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En el presente trabajo se estudian los procesos de carbonatación de frústulas de diatomeas y la consecuente formación de calizas/dolomías diagenéticas, en estratos de diatomitas marinas (Messiniense, sección de Níjar) y lacustres (Turoliense, sección de Tresjuncos). Se realiza un estudio mineralógico y petrológico para conocer el reemplazo de ópalo A por calcita o dolomita, considerando los factores que favorecen este reemplazo en ambos ambientes. Los datos obtenidos son relevantes para la investigación del uso de diatomitas en la captura y alma­cenamiento de dióxido de carbono a través de su carbonatación. Los procesos pseudomórficos de carbonatación de las valvas se llevaron a cabo mediante el reemplazo directo del ópalo A por calcita o dolomita y la cementación de la microporosidad. La perfecta reproducción de las valvas indica un mecanismo de reacción con acoplamiento de disolución-precipitación en la interfase de reacción. Se interpreta que la organización de las microesferas de ópalo A, dentro de las valvas, condicionaría la organización de complejos amor­fos de varios iones o nanopartículas de carbonatos amorfos, que posteriormente recristalizarían a calcita o dolomita. En las diatomitas lacustres, el proceso de carbonatación de las valvas fue favorecido por la materia orgánica proveniente de la gran cantidad de biota (anfibios, crustáceos, insectos, plantas... etc.) que incorporan. En este ambiente, los procesos de sulfato-reducción durante la degradación de la materia orgánica, generaron CO2, que junto con cambios de pH y salinidad facilitaron los reemplazos. Es posible que las carbonataciones de las valvas fueran coetáneas a la precipitación de calcita y dolomita que fosiliza la biota y por lo tanto, diagenéticas tempra­nas. En las diatomitas marinas, el proceso desencadenante del reemplazo de las valvas fue la formación de capas lenticulares y nódulos opalinos, ya que al disolverse los microfósiles calcáreos que las diatomitas incluían, se liberó CO2 y Ca. Pequeñas variaciones de pH en un entorno alrededor de 9, y la presencia de sulfatos facilitarían el reemplazado durante la diagénesis por enterramiento.

This paper describes and interprets the mineral and facies assemblages that occur in carbonate–evaporite shallow lacustrine deposits, considering the importance of the processes pathway (i.e. dolomitization, gypsum calcitization and silicification). The Palaeogene deposits of the Deza Formation (Almazán Basin, central‐northern Spain) are selected as a case study to determine the variety of physicochemical processes taking place in carbonate–evaporite shallow lakes and their resulting diagenetic features. Dolostones are the predominant lithology and are composed mainly of dolomite with variable amounts of secondary calcite (5 to 50%), which mainly mimic lenticular gypsum (pseudomorphs). Five morphological types of dolomite crystal were identified as follows: dolomite tubes, dolomite cylinders, rhombohedral dolomite, spheroidal and quasi‐rhombohedral dolomite, and cocoon‐shaped dolomite. The dolomite cylinders and tubes are interpreted as the dolomitized cells of a widespread microbial community. The sequence of diagenetic processes started with growth of microlenticular interstitial gypsum in a calcareous mud deposited on the playa margin mudflats, and that sometimes included microbial sediments. Immediately following growth of gypsum, dolomite replaced the original calcite (or possibly aragonite) muds, the microbial community and the gypsum. Partial or total replacement of gypsum by dolomite was related mainly to the biomineralization of endolithic microbial communities on gypsum crystals. Later calcitization took place under vadose, subaerial exposure conditions. The development of calcrete in distal alluvial settings favoured the release of silica and subsequent silicification on the playa margin mudflats. Stable isotope compositions of calcite range from −9·02 to −5·83‰ δ13CPDB and −7·10 to 1·22‰ δ18OPDB; for the dolomite, these values vary from −8·93 to −3·96‰ δ13CPDB and −5·53 to 2·4‰ δ18OPDB. Quartz from the cherts has δ18OSMOW values ranging from 27·1 to 31·1‰. Wide variation and relatively high δ18OSMOW values for dolomite indicate evaporitic and closed hydrological conditions; increased influx of meteoric waters reigned during the formation of secondary calcite spar., This research was funded by the Spanish Ministry of Economy and Competitiveness, projects CGL2011‐27826‐CO2‐02 and CGL2014‐54818‐P.

The volcanic rocks of the Cabo de Gata region have been transformed by hydrothermal alteration, forming in some areas, highly silicified and feldspatized rocks. Various samples from outcrops of unaltered and hydrothermally altered volcanic rocks were studied by conventional transmitted-light microscopy to determine their genesis (magmatic, inherited or neoformed) and scanning electron microscope cathodoluminescence to define their spectral features. The magmatic quartz shows emission bands at 400, 440, and 480nm., and these bands disappear or decrease in the secondary hydrothermal quartz, characterized also by an intense emission band at around 570–580 nm, and another around 460 nm. The inherited magmatic plagioclase and neoformed anorthoclase have some similar emission bands and their genesis is not marked by cathodoluminescence features. The magmatic K-feldspars (sanidine), have emission bands at around 425, 440, and 490 nm, constituting a strong blue emission (420–500 nm). This emission is wea...

This study reports the mineralogical and geochemical variations of surface saline deposits and their substrates along an arid valley with an ephemeral river system (Spanish: rambla) affected by pollution from the now closed Cabezo de San Cristobal mine. This mine, which is close to Mazarron, Spain, once produced Pb, Zn, Ag and Fe ores. Saline efflorescences and crusts were found on piles of tailings and on the mudflats surrounding acidic ponds associated with the mine. These were composed of jarosite (10–60%), haematite (up to 20%) and gypsum (5–20%), and the kieserite (20–40%) and halotrichite (5–40%) mineral groups. In the rambla deposits downstream of the mine, the mineralogy of the saline efflorescences was similar to that recorded upstream: gypsum up to 20%, thenardite 5–20%, halite 20–40% and bloedite 20–40%. However, the substrate was very different, being mainly composed of jarosite (40–60%), haematite (5–20%) and goethite (5–20%); these minerals were never found upstream of the mining area, where carbonates predominated. Downstream of the mine, the rambla deposits contained large quantities of Fe (158,251 ppm), Pb (2,720 ppm), Zn (3,064 ppm), Sb (240 ppm) and As (1,092 ppm), along with Co (29 ppm), Ce (323 ppm), V (112 ppm), Th (23 ppm) and Nb (19 ppm) (all figures are maxima). Upstream, these contents were insignificant. Neither were large quantities found in the actual stream water (only Cu, V and Zn were detected, and then at concentrations of

Casa Montero is a mining complex located outside Madrid (Spain), dated from the Early Neolithic (c. 5400-5000 cal bc). An area of some 4 ha has been investigated and some 4000 shafts recorded, of which 324 have been excavated. The characterization of its raw flint materials and the establishment of its diagnostic features are indispensable in the reconstruction of the distribution of the mine's products beyond the immediate site. This work reports the geological study of the mine's Miocene flint layers and their petrological characterization. Archaeological samples from the mine's shafts were classified according to macroscopic features and petrological characteristics.

The thermoluminescence (TL) properties of a well-characterised natural α-cristobalite from Lanzarote (Canary Islands, Spain) have been studied. The natural blue emission (at 400 nm) of this silica polymorph of quartz reveals the appearance of three groups of components peaked at 150, 230–240 and 370 °C, which could be associated, respectively, with (i) structural defects (similar to quartz), (ii) the reversible phase transition from α-cristobalite to β-cristobalite and (iii) electron recombination with unstable holes trapped at oxygen vacancies next to Al ions linked to the formation of β-cristobalite. Similar to quartz, the induced TL (ITL) glow curves display four maxima, peaked at 90, 110, 180 and 220 °C, which could be respectively associated with (i) oxygen vacancies, (ii) recombination of electrons with (H3O4)° centres that can act as hole traps, (iii) [GeO4]− centres that are stabilised with monovalent cations (H+, Li+ or Na+) and (iv) [AlO4]° hole-like centres that are created when alkali ions are...

The Miocene distal alluvial fan and palustrine deposits of the Madrid Basin (Paracuellos de Jarama area) were examined to establish the sequence of its pedogenic–diagenetic processes and the main factors controlling them. In this area the diagenetic processes operated not only on carbonates, as commonly studied, but also in high magnesium clays and opaline cherts. This paper provides a dynamic model for saline–alkaline lake margins that complements the existing freshwater palustrine model. Three sections (BER, PEL and UPC) containing limestones, sepiolite, dolostones and opaline cherts were examined; these represent the entire transition from alluvial fan deposit to lake environment. The alluvial fan deposits (PEL section) show the most complex sequence of processes. After a weak edaphic imprint, the primary mudstone and siltstones are affected by silicification (opaline levels formed mainly by selective replacement of sepiolites) and later dolomitization. Several types of dolomite are present, rounded crystals, spherules and globular bodies being the most characteristic. In the ponds situated at the foot of the alluvial fans or in interfan areas, sepiolite precipitated within free water bodies (BER section) and an intense pedogenesis led to the formation of palustrine sepiolite deposits. Later, the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging. In the shallow lakes (UPC section), palustrine limestones and sepiolite were deposited. The carbonates are partially affected by silicification, the sepiolite becoming completely silicified. The opaline levels clearly preserve the structure of the limestones and sepiolite host rocks. All the opaline levels show local aging processes. The silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies. In the distal alluvial fan sediments of the PEL section, the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures. The silicification environment was therefore not easy to determine. In the BER and UPC section, the main silicification took place in groundwater-influenced environments but evidence was seen of cements and silicifications in vadose environments. In all study settings, the overlapping of pedogenic, vadose and groundwater processes was controlled by changes in the position of the water table. Four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements. The majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin. In all the studied environments, the sepiolite deposits were preferentially silicified; their fibrous structure and high absorption capacity helping to retain interstitial fluids. Moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments. The dissolution of the sepiolite caused an increase in silica in the interstitial fluids, favouring silicification and ruling out the need of a biogenic source. The replacement of limestones by silica is usually driven by variations in pH and salinity, which are very common in surface environments such as shallow lakes and soils.

Root-associated stalactites (rootsicles) in Galeria da Queimada lava tube have a mineralogical composition and developmental association with microbes that render them unique. Samples were examined by X-ray diffraction, micro-Raman spectrometry and scanning electron microscopy/X-ray energy-dispersive spectroscopy. Three types of rootsicle were defined: incipient; hard (white and red); and black spongy. The incipient rootsicles still contained rotten organic material and showed the beginning of mineralization by allophane. The white hard and black spongy types were also composed of allophane, while the red hard type was composed of hydrous ferric oxi-hydroxide minerals (HFO). The allophane and HFO in the andisol covering the cave roof precipitated out of the dripwater running along the roots to form the studied rootsicles. All three types of rootsicle showed black layers, coatings, spots or patches composed of manganese oxide minerals and, occasionally, hisingerite (iron (III) phyllosilicate). An alternation of organic precipitation caused by filamentous bacteria and inorganic precipitation (the latter facilitated by pH changes in the dripwater and the cave's temperature) built up both the porous and compact rings observed in the white and red hard rootsicles. The largely straight filaments seen in the porous rings of the white hard rootsicles may be indicative of the previous presence of Leptothrix spp., while the helical morphologies seen in the red hard rootsicles may be indicative of that of Gallionella spp. The manganese oxide minerals detected probably formed via microbial activity. This study reflects the important role of filamentous bacteria in rootsicle formation, independent of their mineralogy., This work was funded by Project CGL2011-27826-CO2-02 from the Spanish Ministry of Science and Innovation., R.D. was supported by a CSIC JAE-Predoc grant co-financed by the European Social Fund (ESF)., Sí

El presente trabajo estudia los diferentes tipos de sílex que aparecen en la Formación Calizas de Oriñón (Aptiense superior-Albiense inferior), en un zona concreta de la costa oriental de Cantabria (Liendo-Castro Urdiales), debido a su gran abundancia y espectacularidad. Las rocas que los incluyen son principalmente biocalcarenitas (wackestones/packstones de pellets, fragmentos de equinodermos, ostreidos, foraminíferos y espículas calcáreas o silíceas calcificadas), siendo el ambiente de depósito marino de plataforma abierta, y existiendo un hardground regional, donde las silicificaciones son diferentes. La fuente de la sílice proviene de la calcitización o disolución de las espículas de esponjas silíceas. Se determina que la gran mayoría de sílex (mosaicos de cuarzo micro-microcriptalino y calcedonita principalmente), se forman por la silicificación selectiva de estructuras biogénicas, siendo las galerías de Thalassinoidespreferentemente silicificadas, debido a que en su relleno existe mayor proporción de materia orgánica y mayor porosidad y permeabilidad. En las calizas que soportan el hardground además de las silicificaciones selectivas sobre fósiles (rostros de belemnites, ostreidos y equínidos), la sílice se acumula de forma importante en huellas de alimentación y rellenos de perforaciones, dando lugar a su vez a costras sobre bioturbaciones y perforaciones que a pesar de su importancia no han podido ser clasificadas. La silicificación de todas las estructuras biogénicas, originó inicialmente fases opalinas y tuvo lugar durante la diagénesis temprana cuando la oxidación de la materia orgánica estaba todavía activa, conservándose en los Thalassinoides silicificados formas (filamentos y cocoides) que podrían ser consideradas microbianas. La transformación ópalo-cuarzo fue temprana, preservándose dichas formas. En las galerías silicificadas de Thalassinoides los procesos diagenéticos son diferentes a los de los sedimentos carbonáticos marinos que las incluyen, y al resto de estructuras biogénicas silicificadas, existiendo en estas galerías neoformación de dolomita y calcita, en un microambiente cerrado donde las condiciones cambian de oxidantes a reductoras.

During the Miocene the Madrid basin was a closed basin in which a succession of alluvial and lake deposits were replaced by different types of crusts (mainly calcretes, dolocretes, and silcretes). In a geographically restricted area to the south, Mg-clay deposits were precipitated in ponds on the sand flats. A discontinuous nodular and stratiform Mg-rich chert appears at the top of the Mg-clay deposits and displays visual, mineralogical, and petrological characteristics that are very different from the other silcretes of the Madrid basin. As revealed by X-ray diffraction, the cherts consist mainly of quartz and moganite, which are commonly found in Magadi-type cherts. The petrological characteristics reveal that the chert consists of a groundmass crossed by several networks of quartz veins. The groundmass is formed by mosaic quartz with different sizes of crystals (usually less than 20 µm) and length-slow chalcedony. The presence of gridwork extinction patterns, the differentiation of silica flakes, and the preservation of the micromorphological characteristics of the Mg-clay deposits indicate an early replacement by quartz and moganite. The quartz networks show veins of different shapes (rectilinear, irregular, branched, and V-shaped), sizes (from a few microns to several millimeters), and textures (mainly megaquartz, microquartz, and length-slow chalcedony). The V-shaped veins, and some irregular and branched veins, are considered to be a consequence of the silicification of cracks in the host rocks. The other veins represent silica filling cracks that formed by shrinkage during the various stages of chert formation. Many macroscopic attributes (e.g., some kinds of cracks), mineralogical features (the presence of moganite), and petrological characteristics (e.g., rectilinear or gridwork orientation of quartz crystals) are similar to those of Magadi-type cherts. In spite of this, the origin and the environment of formation of this Mg-rich chert, which crystallized by silicification of Mg-clays during deposition of silcrete, are very different from those of the typical Pleistocene Magadi cherts of African lakes. A pronounced shrinkage during silicification in a zone just below the water table and the presence of the moganite are the two factors that caused these Mg-rich cherts to appear similar to Magadi-type cherts.

Four regressive sequences are present in the opaline rocks and related deposits of the Miocene Intermediate Unit of the Madrid Basin. The sequences consist of silty mudstones and argillaceous opals, separated by transitional facies. The silty mudstone consists mainly of dioctahedral smectites, whereas the argillaceous opal is principally opal-CT and variable amounts of sepiolite. In the transitional facies, lamina of dioctahedral smectite co-exist with neoformed opal-CT and sepiolite. Petrological and geochemical features (major, trace and REE elements) indicate that the opaline levels and the transitional facies are related and are a consequence of silcrete formation in an argillaceous playa deposit. The isocon method was used to calculate changes in element concentration associated with silcrete formation. The geochemical data suggest silicification in an arid environment. The silcrete profile occurs four times, possibly as a result of highstand–lowstand fluctuations of the lake level caused by climatic changes. Structures and cements in the silcretes indicate that, although silicification may have commenced at the top of a groundwater table, it continued in the unsaturated zone above the water table.

Unusual mineralized and silicified carbonate-rich geodes were found hosted in Lower Triassic red mudstones in Central Spain. From their borders to their centres, the geodes display (a) a millimetric carbonate crust, (b) a quartz rim, (c) massive dolomite, (d) gold-bearing iron-rich infills (hematite laths and goethite with up to 7000 ppm of gold) and (e) calcite cements that sometimes seal the central hollow. Textural evidence indicates that the geodes were originally anhydrite, which was subsequently replaced by quartz and dolomite. The resultant porosity from this replacement, or by later dissolution, has been filled by epithermal gold-bearing iron-oxide hydroxides, romanechite and calcite.δ13C values indicate the participation of meteoric waters in an environment which was characterized by both a sub-desert climate and a temperate–tropical climate. Oxygen signatures reflect very variable temperatures for all minerals, with the exception of calcite, which appears to have precipitated at

In the eastern sector of the Betic Cordillera outcrops of the Internal Subbetic display chert lenses in the upper parts of oolitic limestones (Camarena Formation, Mid-Jurassic). The stratigraphy indicates a sedimentary record related to synsedimentary tectonics (tilting). The sediments were deposited in different tectonic blocks, at varying depths. The Camarena Formation is mainly made up of shallow platform oolitic limestones and was deposited on a `pelagic' swell, a shallow isolated platform that developed far from continental areas. The chert lenses appear in those stratigraphic units deposited in the lowermost sunken fault blocks which display biosiliceous facies overlying the Camarena Formation. The chert mainly consists of megaquartz, formed by replacement of the oolitic host rocks without an opaline precursor. The part of the host that has been most affected by the silicification is the carbonate cement. The petrological features indicate a slow replacement from solutions which were poor in silica and cations. Comparing the textures and fabric of the cherts and their host rocks, it is deduced that the silicification took place after compaction, cementation, fracturing and local dissolution of the oolitic limestones. The stable isotopic data (δ18O, δ13C) obtained from the chert, oolitic host rock and overlain biosiliceous rocks display significant differences. The host rocks (Camarena Formation) and overlying biosiliceous rocks have isotopic signatures that are typical of marine environments or shallow-marine diagenesis. However, the chert isotopic values are interpreted to suggest meteoric water participation in the silicification process. The timing of the chertification is suggested to have been between the Bathonian (beginning of the deposition of the biosiliceous source facies) and the Aptian (first appearance of this chert as clasts in breccia beds). In palaeogeographic terms the optimum timing would have been during emergence (i.e. between the Bathonian and Tithonian). The exposed areas could have acted as recharge areas of meteoric water into the regional aquifer system. The involvement of meteoric water in the silicification processes gives new support to the hypothesis that pelagic swells were subjected to episodic exposure.

The biostratigraphy and lithological composition of diatomaceous marls have been defined in 93 outcrops of the Guadalquivir Basin. They are included in different lithological formations affected by diachronous tectonic events. The diatom assemblages are dominated by the following upwelling and oceanic species: Thalassionema nitzschioides, Thalassiothrix longissima, Crucidenticula nicobarica, Crucidenticula punctata, Denticulopsis hustedtii and several taxa from Coscinodiscus and Thalassiosira genera. Benthic diatoms are scarce. Three diatom sedimentation episodes have been defined. The Lower Episode includes the Cestodiscus peplum and Coscinodiscus lewisianus Zones and is assigned to the middle Langhian-early Serravallian. After a period of no diatom occurrence (the Coscinodiscus gigas var. diorama Zone is missing), the Middle Episode is defined by the Craspedodiscus coscinodiscus and the Actinocyclus moronensis Zones and covers the late Serravallian-early Tortonian. The Upper Episode corresponds to the Thalassiosira yabei Zone and is assigned to the middle Tortonian. The diatomaceous marls are comprised of opal A (10–85%) (mainly diatoms), calcite (5–80%) (principally foraminifers and nannoplakton), clay minerals (5–40%) and terrigenous quartz (

This paper reports a detailed study of the calcrete and silcrete profiles in the Miocene detrital deposits in the western area of Madrid, at the boundary of two main sedimentary units. The aims of this work were to better understand the pedogenic and diagenetic environments in which these profiles formed and to determine the cause(s) of their enrichment in uranium. Calcrete and silcrete duricrusts are characteristic features of closed continental basins in semiarid climates; this paper discusses the significance of duricrusts as indicators of important change in such basins. The detailed macromorphological, micromorphological, and geochemical study of three duricrust profiles revealed the sequence of pedogenic, vadose, and groundwater processes responsible for their formation. During the first stage of their development, carbonate laminae formed a white ¿grill-like¿ structure within the detrital parent materials. The microstructure and macrostructure of the carbonate, which includes alveolar septal structures and needle-fiber calcite, indicates the important role of roots and their associated microorganisms in calcrete formation. Early silicification occurred in the pedogenic-vadose environment affecting the detrital parent material, roots, and calcretes, forming an early silcrete defined by opaline glaebules and silica rhizoliths. The detailed preservation of the cells in the silicified roots denotes the early replacement of root organic matter. The green or green-yellowish fluorescence of the silicified root structures under short-wavelength UV shows their preferential enrichment in uranium. Calcitization and silicification coexisted in the pedogenic vadose environment, leading to several reversible replacements of calcite and silica. Later, the rise of the water table promoted silicification under phreatic conditions, as indicated by the good preservation of the texture of the detrital host rocks and calcretes. Other silcrete textures, such as ovoidal opaline accumulations, intraclasts produced by autobrecchification, and vadose silica cements, indicate later vadose environments, and consequently variations in the water table. The geochemical features of the calcretes and silcretes (major, minor, and rare earth elements) were inherited from their parent materials. The rare-earth-element patterns of some silcretes show them to have a positive Ce anomaly, suggesting that oxidizing conditions reigned during their formation. The good correlation between silica and uranium suggests that the silica phases acquired uranium through the direct silicification of roots that had fixed uranium from organic matter. This study shows that calcrete-silcrete duricrusts provide detailed information regarding the processes occurring in semiarid continental basins. In the studied basin, roots played a key role in both the development of the duricrust profiles and their enrichment in uranium. These duricrusts provide important information for understanding the overall stratigraphy of the studied basin and its large-scale sequential evolution., This work was funded by Projects CGL2011-27826-CO2-01 and CGL2011-27826-CO2-02 from the Spanish Ministry of Science and Innovation.

Varieties of chalcedony (calcedonite, quartzine, and lutecite) and other optical micro- and cryptocrystalline textures of quartz and opal from Miocene silcretes of the Madrid Basin were studied using a Thermo Fisher DXR Raman microscope. The microscope has a point-and-shoot Raman capability of 1 µm spatial resolution and was coupled with a standard optical microscope. Our results show that all the varieties of chalcedony can have a composite Raman spectrum of both quartz and moganite. The spectra are independent of the chalcedony origin by ageing, direct replacement, and cementation processes. Moganite was absent only in some calcedonite cements. The presence of moganite is independent of the surface sedimentary setting of the host rocks in which the silcretes are formed

13 pages, figures and tables statistics., The ochreous speleothems examined here were obtained from a lava tube on Terceira Island (Azores) and show compact mineralized areas intermixed with zones in which bacterial structures are evident. Bacterial layers of filament-like structures are common throughout the deposits but differences in microstructure among the different speleothems were observed, reflecting a broad morphological range of deposits. The structures and minerals detected in the speleothems betray their biogenic origin. Gallionella and Leptothrix were the two most frequently observed morphotypes and probably the main contributors to speleothem formation. However, DGGE analysis indicated the presence of another bacterial population (with a predominance of proteobacteria) that could also contribute to iron hydroxide–oxide precipitation. The sheaths of Leptothrix cells and stalks of Gallionella cells were associated with large amounts of extrapolymeric substances (EPS), which play a role in biomineralization processes. Independently of the taxa present, mineral deposits were composed of poorly ordered Si-rich ferrihydrite, a typical mineral phase of biogenic Fe precipitates. In this microscopy study of interrelationships between mineral precipitates and associated microorganisms and their structures, we attribute bacteria to have an important role in constructing these speleothems. Through their metabolic activity, these bacteria cause the precipitation of ferrihydrite but their mineral structures could also act as nucleation points for passive mineral precipitation. Finally, the build-up of bacterial mineral structures and later cementation processes seems responsible for the formation of mineral layers that confer consistency to the speleothem. Our findings point to bacterial activity as the main factor determining speleothem structure and formation.

The xenoliths of dunites and other ultramafic rocks (peridotites ,wherlites..etc) are generated by contact metamorphism from sedimentary rocks, proyecto FEDER "Aspectos educativos del volcanismo canario Ministerio Educación Y Ciencia proyecto BTE2002-04017-Co2-1

7 páginas, 3 figuras, 3 tablas., Green opals have been identified in veins and vugs in hydrothermalized basalts from the area of Taganana (Tenerife). The aim of this paper is to document the petrology and geochemistry of these opals and to describe the formation process from amorphous components. Green opals were studied using optical microscopy, powder X-ray diffraction, electron microprobe, atomic absorption spectroscopy and induced coupled plasma mass spectrometer. Our textural and X-ray powder diffraction analyses indicate that green opals are composed of a major groundmass fraction of crystalline (opal CT and celadonite) and amorphous components (opal A and Fe-(hydro)oxides), which host minor siderite and dolomite crystals. The results indicate green opals were formed during the hydrothermal process from a Fe-rich silica amorphous phase (pseudoglass) which aging and alteration transformed into a groundmass of celadonite and opaline mixture rich in Fe-(hydro)oxides. The opaline–celadonite groundmass, or its amorphous precursor, served as a substrate for the growth of siderite and dolomite crystals., The research was supported by the project PB-98-0668-CO2-01 and the project about the research of terrestrial models for the exploration of Mars of the Centro de Astrobiologia (CSICINTA, Associated to the NASA Astrobiology Institute).

Repetitive sequences of carbonate deposits, occurrence include in the lower part of the Paleogene Carbonate Unit (northeast border of the Madrid Basin), have been studied, defining regressive lacustrine sequences and early diagenetic processes. Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray observations, and X-ray diffraction analyses joint to isotopic studies (d18O and d13C) have been used to characterise the facies and environments. The sequences consist of a lower uncemented carbonate mud unit, calcitic or dolomitic in composition, and an upper carbonate unit (arenites, bioclastic limestones, and microbial laminated limestones with cherts). Visual features (vertical prismatic structures, fissures, massive nodulization, rhizoliths, brecciation) and microscopic features (micrite micronodules, vug porosity, circumgranular cementation, gypsum lenticular crystals) outline pedogenic processes. These features, found locally in bioclastic and microbial laminated limestones or in dolomite uncemented muds, define, respectively, palustrine limestones or dolocretes. Facies analysis allows us to define several lacustrine sub-environments (basinal, littoral, eulittoral and supralittoral) and characterises different types of shallowing upward lacustrine sequences, either with or without subaerial exposure. The isotopic values of the bioclastic and microbial laminated limestone (from 5.77xto 6.78xfor d13C, and from 5.25xto 5.53xfor d18O) and those of uncemented calcitic muds (from 5.80xto 7.01xfor d13C, and from 4.98xto 5.58xfor d18O) establish that both types of carbonates precipitated in equilibrium with meteoric waters. In the palustrine carbonate deposits, the d13C values suggest a strong organic contribution. The dolomitization that only affected the uncemented carbonate muds is early interpreted because of the structural and compositional characteristics of the dolomite (micro-rhombic dolomicrite, nearly stoichiometric and poorly ordered), and because the calculated average D18O dol–cal for calcitic and dolomitic uncemented muds is about 6x. The d18O isotopic values indicate that the dolomite precipitated from water that was slightly more enriched in heavy isotopes than the calcite, because of an increase in evaporation rates. Nodules and nodular levels of cherts occur in the upper units of littoral and eulittoral sequences, probably as a consequence of the existence of microbial mats that could include the silica source. According to their mineralogy (Opal CT and quartz/moganite) and structures (double nodules, lamination and bioturbation), three types of chert are described (TB, MB and WO). These types define three general stages of silicification during the early diagenesis, from the recently buried to the postcompactation of carbonate deposits. The d18O values of quartz show that the silicification and ageing of opaline phases occurred from meteoric waters, which were lighter than the calcite or dolomite precipitating waters. The cherts included in palustrine limestones show d18O values of quartz that record more evaporated waters than those of the general stages of silicification.

The volcanic caves of Algar do Carvão (a pit with a complex genesis) and of Branca Opala (a lava-tube cave), differ from other caves on the island of Terceira by having speleothems composed only by opal. This kind of speleothems, in volcanic caves, suggests that the silica origin can be related to hydrothermal activity. The objective of this study is to determine the speleothem isotopic composition in δ¹⁸OSMOW and δDSMOW to determine their geochemical signature and conditions of formation. The studied samples from Algar do Carvão pit are stalactites and flowstones/gours that have been formed in subaerial conditions, while those from the Branca Opala lava tube have been defined as underwater silica stromatolites. dD data of speleothems from both caves are relatively similar (−102.0 ± 10.5‰ in Branca Opala and −103.7 ± 11.1‰ in Algar do Carvão) and it can be assumed that the fluids involved in the silica deposition may be similar. Moreover the δ¹⁸OSMOW data show that the Algar do Carvão speleothems have higher values (δ¹⁸OSMOW 39,4 ± 0,3‰), relatively to the Branca Opala stromatolites (δ¹⁸OSMOW 35,2 ± 0,8‰). The diagenesis that affected the opal A may explain, in part, the observed differences. However, other genetic factors are also influencing such as the fact that Algar do Carvão stalactites are formed in sub aerial conditions, under the influence of evaporation processes, while Branca Opala speleothems were formed in underwater conditions. However, it is also possible, that the water that originated the Branca Opala speleothems was 10,7 °C warmer than the water that formed the Algar do Carvão stalactites.Las cuevas de Algar do Carvão (una sima volcánica de génesis compleja) y Branca Opala (un tubo volcánico) se diferencian de otras cuevas de la isla de Terceira por tener espeleotemas formados sólo por ópalo. Esta composición, y el hecho de ser cuevas volcánicas, hacen pensar que el origen de la sílice pueda estar ligado a la actividad hidrotermal. El objetivo de este estudio es determinar la composición isotópica en δ¹⁸OSMOW, y en δDSMOW de los espeleotemas de sílice, para determinar su impronta geoquímica y las condiciones de formación. Las muestras estudiadas de la sima de Algar do Carvão son estalactitas y revestimientos de paredes, que se han formado en ambiente subaéreo, mientras que, las estudiadas en el tubo volcánico de Branca Opala, han sido definidas como estromatolitos silíceos subacuáticos. Los datos de δD de los espeleotemas de ambas cuevas son relativamente parecidos (−102,0 ± 10,5‰ en Branca Opala y −103,7 ± 11,1‰ en Algar do Carvão) y se puede asumir que los fluidos involucrados en su formación puedan ser similares. Por otra parte, los datos del δ¹⁸OSMOW revelan que los espeleotemas de Algar do Carvão presentan valores relativamente más altos (δ¹⁸OSMOW 39,4 ± 0,3‰) que los estromatolitos opalinos de Branca Opala (δ¹⁸OSMOW 35,2 ± 0,8‰). La diagénesis que sufre el ópalo A puede explicar, en parte, las diferencias observadas, pero, otros factores genéticos también están influyendo, como pueda ser el hecho de que las estalactitas de Algar do Carvão se formaran en condiciones subaéreas, con la influencia de la evaporación, mientras que los espeleotemas de Branca Opala fueran subacuáticos, o bien, que las aguas de precipitación de los espeleotemas de Branca Opala fueran hasta 10,7 °C más calientes que las de Algar do Carvão.

La composicion quimica de elementos mayores es utilizada para analizar las caracteristicas composicionales y sedimentologicas en 24 afloramientos de sedimentos miocenos con diatomeas de la Cuenca del Guadalquivir. En funcion de las biozonas de diatomeas, se consideran tres episodios de sedimentacion diatomitica (Inferior, Medio y Superior) que reflejan un medio de sedimentacion de alta productividad biogenica ocasionada por corrientes ascensionales costeras ("coastal upwelling"). Los grandes margenes de variacion de los tres oxidos mas importantes SiO2 (12.18-78.00), CaO (1.60-43.73) y Al2O3 (2.76-10.32) definen la gran variacion que presentan los tres componentes mayoritarios de estos sedimentos: Opalo A, calcita y minerales de la arcilla-terrigenos. A traves de calculos normativos en funcion de la composicion mineralogica de las muestras y de los analisis de difraccion de RX, se ha definido el porcentaje de silice biogenica. El estudio de los maximos y minimos de silice biogenica ha puesto de manifiesto su relacion con tipos dominantes de diatomeas, la estructura de la roca y la posicion geografica dentro de la Cuenca. La proyeccion de las muestras en los diagramas triangulares SiO 2 biogenica-Al2O3-CaO ha revelado que el Episodio Superior es el que manifiesta mayor proporcion relativa de Al2O32 biogenica/CaO versus Al2O3 han demostrado que existe frecuentemente una asociacion preferente entre el plancton calcareo y los minerales de arcilla-terrigenos. La proyeccion de las muestras en el diagrama Fe2O3/TiO2 versus Al2O3/Al2O3+FeO, junto con la pendiente de la recta de regresion Fe (y) - Al (x) indican que todas las series se pueden considerar de plataforma continental existiendo zonas con mayor o menor influencia del continente. La relacion MnO/Al 2O3, se ha utilizado para medir la tasa sedimentacion; los valores obtenidos (entre 1,7 y 180 mm/103 anos) concuerdan con los hallados en otras zonas de corrientes ascensionales costeras, encontrandose diferencias en funcion de la estructura de la roca, las diatomeas dominantes y los episodios considerados. (arcillas-componentes terrigenos). Los diagramas SiO

Electron microscope observations of opaline silcretes found in the playa of a Miocene shallow lake reveal new opal CT microstructures. These microstructures are made up of fibres, filaments or rods, which are formed, in tum by coalesced microspheres or microhemispheres (0,1-0,2 µ diameter). They are up to 10 µ in length although larger ones could exist. They are straight, bent, wavy or even hairpin shaped. These microstructures comprise part of the opaline mass of the silcretes and are very similar to both neogenic opaline structures which are formed by bacterial intluence and to silicified biofilms. Therefore microbial mediation could exist in the genesis of this opal CT, although a possible inorganic origin by silicification of sepiolite is also considered.

The lithological composition and biostratigraphy of diatomaceous marls have been defined in 93 outcrops of the Guadalquivir Basin. The diatomaceous marls display a wide variety of lithological types, mainly comprised of opal A (10-85 %) (mainly diatoms and to a lesser extent silicoflagellates, spicules and radiolarians), calcite (5-80 %) (principally foraminifers, nannoplankton, sparite, micrite and microspar), c1ay minerals (5-40 %) (smectite in greater quantities than illite; and kaolinite in small quantities) and terrigenous quartz ( Thalassiosira nitzshoides, Thalassiotrix longuissima and Thalassionema baci//aris . Benthic diatoms are scarce but they have great species diversity. However, in sorne outcrops the genus Delphineis is more frequent thus indicating environments c10ser to shore areas. Three episodes based on Barron's (1985) low latitude zonation have been defined. The Lower Episode includes Cestodiscus peplum and Coscinodiscus lewisianus Zone and is assigned to the upper Langhian- lower Serravalian. After a period of no diatom occurrence (e. gigas var. diorama Zone is missing), the Middle Episode is defined by Craspedodiscus coscinodiscus and Actinocyclus moronensis Zones and covers the latest Serravalian-early Tortonian. The Upper Episode corresponds to Thalassiosira yabei Zone and it is assigned to middle Tortonian. The fundamental types of dilutants in diatomic sedimentation (biogenic calcareous, terrigenous and c1ay minerals), do not vary throughout the three episodes but there are great variations in the proportions. The coastal upwelling regime produces tongue-like patterns of sedimentation and the sedimentary record displays different changes of facies. The Upper Episode, where the c1ay-terrigenous dilutants predominate over calcareous dilutants and Delphineis appear in greater quantities, is interpreted as having been formed in an environment closer to the shore than other episodes.

In Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro- and cryptocrystalline quartz, with local chalcedonic quartz (both length-fast and length-slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro- and cryptocrystalline quartz, length-slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg-rich environment. Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica-rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.

The radiolarite sequence of the middle Subbetic (Betic cordillera, southern Spain) is composed of three main lithologies: radiolarite, siliceous mudstone and marl, and pelagic limestone with radio larians. Virtually all transitional lithologies among the three listed exist. Clay minerals, calcite, and hematite make up the radiolarite impurities. Smectite, illite, and locally kaolinite are the clay minerals present, smectite always being most abundant. No radiolarite bed with more than 85% silica has been found; the total SiO2 content ranges from 61 to 85%. Frequently the compact appearance of the radiolarite results from a high carbonate content. The ratios Si/Si + Al + Fe + Ca, Si/Si + Al + Fe, and Al/Al + Fe + Mn are calculated and used to compare the beds of couplets in rhythmic sequences and to obtain the relative distance of sections from their terrigenous source. These three chemical indices do not give consistent results. Fe enrichment of radiolarite relative to siliceous mudstone is probably related to the low mean sedimentation rate of these sequences from the Subbetic (3.5 mm/1,000 y). Pelagic settling in a basin with irregular bottom topography and with areas topographically protected from sediment input is proposed for deposition of these deposits. Productivity cycles and terrigenous dilution gave rise locally to rhythmically bedded sequences, usually composed of radiolarite mudstone alternations. Locally, contour and distal dilute turbidity currents reworked and deposited the sediment, and gave rise to thin sequences of layered sedimentary rocks and to laminated beds. Deposition above a relatively shallow CCD is deduced, and a water depth shallower than previously thought is suggested for these deposits, which are considered as the deepest water deposits of the Betic External Zones.

15 páginas, 1 figura, 3 láminas., [ES] El análisis sedimentológico y petrológico del conjunto de los sedimentos calcáreos y las rocas silíceas continentales del Mioceno medio, al sur de Villaluenga, da nuevas perspectivas a la interpretación y génesis de estas rocas. Las principales facies calcáreas encontradas han sido: 1) micritas complejas con laminación difusa (¿criptoalgar ?), pseudomorfos de cristales lenticulares de yesos e indicios de posibles diatomeas; 2) biomicritas con alta proporción de charáceas, moluscos y ostrácodos que se disponen con estructura laminada por acumulación de los componentes esqueléticos en niveles; 3) biomicritas con los mismos componentes anteriores, pero desorganizados, existiendo además en algún caso diatomeas; 4) pseudoesparitas y microesparitas con pseudomorfos calcáreos de cristales lenticulares de yeso; 5) margas sepiolíticas, El ambiente de sedimentación es, en general, muy superficial, de baja salinidad, calcáreo, altamente productivo, con abundante vegetación (a veces diatomeas) y ocasionalmente llegan a existir períodos de exposición subaérea. Todos los tipos de facies, desde las lacustres; poco profundas a las que manifiestan subaerismo, presentan fenómenos de silicificación, aunque éstos pueden ser diferentes. Se determina que hay una silicificación penecontemporánea, discontinua que origina sólo minerales de la sílice y que puede estar relacionada con la disolución de las frústulas de diatomeas incluidas en la caliza. Muy tempranamente en la diagénesis, pero posterior a la anterior, tiene lugar una nuevas silicificación más compleja que ha originado los niveles continuos ópalo-arcillosos, homogéneos o con estructuras Liesegang, y en los que, además de los minerales exclusivamente silíceos, se ha producido Sepiolita. Se piensa que esta segunda silicificación se origina en la interfase agua-sedimento, cuando en las pequeñas depresiones, en las que se origina la sedimentación lacustre-palustre, el espesor de la lámina de agua es muy pequeña, llegando, a veces , a la desecación. Las cementaciones silíceas sobre diferentes porosidades de la roca (fenestral, pequeños huecos kársticos, etc.) indican que aunque las dos silicificaciones son muy tempranas, posteriormente hay removilizaciones que originan cementaciones y posiblemente silicificaciones tardías de cuarzo que hacen muy compleja la historia diagenética de la roca calco-silícea., [EN] The petrological and sedimentological analysis Middle Miocene carbonate sediments and siliceous rocks, outcropping North Toledo, provides new ideas on the interpretation of these lacustrine and paludine sedimentation. The main lithofacies found are: 1) micrite with crytalgal (?) laminite, calcite gypsum pseudomorphs, and trace of diatoms; 2) biomicrite with varying amount of lacustrine mollusks, ostracodes and charophytes, which form laminar structure; 3) biomicrite with the same skeletal components (charopytes, ostracodes, etc.) and sometimes diatoms but disorganized, 4) pseudospar and microspar calcite with lenticular gypsum pseudomorphs; 5) sepiolitic marls.

This article reports the results of a petrological and geochemical study of silica nodules from Quaternary alluvial fallS of the Olaroz Salar. The nodules are mainly constituted by C-T opal, d. (101) spacing to 4,11 A, implying a high-disorder; this is commonly observed in opa! rocks formed in near surface environments, without burial and heating. The chemical composition of the nodules is anomalous when compared with other typical silica-rocks formed in arid continental closed basin, without any eruptive environments (for instance the Cuenca del Tajo in Spain). The siliceous nodules do not have the characteristics of the silcretes and are not associated to the evaporitic facies of the salar. Their genesis is probably related to contamination of groundwater of the alluvial fan with high-silica thermal water. The C-T opal would thus precipitate by either displacing tbe terrigenous host-rock or replacing previous ulexite nodules.