Carlos Rodriguez-Navarro

Carbonate and silicate phase reactions during ceramic firing

Mineralogical, textural and chemical analyses of clay-rich materials following firing, evidence that initial mineralogical differences between two raw materials (one with carbonates and the other without) influence the tex- tural and mineralogical evolution of the ceramics as T increases from 700 to 1100° C. Mineralogical and textural changes are interpreted considering local marked disequilibria in a system that resembles a small-scale high- T meta- morphic process ( e.g., contact aureoles in pyrometamorphism). In such conditions, rapid heating induces significant overstepping in mineral reaction, preventing stable phase formation and favoring metastable ones. High- T transfor- mations in non-carbonate materials include microcline structure collapse and/or partial transformation into sanidine; and mullite plus sanidine formation at the expenses of muscovite and/or illite at T ‡ 800° C. Mullite forms by mus- covite-out topotactic replacement, following the orientation of mica crystals: i.e., former (001) muscovite are ^ to (001)mullite. This reaction is favored by minimization of free energy during phase transition. Partial melting followed by fingered structure development at the carbonate-silicate reaction interface enhanced high- T Ca (and Mg) silicates formation in carbonate-rich materials. Gehlenite, wollastonite, diopside, and anorthite form at carbonate-silicate interfaces by combined mass transport (viscous flow) and reaction-diffusion processes. These results may add to a better understanding of the complex high- T transformations of silicate phases in both natural ( e.g., pyrometamor- phism) and artificial ( e.g., ceramic processing) systems. This information is important to elucidate technological achievements and raw material sources of ancient civilizations and, it can also be used to select appropriate clay com- position and firing temperatures for new bricks used in cultural heritage conservation interventions.

Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Abstract Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2DXRD), and transmission electron microscopy coupled with selected area electron diffraction (TEMSAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (101̅4)calcite display the following topotactic relationships: (1) (12̅10)calcite//(110)CaO; (2) (1̅104)calcite┴ (110)CaO; (3) (1̅018)calcite//(110)CaO; and (4) (01̅14)calcite┴(110)CaO; with [841]calcite//[11̅0]CaO in all four cases. At this stage, the reaction mechanism is independent of PCO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy Eα = 176 ± 9 kJ/ mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

Role of particulate matter from vehicle exhaust on porous building stones (limestone) sulfation

Abstract This work, for the first time, experimentally demonstrates the relationship between motor vehicle emissions and the decay of ornamental calcareous stone, by means of sulfation processes (the well-known phenomenon of Black-crust formation). The critical catalytic effects of carbon (soot) and metal-rich particles from vehicle exhaust result in the acceleration of the rate of fixation of atmospheric SOZ to form gypsum on the limestones (calcarenites) used to build Granada Cathedral (Spain). The analysis of particulate matter deposited on the building (carbonaceous and metal-rich particles), as well as of emissions from both leaded-gasoline and diesel motor vehicles confirms that the origin of the particulate matter found in the surface of decayed building stones from Granada Cathedral is consistent with having been contributed by motor vehicle exhaust. Experimental data indicate the role played by this particulate matter in the fixation of atmospheric SOZ as sulfates (gypsum) on calcareous materials in the presence of humidity. We have also experimentally demonstrated that there is a close relationship between the composition of the particulate matter and the fixation rates of the SO 2 in the form of sulfate: (a) diesel engine exhaust, which is primarily composed of soot and metallic particles bearing Fe and Fe-S as major elements and of Cr, Ni, Cu, and Mn as trace elements, plays the largest part in the catalytic oxidation rates of SO 2 ; (b) the emissions from gasoline engines, composed of minor quantities of soot and high concentrations of Pb- and Br-bearing particles, cause a lower rate of SO 2 fixation as gypsum on limestones. From these experimental findings, a new hypothesis is proposed concerning the sulfation of the limestones.

Precipitation and Growth Morphology of Calcium Carbonate Induced by Myxococcus Xanthus: Implications for Recognition of Bacterial Carbonates

Abstract It is thought that morphologies of bacterial carbonates can be used to identify microbial fossils and/or precipitates in sediments and rocks. This study shows that calcite and vaterite formed in a gel medium in the presence of Myxococcus xanthus display a range of morphologies that depend on whether the bacteria are live or dead. Metabolic activity of the bacteria induced: (1) aggregates of calcified bacteria formed at maximum supersaturation; (2) vaterite spheres (final growth stage of dumbbell fibrous-radiated aggregates); and (3) dipyramid- and disphenoid-like calcite crystals (combination of {011} and {0001} forms). Morphologies (2) and (3) developed at a lower supersaturation and are typically found in gel-like media. Dipyrimidal-like calcite crystals were also obtained abiotically in gel medium. Dead M. xanthus cells induced heterogeneous precipitation of calcite with rhombohedral morphologies at low supersaturation. A growth mechanism resulting from self-assembly of calcium carbonate nanocrystals may account for the observed morphologies, crystal microstructure, and crystallite size measurements. All of the above-mentioned morphologies of bacterial carbonate have been observed in other laboratory experiments and in continental and marine environments. However, all of them have also been produced abiotically, with the exception of calcified bacterial cells. This may make it more difficult to identify bacterial activity in the rock record. Nonetheless, bacterially induced alkalinization appears to be a prerequisite for the development of spherulitic and dipyramid- or disphenoid-like forms in natural mucilaginous biofilms and microbial mats. The morphologies reported here may facilitate the recognition of early and recent marine and continental microcrystalline bacterial carbonates and cements.

Mechanism of leached layer formation during chemical weathering of silicate minerals

The dissolution of most common multicomponent silicate minerals and glasses is typically incongruent, as shown by the nonstoichiometric release of the solid phase components. This results in the formation of so-called surface leached layers. Due to the important effects these leached layers may have on mineral dissolution rates and secondary mineral formation, they have attracted a great deal of research. However, the mechanism of leached layer formation is a matter of vigorous debate. Here we report on an in situ atomic force microscopy (AFM) study of the dissolution of wollastonite, CaSiO 3 , as an example of leached layer formation during dissolution. Our in situ AFM results provide, for the first time, clear direct experimental evidence that leached layers are formed in a tight interface-coupled two-step process: stoichiometric dissolution of the pristine mineral surfaces and subsequent precipitation of a secondary phase (most likely amorphous silica) from a supersaturated boundary layer of fluid in contact with the mineral surface. This occurs despite the fact that the bulk solution is undersaturated with respect to the secondary phase. Our results differ significantly from the concept of preferential leaching of cations, as postulated by most currently accepted incongruent dissolution models. This interface-coupled dissolution-precipitation model has important implications in understanding and evaluating dissolution kinetics of major rock-forming minerals.

Influence of Substrate Mineralogy on Bacterial Mineralization of Calcium Carbonate: Implications for Stone Conservation

ABSTRACT The influence of mineral substrate composition and structure on bacterial calcium carbonate productivity and polymorph selection was studied. Bacterial calcium carbonate precipitation occurred on calcitic (Iceland spar single crystals, marble, and porous limestone) and silicate (glass coverslips, porous sintered glass, and quartz sandstone) substrates following culturing in liquid medium (M-3P) inoculated with different types of bacteria (Myxococcus xanthus, Brevundimonas diminuta, and a carbonatogenic bacterial community isolated from porous calcarenite stone in a historical building) and direct application of sterile M-3P medium to limestone and sandstone with their own bacterial communities. Field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), powder X-ray diffraction (XRD), and 2-dimensional XRD (2D-XRD) analyses revealed that abundant highly oriented calcite crystals formed homoepitaxially on the calcitic substrates, irrespective of the bacterial type. Conversely, scattered spheroidal vaterite entombing bacterial cells formed on the silicate substrates. These results show that carbonate phase selection is not strain specific and that under equal culture conditions, the substrate type is the overruling factor for calcium carbonate polymorph selection. Furthermore, carbonate productivity is strongly dependent on the mineralogy of the substrate. Calcitic substrates offer a higher affinity for bacterial attachment than silicate substrates, thereby fostering bacterial growth and metabolic activity, resulting in higher production of calcium carbonate cement. Bacterial calcite grows coherently over the calcitic substrate and is therefore more chemically and mechanically stable than metastable vaterite, which formed incoherently on the silicate substrates. The implications of these results for technological applications of bacterial carbonatogenesis, including building stone conservation, are discussed.

Formation of amorphous calcium carbonate and its transformation into mesostructured calcite

Amorphous calcium carbonate (ACC) is a key precursor of crystalline CaCO3 biominerals and biomimetic materials. Despite recent extensive research, its formation and amorphous-to-crystalline transformation are not, however, fully understood. Here we show that hydrated ACC nanoparticles form after spinodal liquid–liquid phase separation and transform via dissolution/(re)precipitation into poorly hydrated and anhydrous ACC nanoparticles that aggregate, forming a range of 1D, 2D and 3D structures. The formation of these structures appears to be achieved by oriented attachment (OA), facilitated by the calcite medium-range order of ACC nanoparticles. Both electron irradiation processes in the TEM and under humid air exposure at room temperature of the latter ACC structures result in pseudomorphs of single crystalline mesostructured calcite. While the high-vacuum/e-beam heating leads to solid-state transformation, the transformation in air occurs via an interface-coupled dissolution/precipitation mechanism. Our results differ significantly from the currently accepted model, which considers that the low T ACC-to-calcite transformation in air and during biomineralization is a solid-state process. These results may help to better understand how calcite biominerals form after ACC and offer the possibility of biomimetically preparing single crystalline calcite structures after ACC by tuning pH2O at room temperature.

TEM study of mullite growth after muscovite breakdown

Abstract Mullite (Mul) formation after high-T muscovite (Ms) breakdown has been studied in phyllosilicaterich bricks. At T ≥ 900 °C Ms dehydroxylation is followed by partial melting that triggers the nucleation and growth of Mul acicular crystals. An analytical electron microscopy study reveals that the Mul is a 3:2-type with a [6](Al1.686Ti0.031Fe0.159Mg0.134)[4](Al2.360Si1.649)O9.82 formula and an O atom vacancy of x = 0.18. This is consistent with X-ray diffraction results [i.e., unit-cell parameters: a = 7.553(7), b = 7.694(7), and c = 2.881(1) Å, V = 167.45 Å3]. The initial stage of the process resulting in Mul growth followed the balanced reaction Ms → 0.275Mul + 0.667Melt + 0.244K2O + 0.01Na2O + 0.125H2O, yielding an alkali-poor peraluminous melt. H2O with K (and Na), which are lost along the (001) planes of dehydroxylated Ms, play a significant role as melting agents. The c-axes of the Mul crystals are oriented parallel to [010]ms or to the symmetrically equivalent <310>ms zone axis, while the (120)mul or (210)mul planes are subparallel to (001)ms (TEM results). These systematic orientations point to epitaxial Mul nucleation and growth on the remaining Ms substrate, which acts as a template for Mul heterogeneous nucleation. Randomly oriented Mul growth is also observed during the late stages of the process (i.e., melt cooling). The epitaxial nature of Mul growth after dehydroxylated Ms melting minimizes the energy requirement for nucleation. In addition, the water released after Ms breakdown and the multicomponent nature of the melt enable this high-T aluminum silicate to grow at T ~ 900 °C, almost 100 °C below the SiO2-Al2O3-K2O ternary system eutectic (after a melt with an end-member Ms composition).

The mechanism of thermal decomposition of dolomite: New insights from 2D-XRD and TEM analyses

Abstract Despite being studied for more than one century, no consensus exists regarding the ultimate mechanism(s) of the thermal decomposition of dolomite [(CaMg(CO3)2]. To shed light on such a reaction, dolomite single crystals were calcined in air between 500 and 1000 °C, and in situ, in a TEM (high vacuum), following irradiation with the electron beam. In situ TEM shows that the decomposition involves the initial formation of a face centered cubic mixed oxide (Ca0.5Mg0.5O) with reactant/product orientation relationships [001]dolomite//<111>oxide, <4̄41>dolomite//<100>oxide, {112̄0}dolomite//{110}oxide, {112̄8}dolomite//{110}oxide and {101̄4}dolomite^{100}oxide~12°. This phase undergoes de-mixing into oriented crystals of Mg-poor CaO and Ca-poor MgO solid solutions upon long-term e-beam exposure. Ex situ TEM, XRD, 2D-XRD, and FESEM analyses show the formation of porous pseudomorphs made up of oxide nanocrystals with similar parent/product orientation relationships, but with limited Ca/Mg substitution (up to ~9-11%) due to de-mixing (spinodal decomposition) of the metastable (Ca,Mg)O precursor. High ion diffusivity at T > 500 °C (ex situ experiments) favors the formation of pure CaO and MgO crystals during coarsening via oriented aggregation and sintering. These results show that the thermal decomposition of dolomite is topotactic and independent of pCO2. Formation of Mg-calcite nanocrystals (up to ~8 mol% Mg) during the so-called “half decomposition” is observed at 650-750 °C. This transient phase formed topotactically following the reaction of CaO nanocystals (solid solution with ~9 mol% Mg) with CO2 present in the air and/or released upon further dolomite decomposition. With increasing T, Mg-calcite transformed into calcite, which underwent decomposition following the known topotactic relationship: {101̄4}calcite//{110}CaO and <4̄41>calcite//<110>CaO. These observations solve the long standing controversy on the mechanism of the “two-stage” decomposition of dolomite, which assumed the direct formation of calcite during the so-called “half decomposition.”

Bioconservation of deteriorated monumental calcarenite stone and identification of bacteria with carbonatogenic activity

The deterioration of the stone built and sculptural heritage has prompted the search and development of novel consolidation/protection treatments that can overcome the limitations of traditional ones. Attention has been drawn to bioconservation, particularly bacterial carbonatogenesis (i.e. bacterially induced calcium carbonate precipitation), as a new environmentally friendly effective conservation strategy, especially suitable for carbonate stones. Here, we study the effects of an in situ bacterial bioconsolidation treatment applied on porous limestone (calcarenite) in the sixteenth century San Jeronimo Monastery in Granada, Spain. The treatment consisted in the application of a nutritional solution (with and without Myxococcus xanthus inoculation) on decayed calcarenite stone blocks. The treatment promoted the development of heterotrophic bacteria able to induce carbonatogenesis. Both the consolidation effect of the treatment and the response of the culturable bacterial community present in the decayed stone were evaluated. A significant surface strengthening (consolidation) of the stone, without altering its surface appearance or inducing any detrimental side effect, was achieved upon application of the nutritional solution. The treatment efficacy was independent of the presence of M. xanthus (which is known as an effective carbonatogenic bacterium). The genetic diversity of 116 bacterial strains isolated from the stone, of which 113 strains showed carbonatogenic activity, was analysed by repetitive extragenic palindromic–polymerase chain reaction (REP-PCR) and 16S rRNA gene sequencing. The strains were distributed into 31 groups on the basis of their REP-PCR patterns, and a representative strain of each group was subjected to 16S rRNA gene sequencing. Analysis of these sequences showed that isolates belong to a wide variety of phylogenetic groups being closely related to species of 15 genera within the Proteobacteria, Firmicutes and the Actinobacteria. This study shows that the abundant carbonatogenic bacteria present in the decayed stone are able to effectively consolidate the degraded stone by producing new calcite (and vaterite) cement if an adequate nutritional solution is used. The implications of these results for the conservation of cultural heritage are discussed.