Cristalización de sulfatos de estroncio en presencia de sílice disuelta

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEUniversidad Complutense de MadridMadrid (Comunidad Autónoma)pu

Crecimiento epitaxial de calcita sobre las caras (104) de dolomita y Kutnahorita

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Estudio nanotribológico de superficies minerales en condiciones acuosas

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Los cuasicristales muestran simetrías descritas por un mineralogista del siglo XVIII : [entrevista realizada por la Oficina de Transferencia de Resultados de Investigación (OTRI), Unidad de Información Científica y Divulgación de la Investigación, de la Universidad Complutense de Madrid a los profesores María Victoria López-Acevedo Cornejo y Carlos Pina Martínez, de la Facultad de Ciencias Geológicas de la UCM]

Tanto los cristales como los cuasicristales están formados por infinidad de átomos ordenados. La diferencia radica en que, en los primeros, este orden es periódico, mientras que en los cuasicristales, no lo es y muestran simetrías consideradas “prohibidas” en la Cristalografía. Su descubrimiento fue merecedor del Premio Nobel de Química en 2011. Curiosamente, dos siglos antes, el científico francés Jean-Baptiste Romé de L´Isle construyó una seriede modelos con la simetría de los cuasicristales. Victoria López-Acevedo y Carlos Manuel Pina, investigadores de la facultad de Ciencias Geológicas de la Universidad Complutense de Madrid (UCM), han analizado las morfologías cuasicristalinas descritas por Romé, tratando de esclarecer cómo pudo tener esta premonición científic

High resolution imaging of the dolomite (104) cleavage surface by atomic force microscopy

In this paper we present high resolution atomic force microscopy (AFM) images of dolomite (104) cleavage surfaces immersed in pure water. These images show a rectangular lattice with surface unit cell dimensions in general agreement with those derived from the dolomite bulk structure. Furthermore, the twodimensional fast Fourier transform (2D-FFT) plots of the high resolution images exhibit a pattern of periodicities consistent with both the alternate orientation of the carbonate groups and the positions for calcium and magnesium atoms on the dolomite (104) surface. However, the Mg2+ and Ca2+ sublattices could not be resolved. Finally, the images in both the real and the Fourier space do not reveal any clear evidence of reconstruction of the dolomite (104) surfaces

On The Surface Processes Of Crystal Growth At Low Temperatures

This paper deals with some microscopic phenomena, which occur on crystal surfaces during growth from aqueous solutions. Since crystallisation and crystal-solution interactions are ubiquitous in nature, their study is of relevance for a better understanding of processes in mineralogy, environmental sciences and biomineralisation. By using Atomic Force Microscopy we have been able to image the evolution of surfaces during the growth of pure substances and solids with chemical variability (solid solutions), and also during dissolution-crystallisation reactions. In order to interpret, and in some cases predict, the crystal growth behaviour at a molecular scale we have taken into account the relationships between supersaturation, growth mechanisms and crystallographic features of the surfaces

Los fundamentos de la Cristalografía: una reseña histórica

La Cristalografía es la ciencia que estudia la estructura, propiedades y formación de los cristales, así como la interacción de éstos con la radiación. El desarrollo de la Cristalografía como una ciencia moderna es el resultado de un esfuerzo experimental y teórico que abarca más de trescientos años de investigación. Desde las primeras mediciones de las caras de cristales de cuarzo realizadas por Nicolaus Steno en el siglo XVII hasta las recientes determinaciones de las estructuras cristalinas de complejas moléculas biológicas, los científicos han recorrido un apasionante camino en el que poco a poco se han ido revelando los secretos del orden y las propiedades de los cristales. En este artículo presento una breve historia de la Cristalografía y explico algunos de los descubrimientos que han contribuido de forma decisiva al desarrollo de esta ciencia

The composition of solid solutions crystallising from aqueous solutions: the influence of supersaturation and growth mechanisms

In this paper we present a new approach to the problem of the crystallisation in solid solution–aqueous solution (SS–AS) systems, in which two main controlling factors have been considered: (i) the supersaturation state of the multicomponent solution in contact with the growing crystal and (ii) the growth mechanisms, operating at a molecular scale on the various faces of the crystal. Supersaturation has been evaluated as a function of the solid solution (the β function) and the transitional supersaturation between spiral growth and two dimensional nucleation mechanisms has been considered as a linear function of the solid composition (the β* line). By superimposing β functions and β* line on a supersaturation–solid composition diagram, we can define compositional regions growing according to different growth mechanisms. In order to test our model, a number of in situ Atomic Force Microscope (AFM) experiments have been conducted in the Ba2+–Sr 2+–SO24–H2O system, using barite (001) as the substrate. The general growth behaviour observed is consistent with the predictions given for a number of initial aqueous solution compositions. Microprobe analysis shows that the new (001) layers grown under conditions where the maximum supersaturation corresponded to intermediate compositions of the (Ba,Sr)SO4 solid are very Sr-rich. A qualitative explanation for such a compositional shift is given on the basis of nucleation rate calculations in the Ba2+–Sr 2+–SO424–H2O system. Finally we discuss the effect of the substrate on the formation and distribution of two-dimensional (Ba,Sr)SO4 nuclei on a barite (001) surface

The formation of the dolomite-analogue norsethite: Reaction pathway and cation ordering

Reaction pathways and cation ordering mechanisms involved in the formation of the mineral dolomite in nature still remain poorly understood. This is mainly due to the experimental problems posed by the synthesis of dolomite at ambient conditions, which preclude monitoring its formation in reasonable time scales. However, processes leading to the crystallization of fully-ordered dolomite-like structures can be studied by conducting experiments with mineral analogues, which are more readily precipitated. In this paper we present a study of the formation of the dolomite-analogue norsethite [BaMg(CO3)2] from a slurry which was aged at room temperature during 14 days. We found that norsethide forms by two dissolution–crystallization reactions from an initial amorphous nano-sized precipitate. The first reaction produces a mineral assemblage composed by witherite [BaCO3], northupite [Na3Mg(CO3)2Cl] and norsethite. The second dissolution–crystallization process leads to the almost complete depletion of witherite and northupite in favor of norsethite. While the composition of norsethite crystals rapidly reaches a Ba/Mg = 1 ratio, X-ray diffraction peaks indicate an increase in the crystallinity of those crystals during the first 48 h of reaction. Simultaneously, Ba–Mg cation ordering increases, as shown by the evolution of intensity ratios of certain superstructure and structure reflections. Altogether, these results demonstrate that the formation of fully-ordered norsethite occurs by a sequence of solvent-mediated processes which involve a number of precursors. Our study also suggests that similar processes might lead to the formation of dolomite in natural environments

Quasicrystals and Other Aperiodic Structures in Mineralogy

In this article, we first present and discuss eighteenth-century descriptions of minerals that contributed decisively to the development of crystallography. Remarkably, these old crystallographic descriptions included morphologies with symmetries incompatible with an internal periodic order of atoms, which, however, have been recognised to be characteristics of quasicrystals. Moreover, we also review a number of studies of minerals with aperiodic crystal structures, including recently reported natural quasicrystals of extra-terrestrial origin. Finally, we discuss the current investigations addressing the search for new quasicrystalline minerals in nature