J. M. González López

Mineral quantification in sepiolite-palygorskite deposits using X-ray diffraction and chemical data

Abstract A quantification technique useful in analysis of clay-rich deposits containing sepiolite and/or palygorskite has been developed by combining diffractometric and chemical data and represents a considerable improvement on classic quantification based on X-ray diffraction. The required data are: intensity ratios between certain diffraction peaks of calcite-dolomite and sepiolitepalygorskite- smectites, and also the CaO, MgO, K2O, Al2O3 and SiO2 contents. The validity of this method was tested by analysis of artificial mixtures of pure phases of the minerals found in such deposits.

TEM study of mineral transformations in fired carbonated clays: relevance to brick making

Abstract This study uses transmission electron microscopy (TEM) and analytical electron microscopy (AEM) supported by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the mineralogical and textural changes produced in carbonated clays by firing. Sample bars were prepared using raw clays composed of quartz, illite and carbonates with minor amounts of smectite, chlorite, feldspars and Fe oxides. The raw samples were then fired at temperatures of between 800 and 1050°C. The XRD data show that increases in firing temperature result in dehydroxylation of clay minerals, carbonate decomposition and the formation of Ca-bearing silicates (e.g. gehlenite, wollastonite, pyroxenes and anorthite). The sizes of the Ca-silicate crystals make the use of the SEM inappropriate since they lie below the resolution threshold. However, TEM/AEM do provide the required textural and compositional characterization, revealing that there is a broad range of pyroxene compositions, some of which resemble fassaite, and that Ca/Mg ratios increase with temperature. The TEM also shows significant dehydroxylation and vitrification of the clay-rich matrix at T of ~800°C. Observed mineralogical and textural changes probably occurred in a system with a local disequilibrium much like small-scale, high-temperature metamorphic reactions (i.e. pyrometamorphism). The importance of these results is that they enable the selection of more appropriate raw clay composition and firing dynamics (temperature, firing duration and cooling rate) for both the brickmaking industry and brick conservation in the field of cultural heritage.

Transmission electron microscopy study of illitization in pelites from the Iberian range, Spain : Layer-by-layer replacement?

A sequence of interstratified illite-smectite (I-S) and illite in Paleozoic pelites and metapelites from the Iberian Range, Spain, was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The fine-grained matrix of diagenetic pelites is composed of I-S with sequences of illite- and smectite-like layers in a given sample. The Reichweite (R) values as determined by XRD and averaged over heterogenous I-S layer sequences increase with increasing grade, apparently continuously, in sharp contrast with TEM observations of other published sequences. Changes in I-S sequences along layers are rarely observed. In the higher-grade diagenetic pelites, I-S coexists with illite. Each I-S phase has a composition similar to that of illite, implying unique Al-Si distributions in contrast to smectite and muscovite. Selected area electron diffraction (SAED) patterns of I-S and illite are diagnostic of 1Md polytypism. Anchizonal metapelites consist of larger packets of well-crystallized muscovite, with SAED patterns corresponding to a two-layer polytype.The continuous sequence of changes studied by TEM in I-S sequences and lateral transitions among these units is consistent with illitization by layer-by-layer replacement, although other processes are possible also. Replacement of individual layers probably occurs via fluids at reaction interfaces, in contrast to solid-state reactions, sensu strictu. The transition from the diagenetic to anchizonal rocks (transition in textures and formation of muscovite-2M1) occurred via dissolution/crystallization, however, presumably by tectonic stress. XRD and TEM data imply a consistent prograde trend in the sequence, the XRD data denning the average, long-range Reichweite ordering sequence, whereas the TEM data define the short-range layer sequences.

Genesis of kaolinite from Albian sedimentary deposits of the Iberian Range (NE Spain): analysis by XRD, SEM and TEM

Abstract The kaolinite from Albian sedimentary deposits (Escucha and Utrillas Formations) of the Iberian Range (Spain) have been investigated. This research has shown the presence of different types of kaolinites (detrital and diagenetic) along with micaceous phases in these deposits. Detrital kaolinites show anhedral morphology, low crystallinity and a degree of ordering as well as the presence of interstratified smectite layers. They constitute the matrix of the claystones and siltstones and were probably formed as a consequence of intense weathering processes in the source area during the warm period of the early Cretaceous. Diagenetic kaolinites have been recognized in the sandstones and siltstones, with kaolinite growing between ‘expanded’ mica flakes and vermiform and euhedral kaolinite forming the matrix. They have euhedral morphologies, high crystallinity and a high degree of ordering. They grew in situ as a response to incipient diagenesis by K-feldspar dissolution and/or organic acid-rich fluids derived from the maturation of organic matter in shales.

Variations in the chemistry of smectites from the Calatayud Basin (NE Spain)

Abstract Smectites of sedimentary series from a playa lake system from Calatayud Basin have been studied by XRD and TEM chemical microanalyses. Clay particle microanalyses show complete continuity between dioctahedral aluminous and trioctahedral magnesian smectites. Aluminous smectites have been classified as montmorillonites and ferribeidellites, both with compositional variability. They represent detrital phases resulting from weathering in the source area. Compositionally-intermediate smectites (beidellite-saponite and montmorillonite-stevensite) seem to correspond to weighted mean compositions of di- and trioctahedral phases stacked together in the same particle. They represent intermediate transitional stages from detrital to authigenic smectites in more distal basin facies. Trioctahedral smectites, although called saponites and stevensites, are actually random mixed-layer kerolite-Mg-smectite with a high percentage of expandable layers. They represent authigenic clays formed from solutions with high pH and Mg content as a result of evaporative concentration. The variability in composition of smectites, both within the same sample and amongst different samples, is a possible consequence of the heterogeneity in the local chemical environment, and so, equilibrium may be only reached locally in a large variety of microsystems.

Mineralogical and trace element composition of clay-sized fractions from Albian siliciclastic rocks (Oliete Basin, NE Spain)

Abstract Mineralogical and geochemical techniques have been used to determine the role of minerals in controlling the trace element composition of a set of clay-sized fractions from Albian siliciclastic rocks in NE Spain. These clay-sized fractions are composed of kaolinite and illite, minor quantities of quartz, and accessory heavy minerals. Kaolinite has a smaller crystal size than illite, accounting for its relative concentration in these fractions. The mm-sized heavy minerals are rutile, Fe-Ti- and Fe-oxides, zircon, pyrite, cassiterite, monazite and xenotime. Geochemical data indicate that most of the trace elements are relatively concentrated in clay-sized fractions, except for Zr, Hf, Y and HREE. Statistical treatment shows three different associations in the clay-sized fractions: (1) Rb, Cs and Ba with clay phyllosilicates, especially illite; (2) REE with Th, Y and P phases; and (3) Sc, Cr and V with Ti- and Nb oxides. Therefore, these data do not support the dominant REE control by clay mineralogy that other authors have reported.

Mineralogy and geochemistry of Devonian detrital rocks from the Iberian Range (Spain)

Abstract Two profiles in Devonian marine deposits have been studied, consisting of pelites, subgreywackes, greywackes and quartzites. Quartz and clay minerals are major components and feldspar and calcite are minor ones. Phyllosilicates in the fine fractions are kaolinite and illite; kaolinite has a high degree of ordering; illite is predominantly of a 1Md polytype, with low Na content and poor crystallinity and has a phengitic composition in greywackes, whereas in pelites it is muscovitic in composition. Both phyllosilicates may be inherited from a source area with intensive weathering processes, although illite may also be a diagenetic phase. These mineral characteristics indicate that the Devonian rocks did not reach the anchizone boundary in their post-depositional evolution. The chemical composition of pelites and subgreywackes reveals a high degree of chemical maturity. Chondrite-normalized REE patterns indicate a higher degree of weathering of these Devonian sediments than of Post-Archaean Australian Shales (PAAS), possibly as a consequence of sedimentary recycling processes. The REE patterns of the Devonian rocks in addition to the high Th/Sc, La/Sc and Th/Co ratios suggest a felsic composition of the primitive source area, probably a K-rich granite.

Geochemical, and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena, central Pyrenees (Spain): evidence for their diagenetic environments

Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light δ18O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more 18O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in δ13C of micrites represent primary secular trends, according to published δ13C variations. The δ13C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.