which δ 13 C ranges from −10.1‰ to −10.9‰ and δ 18 O ranges from −5.7‰ to −7.0‰. These values are very similar to those from palaeosols of other ages and localities, which is an important argument favouring emersion stages, with continental deposition in an island with an arid or semi-arid climate. Nevertheless, these calcretes outcrop in association with a palaeokarstic relief reflecting wet climate before the calcrete formation.

Geochemistry of speleothems from Jurassic palaeokarst (Subbetic, southern Spain)

Abstract Speleothems growing on Jurassic palaeokarstic cavity walls from the Subbetic (Southern Spain) have been geochemically studied. These speleothems contain more than one generation of low-magnesian calcite (LMC) cements which grow into the cavities. The simplest case is composed of two generations of precipitate: the first cement is a non-luminescent (NL) radiaxial fibrous calcite (RFC) with a Fe Mn ratio between 2.8 and 6.1; the second cement is a zoned dull luminescent (ZDL) columnar, palisade calcite (CPC) with a lower Fe Mn ratio. The more complex case is composed of several generations of precipitate (NL-RFC and ZDL-CPC), with interbedded mechanically deposited internal sediments. Petrographic and isotopic data of the host rocks suggest that they have been formed in the marine phreatic environment. In contrast, the isotopic composition of the internal sediment and of non-skeletal grains of the host rocks suggest that they underwent diagenetic alteration in the marine-meteoric mixing zone. The isotopic composition of cements (in speleothems and complete infillings of small cavities) shows a typical meteoric “inverted J” trend. The alternation of the NL and ZDL cement stages in the speleothems is interpreted to be a result of hydrodynamic changes in the cementation environments. These changes can be related to sea-level fluctuations which affected the Subbetic continental margin during the Jurassic.

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.

Ion Exchange Behavior of the Febex Bentonite: 2. Batch Experiments and Geochemical Modeling

Ion exchange experiments have been performed with the FEBEX bentonite. Five grams of dry powder of this clay were put inside dialysis bags, which were located inside PFA reactors filled with 125 ml of water of a given chemical composition (Moody, Grimsel, and bentonitic-granitic type waters). The reactors containing the clay powder/water mixture were heated to different temperatures (from room temperature up to 80°C) along a time span ranging from 1 day to 1 year. Water was renewed according to a prescribed schedule but not the clay, which remained in place for the whole extent of each test. After each water renewal, major cations, silica, total inorganic carbon, and pH were analyzed. At the end of each test, the exchange complex and CEC of the bentonite were measured. These experiments have been modeled with a conveniently modified version of the EQ3/6 software package where ion exchange reactions were formulated as half reactions and added to its database. In general, model results are in fairly good agreement with experimental data, especially in the case of dissolved cations. Computed values of exchanged concentrations also match the measurements, although in some cases they deviate from them. The fact that the numerical results reproduce the observed patterns of exchange tests indicates that the adopted geochemical conceptual model is appropriate. Some features of the geochemical evolution of these tests also take place at the “mock-up” and “in situ” FEBEX tests.

Climate Variability During the Middle-Late Pleistocene Based on Stalagmite from Órganos Cave (Sierra de Camorra, Southern Spain)

Paleoclimatic reconstruction in southern Spain has been investigated in this study using stable isotope analyses from Organos Cave (Southern Spain). A combination of δ18O and δ13C analyses with uranium-series dating and δD values of the water extracted from stalagmite fluid inclusions makes it possible to obtain an isotopic record during 340–370 ky BP (MIS10). The profile shows the climatic evolution in the area and we can see a colder early stage with small fluctuations that change to warmer conditions at the end, coinciding with the start of the interglacial stage MIS9.