Angel La Iglesia

Zaccagnaite-3R, a new Zn-Al hydrotalcite polytype from El Soplao cave (Cantabria, Spain)

Abstract We have recently discovered significant amounts of zaccagnaite, a natural Zn-Al-CO3 hydrotalcite in the El Soplao cave (north Spain). The El Soplao zaccagnaite is speleothemic, i.e., formed in the cave, and therefore it represents a new cave mineral. The origin of El Soplao zaccagnaite is related to the diagenesis of Zn- and Al-rich ferromanganese speleo-stromatolites, where it occurs as a porefilling cement that likely precipitated at low temperature (≤ ~11 °C). In some stromatolite layers, the abundance of zaccagnaite crystals is large enough to enable their physical separation. This has allowed us to obtain its X-ray powder-diffraction pattern, infrared spectrum, and differential thermal/ thermogravimetric profiles. The cell parameters of the El Soplao zaccagnaite, refined from X-ray powder diffraction data are: a = 3.06616(1) and c = 22.6164(1) Å [α = β = 90°, γ = 120°; V = 184.139(1) Å3; Z = 3], consistent with a new trigonal polytype of zaccagnaite: zaccagnaite-3R. Besides, the El Soplao zaccagnaite shows some features previously unknown in natural hydrotalcites, such as octahedral-like morphologies and fluorescence zoning. Electron microprobe analyses revealed that the El Soplao zaccagnaite-3R has an unusual chemistry for natural hydrotalcites, as it is significantly more rich in Al (Zn2+/Al3+ = 1.6) than the hexagonal (2H) polytype (Zn2+/Al3+ = 2.0). The simplified chemical formula deduced from electron microprobe analysis is (Zn0.6Al0.4)(OH)2(CO3)0.2·0.5H2O, where C and water were calculated by stoichiometry. The carbon content calculated by stoichiometry (2.2 wt%) is in good agreement with that measured with the electron microprobe on gold-coated samples (2.5 wt%). The presence of interlayer water and CO3 groups was confirmed by thermogravimetric analysis coupled to mass spectroscopy, and by the analysis of the infrared spectrum.

The physicochemical weathering of monumental dolostones, granites and limestones; dimension stones of the Cathedral of Toledo (Spain)

Abstract Physical and chemical response to weathering has been evaluated for the building stones (granites and carbonate rocks) of the Cathedral of Toledo, Spain. Several aspects have been studied in detail: distribution of porosity and its relation to the fabric within the weathered material, chemical composition of weathering-related surficial crusts, and the order of crystallization of the soluble salts derived from the weathering processes. In addition, the study places some emphasis on the calculation of the crystallization pressures of these salts within the porous material resulting from weathering. Highest pressures are due to gypsum crystallization and, in decreasing order, to: anhydrite, mirabilite, epsomite, bloedite, thenardite and kieserite.

Occurrence of silica polymorphs nanocrystals in tuffaceous rocks, Province of the Mesa Central, Mexico, and their formation from subcritical Si-rich fluids

Abstract Cristobalite-tridymite blade nanocrystals cemented by SiO2-glass and tridymite nanocrystals fill separately SiO2-glass spherules released by explosive volcanism, in rhyolitic tuffs from the Province of the Mesa Central, Mexico. This paper presents the mineralogy of silica polymorphs, occurrence, and process of formation in the Province of the Mesa Central. The understanding of the origin of these pure SiO2-polymorphs, their association with high-temperature minerals, fractionation of magmas, and role of volatiles contribute to our knowledge on silica minerals, their technological implications and damaging health effects. Results indicate that a precursor magma, from which kyanite crystallized, partitioned into an immiscible Fe-rich magmatic liquid that crystallized Fe-cordierite, Fe-amphiboles, and fayalite and into a siliceous melt that led to low-temperature glasses of 78.22-80.01 wt% SiO2 and Si/Al ratio 4.07-5.65. Presence of amphiboles, sulfur in cristobalite-tridymite crystals and alunite suggest association of volatiles. The crystallization of silica polymorphs is associated with the dissolution of water vapor and volatiles in the precursor magma, establishing a silicate melt-water system of two critical points, one of them at pressure and temperature near the critical point of water and another close to the critical point of SiO2, and defining a critical curve between them and supercriticality at pressures and temperatures lower than the critical point of SiO2. Decreasing the ambient conditions from supercritical to subcritical would have allowed the separation of liquid and gases and the crystallization of cristobalite-tridymite and tridymite nanocrystals from the vapor phase. Cristobalite single crystals were not formed and transformation of cristobalite and trydimite to quartz did not occur. We conclude that glasses did not crystallize cristobalite or tridymite. Components in excess of the pure phases precipitated as nanoparticles of siliceous glass forming agglomerates, some containing iron hydroxides and alunogen.

The status of zaratite: investigation of the type specimen from Cape Ortegal, Galicia, Spain

Three zaratite samples from Cape Ortegal (type locality, Spain), Texas, Lancaster County (Pennsylvania) and Heazlewood (Tasmania) were analyzed by electron-probe micro-analysis (EPMA), environmental scanning electron microscopy with energy-dispersive spectrometer (ESEM-EDS), X-ray diffraction (XRD), differential thermal analysis and thermogravimetry (DTA-TG), micro-Raman and Fourier-transform infrared spectrometry (FTIR). The empirical formulae calculated from EPMA exhibit different molar ratios (Ni/CO3, Ni/OH and H 2 O/Ni), showing that these three samples referred to as ‘zaratite’ are chemically distinct; they are actually close to either anhydrous zaratite (Ortegal), nullaginite (Tasmania) or gaspeite (Pennsylvania). The XRD patterns of the three samples only validate their low-crystallinity character with variable mineral inclusions. Raman and FTIR spectra confirm large similarities among the three samples, in line with the general chemical kinship of these nickel hydroxycarbonate (± hydrate) materials. Our data suggest that the natural nickel hydroxycarbonate materials, collectively referred to as ‘zaratite’ on the basis of colour, occurrence and poor crystallinity, are in fact so diverse in terms of Ni/C/H ratios that the ‘zaratite’ term should not deserve a species status and so remains highly questionable.

A simple thermodynamic model on the cracking of concrete due to rust formed after casting

Purpose The purpose of this study is to use thermodynamic data to estimate the pressure exerted by the crystallization of iron oxyhydroxides following the equation proposed by Correns and Steinborn. Design/methodology/approach Standard free energy and molar volume data have been considered for goethite, lepidocrocite, magnetite and hematite, which are described in the literature as the most commonly found mineral phase rust constituents. Findings The studied mineral phases generate higher to lower crystallization pressure values in the following order: goethite > lepidocrocite > hematite > magnetite. The crystallization pressures calculated for these phases are in the 32-350 MPa range, which is higher than the tensile strength of concrete (of the order of 0.2-10 MPa) and thus leads to failure of the cover concrete. Originality/value The aim of this paper is to shed light on this issue by calculating the stresses generated by the crystallization of iron oxide from a supersaturated solution using thermodynamic data. A deliberately simplistic method was proposed, taking as reference the Correns–Steinborn model (Correns and Steinborn, 1939; Correns, 1949). The crystalline phases considered in this paper are those most commonly found in the literature as rust constituents, that is, goethite (α-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe3O4) and hematite (α-Fe2O3). The FeO synthetic phase was also included as a reference.

Geochemistry of reversible hydratable tephra from the Trans Mexican Volcanic Belt

Abstract Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72°C, composed of sodic and non-sodic glasses of surface activity caused by IVAl substituting in Q4(1Al) and Q4(2Al) positions, minor VAl, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H2Om in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H2Om molecules. Sodic glasses contain 71.80-77.77 wt% SiO2, are partially devitrified to crystallites (~5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (~10 μm size). Sodium-free glasses are more siliceous, with 74.84-83.88 wt% SiO2, show partial devitrification to crystallites (~5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (~10 μm size)