Antonio Sánchez-Navas

Carbonate and Phosphate Precipitation by Chromohalobacter marismortui

The ability of Chromohalobacter marismortui to precipitate carbonate and phosphate minerals has been demonstrated for the first time. Mineral precipitation in both solid and liquid media at different salts concentrations and different magnesium/calcium ratios occurred whereas crystal formation was not observed in the control. The precipitated minerals were studied by X-ray diffraction, scanning electron microscopy and EDX, and were different in liquid and solid media. In liquid media aragonite, struvite, vaterite and monohydrocalcite were precipitated forming crystals and bioliths. Bioliths accreted preferentially close to organic pellicles, whereas struvite preferentially grows in microenvironments free of such pellicles. Magnesian calcite, calcian-magnesian kutnahorite, “proto-dolomite” and huntite were formed in solid media. The Mg content of the magnesian calcite and of Ca-Mg kutnahorite also varied depending on the salt concentration of the culture media. This is the first report on bacterial precipitation of Ca-Mg kutnahorite and huntite in laboratory cultures. The results of this research show the active role played by C. marismortui in mineral precipitation, and allow us to compare them with those obtained previously using other taxonomic groups of moderately halophilic bacteria.

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).

Nacre and false nacre (foliated aragonite) in extant monoplacophorans (=Tryblidiida: Mollusca)

Extant monoplacophorans (Tryblidiida, Mollusca) have traditionally been reported as having an internal nacreous layer, thus representing the ancestral molluscan condition. The examination of this layer in three species of Neopilinidae (Rokopella euglypta, Veleropilina zografi, and Micropilina arntzi) reveals that only V. zografi secretes an internal layer of true nacre, which occupies only part of the internal shell surface. The rest of the internal surface of V. zografi and the whole internal surfaces of the other two species examined are covered by a material consisting of lath-like, instead of brick-like, crystals, which are arranged into lamellae. In all cases examined, the crystallographic c-axis in this lamellar material is perpendicular to the surface of laths and the a-axis is parallel to their long dimension. The differences between taxa relate to the frequency of twins, which is much higher in Micropilina. In general, the material is well ordered, particularly towards the margin, where lamellae pile up at a small step size, which is most likely due to processes of crystal competition. Given its morphological resemblance to the foliated calcite of bivalves, we propose the name foliated aragonite for this previously undescribed biomaterial secreted by monoplacophorans. We conclude that the foliated aragonite probably lacks preformed interlamellar membranes and is therefore not a variant of nacre. A review of the existing literature reveals that previous reports of nacre in the group were instead of the aragonitic foliated layer and that our report of nacre in V. zografi is the first undisputed evidence of nacre in monoplacophorans. From the evolutionary viewpoint, the foliated aragonite could easily have been derived from nacre. Assuming that nacre represents the ancestral condition, as in other molluscan classes, it has been replaced by foliated aragonite along the tryblidiidan lineage, although the fossil record does not presently provide evidence as to when this replacement took place.

Amorphous Ca-phosphate precursors for Ca-carbonate biominerals mediated by Chromohalobacter marismortui

Although diverse microbial metabolisms are known to induce the precipitation of carbonate minerals, the mechanisms involved in the bacterial mediation, in particular nucleation, are still debated. The study of aragonite precipitation by Chromohalobacter marismortui during the early stages (3–7 days) of culture experiments, and its relation to bacterial metabolic pathways, shows that: (1) carbonate nucleation occurs after precipitation of an amorphous Ca phosphate precursor phase on bacterial cell surfaces and/or embedded in bacterial films; (2) precipitation of this precursor phase results from local high concentrations of PO43− and Ca2+ binding around bacterial cell envelopes; and (3) crystalline nanoparticles, a few hundred nanometres in diametre, form after dissolution of precursor phosphate globules, and later aggregate, allowing the accretion of aragonite bioliths.

Microbial mediated formation of Fe-carbonate minerals under extreme acidic conditions

Discovery of Fe-carbonate precipitation in Rio Tinto, a shallow river with very acidic waters, situated in Huelva, South-western Spain, adds a new dimension to our understanding of carbonate formation. Sediment samples from this low-pH system indicate that carbonates are formed in physico-chemical conditions ranging from acid to neutral pH. Evidence for microbial mediation is observed in secondary electron images (Fig. 1), which reveal rod-shaped bacteria embedded in the surface of siderite nanocrystals. The formation of carbonates in Rio Tinto is related to the microbial reduction of ferric iron coupled to the oxidation of organic compounds. Herein, we demonstrate for the first time, that Acidiphilium sp. PM, an iron-reducing bacterium isolated from Rio Tinto, mediates the precipitation of siderite (FeCO3) under acidic conditions and at a low temperature (30°C). We describe nucleation of siderite on nanoglobules in intimate association with the bacteria cell surface. This study has major implications for understanding carbonate formation on the ancient Earth or extraterrestrial planets.

SEQUENTIAL KINETICS OF A MUSCOVITE-OUT REACTION : A NATURAL EXAMPLE

Abstract A natural example of sequential kinetics for the muscovite dehydration reaction in highly deformed mylonitic gneisses is analyzed. Studied textures consist of deformed pegmatitic muscovite crystals surrounded by fibrolitic sillimanite tightly intergrown with biotite and with potassium feldspar in the pressure shadows. Potassium feldspar, andalusite, biotite, and quartz appearing in the crystal core constitute the products of muscovite breakdown produced by topotactic replacement of muscovite within a single crystal. Non-isochemical decomposition of muscovite is proposed in which diffusion of K+ outward from the crystal took place along the muscovite interlayer and shear planes parallel to (001). The chemical potential gradient calculated for K capable of producing the texture observed within the pegmatitic crystals, results not only from the overstepping of the muscovite-out reaction but also from the difference in free energy between andalusite and sillimanite under P-T conditions where andalusite is stable. The overall reaction within the andalusite stability field consists of the transformation 1 Ms + 1 Qtz → 1 And + 1 Kfs + 1 H2O and of the indirect replacement 0.25 Sil → 0.25 And. Andalusite nucleation and growth took place completely inside muscovite, whereas most of the potassium feldspar was produced outside, preferentially at pressure shadows, after outward intracrystalline diffusion of K+ and fibrolitic sillimanite dissolution in the matrix. The ratelimiting step of this reaction process was initially the migration rate of the muscovite-andalusite and muscovite-potassium feldspar interfaces. However, it was followed by a diffusion-controlled step. The change in the relative rates of the reaction mechanisms was due to the coarsening of the reaction products, the low amount of energy and time required for non-reconstructive transformations (periodic bond chains of the tetrahedral sheet of muscovite are inherited by potassium feldspar and those of the octahedral sheet by andalusite), the magnitude of the heat flow associated with a possible contact metamorphic episode, and the slow diffusion of K+ within the crystal. Intracrystalline diffusion took place through a double monolayer of absorbed molecules of water (~10 Å) localized at microcleavages produced by basal slip in the interlayer level, instead of a proper lattice diffusion process. The derived rate law at constant P and T (considering the cases of 2 kbar and 605 °C and 3 kbar and 639 °C) for the diffusion-controlled process gives a time span of about 10 000 years for the growth of these metamorphic textures, which seems to be a reasonable estimate for a contact metamorphic event.

Andalusite-sillimanite replacement (Mazarrón, SE Spain): A microstructural and TEM study

Abstract At Mazarrón, SE Spain, dacitic lavas of the Neogene Volcanic Province contain numerous xenocrysts and xenoliths with abundant andalusite that displays variable degrees of transformation to both fibrolite and coarse sillimanite. At the onset of replacement, andalusite dissolves along grain boundaries and (110) cleavage planes, probably assisted by fluids or melts. At the same time, fibrolite crystallizes together with plagioclase, cordierite, and graphite in newly formed embayments or in the adjacent matrix. With increasing reaction progress, fibrolite needles coalesce into coarser sillimanite prisms, and direct topotactic replacement of andalusite is observed. The mutual crystallographic orientation of andalusite and sillimanite obtained from TEM investigation deviates slightly from the topotactic relationship proposed in the literature (cAnd || cSil, aAnd || bSil, bAnd || aSil). The two lattices are rotated by ~2.5° around aAnd (= bSil). With this misorientation, the structurally equivalent {032}And and {302}Sil planes, which exhibit the smallest misfit between the two lattices, become parallel. Macroscopic interfaces with such orientations are rare. Microscopically, however, decomposition of faces into {032}And || {302}Sil and {110} facets are common. The mutual crystallographic orientation of the reactant and the product phases is, therefore, controlled by lattice misfit minimization. The prismatic shape of the final coarse sillimanite crystals, however, is controlled by kinetic factors. The reaction seems to proceed fastest parallel to the octahedral Al chains resulting in the development of crystals elongated along the c axis. The high activation energy and the large overstepping of the equilibrium temperature required for the transformation are probably responsible for the large differences in reaction progress observed in the samples from Mazarrón.

Transformation of Andalusite to Kyanite in the Alpujarride Complex (Betic Cordillera, Southern Spain): Geologic Implications

The crystal growth features of andalusite and the transformation of andalusite to kyanite allow recognition of pre-Alpine and Alpine tectonometamorphic histories in the metapelites of the Alpujarride Complex. Two types of mineral segregations occur in relation to the andalusite → kyanite transformation. One type of segregation consists of a mantle of muscovite around a pre-Alpine andalusite chiastolite core that is partially transformed to fine-grained Alpine kyanite within a matrix particularly rich in biotite + quartz. The second type of segregation consists of Alpine muscovite + kyanite domains that form after dissolution of pre-Alpine andalusite + biotite domains within the matrix. The formation of these two types of mineral segregations involves similar reactions between the fluid and the local mineral assemblage. These reactions progress simultaneously. Each of them acts as source (or sink) for the ions, and intermediate mineral phases are consumed (or produced) by the other reactions, so that a combination of individual reactions produces the andalusite → kyanite net reaction. This reaction is catalyzed by the muscovite and biotite of the matrix, whose dehydration provides the chemical driving force needed to break Si-O bonds for the andalusite → kyanite transformation. Surfaces perpendicular to F-type {110} faces of the andalusite chiastolites associated with layeritic crystal growth mechanisms constitute fast reaction pathways for the andalusite → kyanite reaction. The transformation of pre-Alpine andalusite to Alpine kyanite constitutes the first solid textural evidence of the existence of a polymetamorphic history in the rocks of the Alpujarride Complex (Betic Cordillera, southern Spain).

Crystal Growth of Inorganic and Biomediated Carbonates and Phosphates

where the O-H covalent bond in the oxyacid makes carbonate salts moderately soluble. The most common metal cations forming carbonate minerals are Ca2+, Mg2+, Mn2+, Fe2+, Pb2+, Sr2+, Co2+, Ni2+, Zn2+, Cd2+ and Cu2+. Continental and marine waters are enriched in Ca and Mg and are known to be saturated with respect diverse Ca-Mg carbonates such as calcite (CaCO3), aragonite (CaCO3) and dolomite (MgCa(CO3)2).[3] The concentration of the phosphate species (H3PO4, H2PO4, HPO4, and PO4) is also a function of pH, and their respective oxyacids are stronger than those of carbonic acids.[2] Because of this, phosphates are more stable than carbonates at low pH (<5). Chemical composition of phosphate minerals is more variable than that of carbonate minerals, and crystalchemical substitution of the PO4 group by CO3, OH–, F–, Cl–, etc, is rather common. In addition, numerous metals as Ca2+, Mg2+, Fe2+, Na+, Sr2+, Ce3+, La3+, Ba2+, and Pb2+ can be incorporat‐ ed into the structure of the phosphate minerals.

The Formation of Manganese Dendrites as the Mineral Record of Flow Structures

Manganese oxide patterns known as “pyrolusite dendrites” are explained as the result of the mineral record of flow structures in porous media. This interpretation is supported by a) fractal characterization, b) Mn profiles across the mineral pattern and the matrix rocks, c) structures reminiscent of flow pattern observed at the scale of the grain size of the matrix rocks, d) the lack of long-range order of the manganese oxide particles and e) the existence of clays and quartz grains of colloidal size intimately linked to the manganese particles. This interpretation also explains other fractal and non-fractal patterns accompanying the beautiful treelike fractal forms associated with these manganese oxide patterns.